# HDF5Array performance

# Introduction

The aim of this document is to measure the performance of the HDF5Array package for normalization and PCA (Principal Component Analysis) of on-disk single cell data, two computationally intensive operations at the core of single cell analysis.

The benchmarks presented in the document were specifically designed to observe the impact of two critical parameters on performance:

1. data representation (i.e. sparse vs dense);
2. size of the blocks used for block-processed operations.

Hopefully these benchmarks will also facilitate comparing performance of single cell analysis workflows based on HDF5Array with workflows based on other tools like Seurat or Scanpy.

# Install and load the required packages

Let’s install and load HDF5Array as well as the other packages used in this vignette:

    if (!require("BiocManager", quietly=TRUE))
        install.packages("BiocManager")

    pkgs <- c("HDF5Array", "ExperimentHub", "DelayedMatrixStats", "RSpectra")
    BiocManager::install(pkgs)

Load the packages:

    library(HDF5Array)
    library(ExperimentHub)
    library(DelayedMatrixStats)
    library(RSpectra)

# The test datasets

## Sparse vs dense representation

The datasets that we will use for our benchmarks are subsets of the 1.3 Million Brain Cell Dataset from 10x Genomics. This is a sparse 27,998 x 1,306,127 matrix of counts, with one gene per row and one cell per column. Around 7% of the matrix values are nonzero counts. The dataset is available via the ExperimentHub package in two forms:

1. As a sparse HDF5 file: This is the original HDF5 file provided by 10x Genomics. It uses the CSR/CSC/Yale representation to store the sparse data.

2. As a dense HDF5 file: The same data as the above but stored as a regular HDF5 dataset with (compressed) chunks of dimensions 100 x 100.

The two files are hosted on ExperimentHub under resource ids `EH1039` and `EH1040`:

    hub <- ExperimentHub()

    hub["EH1039"]$description  # sparse representation

    ## [1] "Single-cell RNA-seq data for 1.3 million brain cells from E18 mice. 'HDF5-based 10X Genomics' format originally provided by TENx Genomics"

    hub["EH1040"]$description  # dense representation

    ## [1] "Single-cell RNA-seq data for 1.3 million brain cells from E18 mice. Full rectangular, block-compressed format, 1GB block size."

Let’s download them to the local ExperimentHub cache if they are not there yet:

    ## Note that this will be quick if the HDF5 files are already in the
    ## local ExperimentHub cache. Otherwise, it will take a while!
    brain_s_path <- hub[["EH1039"]]

    brain_D_path <- hub[["EH1040"]]

`brain_s_path` and `brain_D_path` are the paths to the downloaded files.

## TENxMatrix vs HDF5Matrix objects

We use the `TENxMatrix()` and `HDF5Array()` constructors to bring the sparse and dense datasets in R, as DelayedArray derivatives. Note that this does not load the matrix data in memory.

### Bring the sparse dataset in R

    ## Use 'h5ls(brain_s_path)' to find out the group.
    brain_s <- TENxMatrix(brain_s_path, group="mm10")

`brain_s` is a 27,998 x 1,306,127 TENxMatrix object:

    class(brain_s)

    ## [1] "TENxMatrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    dim(brain_s)

    ## [1]   27998 1306127

    is_sparse(brain_s)

    ## [1] TRUE

See `?TENxMatrix` in the HDF5Array package for more information about TENxMatrix objects.

### Bring the dense dataset in R

    ## Use 'h5ls(brain_D_path)' to find out the name of the dataset.
    brain_D <- HDF5Array(brain_D_path, name="counts")

`brain_D` is a 27,998 x 1,306,127 HDF5Matrix object that contains the same data as `brain_s`:

    class(brain_D)

    ## [1] "HDF5Matrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    dim(brain_D)

    ## [1]   27998 1306127

    chunkdim(brain_D)

    ## [1] 100 100

    is_sparse(brain_D)

    ## [1] FALSE

See `?HDF5Matrix` in the HDF5Array package for more information about HDF5Matrix objects.

Even though the data in `brain_D_path` is stored in a dense format, we can flag it as quantitatively sparse. This is done by calling the `HDF5Array()` constructor function with `as.sparse=TRUE`:

    brain_Ds <- HDF5Array(brain_D_path, name="counts", as.sparse=TRUE)

The only difference between `brain_D` and `brain_Ds` is that the latter is now seen as a sparse object, and will be treated as such:

    class(brain_Ds)

    ## [1] "HDF5Matrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    dim(brain_Ds)

    ## [1]   27998 1306127

    chunkdim(brain_Ds)

    ## [1] 100 100

    is_sparse(brain_Ds)

    ## [1] TRUE

Concretely this means that, when blocks of data are loaded from the dense HDF5 file to memory during block-processed operations, they end up directly in an in-memory sparse representation without going thru an in-memory dense representation first. This is expected to reduce memory footprint and (hopefully) will help with overall performance.

Finally note that the dense HDF5 file does not contain the dimnames of the matrix, so we manually add them to `brain_s` and `brain_Ds`:

    dimnames(brain_Ds) <- dimnames(brain_D) <- dimnames(brain_s)

## Create the test datasets

For our benchmarks below, we create subsets of the 1.3 Million Brain Cell Dataset of increasing sizes: subsets with 12,500 cells, 25,000 cells, 50,000 cells, 100,000 cells, and 200,000 cells. Note that subsetting a TENxMatrix or HDF5Matrix object with `[` is a delayed operation so has virtually no cost:

    brain1_s  <- brain_s[  , 1:12500]
    brain1_D  <- brain_D[  , 1:12500]
    brain1_Ds <- brain_Ds[ , 1:12500]

    brain2_s  <- brain_s[  , 1:25000]
    brain2_D  <- brain_D[  , 1:25000]
    brain2_Ds <- brain_Ds[ , 1:25000]

    brain3_s  <- brain_s[  , 1:50000]
    brain3_D  <- brain_D[  , 1:50000]
    brain3_Ds <- brain_Ds[ , 1:50000]

    brain4_s  <- brain_s[  , 1:100000]
    brain4_D  <- brain_D[  , 1:100000]
    brain4_Ds <- brain_Ds[ , 1:100000]

    brain5_s  <- brain_s[  , 1:200000]
    brain5_D  <- brain_D[  , 1:200000]
    brain5_Ds <- brain_Ds[ , 1:200000]

# Block-processed normalization and PCA

## Code used for normalization and PCA

We’ll use the following code for normalization:

    ## Also selects the most variable genes (1000 by default).
    simple_normalize <- function(mat, num_var_genes=1000)
    {
        stopifnot(length(dim(mat)) == 2, !is.null(rownames(mat)))
        mat <- mat[rowSums(mat) > 0, ]
        col_sums <- colSums(mat) / 10000
        mat <- t(t(mat) / col_sums)
        row_vars <- rowVars(mat)
        row_vars_order <- order(row_vars, decreasing=TRUE)
        variable_idx <- head(row_vars_order, n=num_var_genes)
        mat <- log1p(mat[variable_idx, ])
        mat / rowSds(mat)
    }

and the following code for PCA:

    simple_PCA <- function(mat, k=25)
    {
        stopifnot(length(dim(mat)) == 2)
        row_means <- rowMeans(mat)
        Ax <- function(x, args)
            (as.numeric(mat %*% x) - row_means * sum(x))
        Atx <- function(x, args)
            (as.numeric(x %*% mat) - as.vector(row_means %*% x))
        RSpectra::svds(Ax, Atrans=Atx, k=k, dim=dim(mat))
    }

## Block processing and block size

Note that the implementations of `simple_normalize()` and `simple_PCA()` are expected to work on any matrix-like object regardless of its exact type/representation e.g. it can be an ordinary matrix, a SparseMatrix object from the SparseArray package, a dgCMatrix object from the Matrix package, a DelayedMatrix derivative (TENxMatrix, HDF5Matrix, TileDBMatrix), etc…

However, when the input is a DelayedMatrix object or derivative, it’s important to be aware that:

- Summarization methods like `sum()`, `colSums()`, `rowVars()`, or `rowSds()`, and matrix multiplication (`%*%`), are block-processed operations.

- The block size is 100 Mb by default. Increasing or decreasing the block size will typically increase or decrease the memory usage of block-processed operations. It will also impact performance, but sometimes in unexpected or counter-intuitive ways.

- The block size can be controlled with `DelayedArray::getAutoBlockSize()` and `DelayedArray::setAutoBlockSize()`.

For our benchmarks below, we’ll use the following block sizes:

| NORMALIZATION | PCA
--- | --- | ---
TENxMatrix (sparse) | 250 Mb | 40 Mb
HDF5Matrix (dense) | 16 Mb | 100 Mb
HDF5Matrix as sparse | 250 Mb | 40 Mb

## Monitoring memory usage

While manually running our benchmarks below on a Linux or macOS system, we will also monitor memory usage at the command line in a terminal with:

    (while true; do ps u -p <PID>; sleep 1; done) >ps.log 2>&1 &

where `<PID>` is the process id of our R session. This will allow us to measure the maximum amount of memory used by the calls to `simple_normalize()` or `simple_PCA()`.

# Normalization and PCA of the 27,998 x 12,500 test dataset

## Normalization

In this section we run `simple_normalize()` on the three different representations (TENxMatrix, HDF5Matrix, and “HDF5Matrix as sparse”) of the smaller test dataset only (27,998 x 12,500), and we report the time of each run.

See the Comprehensive timings obtained on various machines section below in this document for `simple_normalize()` and `simple_pca()` timings obtained on various machines on all our test datasets and using four different block sizes: 16 Mb, 40 Mb, 100 Mb, and 250 Mb.

### TENxMatrix (sparse)

    dim(brain1_s)

    ## [1] 27998 12500

    DelayedArray::setAutoBlockSize(250e6)  # set block size to 250 Mb

    ## automatic block size set to 2.5e+08 bytes (was 1e+08)

    system.time(norm_brain1_s <- simple_normalize(brain1_s))

    ##    user  system elapsed
    ##  41.776   0.975  45.096

    dim(norm_brain1_s)

    ## [1]  1000 12500

### HDF5Matrix (dense)

    dim(brain1_D)

    ## [1] 27998 12500

    DelayedArray::setAutoBlockSize(16e6)   # set block size to 16 Mb

    ## automatic block size set to 1.6e+07 bytes (was 2.5e+08)

    system.time(norm_brain1_D <- simple_normalize(brain1_D))

    ##    user  system elapsed
    ##  62.725   0.557  63.292

    dim(norm_brain1_D)

    ## [1]  1000 12500

### HDF5Matrix as sparse

    dim(brain1_Ds)

    ## [1] 27998 12500

    DelayedArray::setAutoBlockSize(250e6)  # set block size to 250 Mb

    ## automatic block size set to 2.5e+08 bytes (was 1.6e+07)

    system.time(norm_brain1_Ds <- simple_normalize(brain1_Ds))

    ##    user  system elapsed
    ##  50.173   0.748  50.930

    dim(norm_brain1_Ds)

    ## [1]  1000 12500

## On-disk realization of the normalized datasets

Note that the normalized datasets obtained in the previous section are DelayedMatrix objects that carry delayed operations. These operations can be displayed with `showtree()` e.g. for `norm_brain1_s`:

    class(norm_brain1_s)

    ## [1] "DelayedMatrix"
    ## attr(,"package")
    ## [1] "DelayedArray"

    showtree(norm_brain1_s)

    ## 1000x12500 double, sparse: DelayedMatrix object
    ## └─ 1000x12500 double, sparse: Unary iso op with args
    ##    └─ 1000x12500 double, sparse: Stack of 1 unary iso op(s)
    ##       └─ 1000x12500 double, sparse: Aperm (perm=c(2,1))
    ##          └─ 12500x1000 double, sparse: Unary iso op with args
    ##             └─ 12500x1000 integer, sparse: Aperm (perm=c(2,1))
    ##                └─ 1000x12500 integer, sparse: Subset
    ##                   └─ 27998x1306127 integer, sparse: [seed] TENxMatrixSeed object

The other `norm_brain1_*` objects carry similar operations.

Before we proceed with PCA, we’re going to write the normalized datasets to new HDF5 files. This introduces an additional cost, but, on the other hand, it has the benefit of triggering on-disk realization of the object. This means that all the delayed operations carried by the object will get realized on-the-fly before the matrix data actually lands on the disk, making the new object (TENxMatrix or HDF5Matrix) more efficient for PCA or whatever block-processed operations will come next.

We will use blocks of 100 Mb for all the writing operations.

    DelayedArray::setAutoBlockSize(1e8)

    ## automatic block size set to 1e+08 bytes (was 2.5e+08)

### TENxMatrix (sparse)

    dim(norm_brain1_s)

    ## [1]  1000 12500

    system.time(norm_brain1_s <- writeTENxMatrix(norm_brain1_s, level=0))

    ##    user  system elapsed
    ##   5.268   0.095   5.364

The new `norm_brain1_s` object is a pristine TENxMatrix object:

    class(norm_brain1_s)

    ## [1] "TENxMatrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    showtree(norm_brain1_s)  # "pristine" object (i.e. no more delayed operations)

    ## 1000x12500 double, sparse: TENxMatrix object
    ## └─ 1000x12500 double, sparse: [seed] TENxMatrixSeed object

### HDF5Matrix (dense)

    dim(norm_brain1_D)

    ## [1]  1000 12500

    system.time(norm_brain1_D <- writeHDF5Array(norm_brain1_D, level=0))

    ##    user  system elapsed
    ##   3.398   0.129   3.529

The new `norm_brain1_D` object is a pristine HDF5Matrix object:

    class(norm_brain1_D)

    ## [1] "HDF5Matrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    showtree(norm_brain1_D)  # "pristine" object (i.e. no more delayed operations)

    ## 1000x12500 double: HDF5Matrix object
    ## └─ 1000x12500 double: [seed] HDF5ArraySeed object

### HDF5Matrix as sparse

    dim(norm_brain1_Ds)

    ## [1]  1000 12500

    system.time(norm_brain1_Ds <- writeHDF5Array(norm_brain1_Ds, level=0))

    ##    user  system elapsed
    ##   5.042   0.144   5.188

The new `norm_brain1_Ds` object is a pristine sparse HDF5Matrix object:

    class(norm_brain1_Ds)

    ## [1] "HDF5Matrix"
    ## attr(,"package")
    ## [1] "HDF5Array"

    showtree(norm_brain1_Ds)  # "pristine" object (i.e. no more delayed operations)

    ## 1000x12500 double, sparse: HDF5Matrix object
    ## └─ 1000x12500 double, sparse: [seed] HDF5ArraySeed object

## PCA

In this section we run `simple_pca()` on the normalized datasets obtained in the previous section and report the time of each run.

See the Comprehensive timings obtained on various machines section below in this document for `simple_normalize()` and `simple_pca()` timings obtained on various machines on all our test datasets and using four different block sizes: 16 Mb, 40 Mb, 100 Mb, and 250 Mb.

### TENxMatrix (sparse)

    DelayedArray::setAutoBlockSize(40e6)   # set block size to 40 Mb

    ## automatic block size set to 4e+07 bytes (was 1e+08)

    dim(norm_brain1_s)

    ## [1]  1000 12500

    system.time(pca1s <- simple_PCA(norm_brain1_s))

    ##    user  system elapsed
    ##  52.424   2.266  54.700

### HDF5Matrix (dense)

    DelayedArray::setAutoBlockSize(1e8)    # set block size to 100 Mb

    ## automatic block size set to 1e+08 bytes (was 4e+07)

    dim(norm_brain1_D)

    ## [1]  1000 12500

    system.time(pca1D <- simple_PCA(norm_brain1_D))

    ##    user  system elapsed
    ##  70.895  89.242  62.637

Sanity check:

    stopifnot(all.equal(pca1D, pca1s))

### HDF5Matrix as sparse

    DelayedArray::setAutoBlockSize(40e6)   # set block size to 40 Mb

    ## automatic block size set to 4e+07 bytes (was 1e+08)

    dim(norm_brain1_Ds)

    ## [1]  1000 12500

    system.time(pca1Ds <- simple_PCA(norm_brain1_Ds))

    ##    user  system elapsed
    ## 159.147   3.630 162.805

Sanity check:

    stopifnot(all.equal(pca1Ds, pca1s))

# Comprehensive timings obtained on various machines

Here we report timings (and memory usage) observed on various machines. For each machine, the results are presented in a table that shows the normalization & realization & PCA timings obtained for all our test datasets and using four different block sizes: 16 Mb, 40 Mb, 100 Mb, and 250 Mb. For each operation, the best time across the four different block sizes is displayed in bold.

All the timings (and memory usage) were produced by running the `run_benchmarks.sh` script located in the `HDF5Array/inst/scripts/` folder of the package, using R 4.5 and HDF5Array 1.35.12 (Bioconductor 3.21).

## Timings for DELL XPS 15 laptop

Specs for DELL XPS 15 laptop (model 9520)
OS | Linux Ubuntu 24.04 | Disk performance | Output of `sudo hdparm -Tt <device>`: `Timing cached reads: 35188 MB in 2.00 seconds = 17620.75 MB/sec Timing buffered disk reads: 7842 MB in 3.00 seconds = 2613.57 MB/sec`
RAM | 32GB
Disk | 1TB SSD

Table 1: Timings for DELL XPS 15 laptop
Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | Normalized Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
nrow x ncol (# genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | nrow x ncol (# sel. genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used
|  | 1. NORMALIZATION & selection of 1000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 74 | 1.2Gb | 57 | 1.2Gb | 40 | 1.6Gb | 34 | 1.9Gb | 1000 x 12500 | [s] | 3 | 1.1Gb | 3 | 1.3Gb | 4 | 1.6Gb | 4 | 1.7Gb | 41 | 1.2Gb | 39 | 1.3Gb | 51 | 1.5Gb | 39 | 1.6Gb
[D] | 51 | 1.3Gb | 50 | 1.4Gb | 52 | 1.6Gb | 46 | 2.2Gb | [D] | 3 | 1.3Gb | 2 | 1.2Gb | 2 | 1.7Gb | 2 | 2.2Gb | 49 | 1.4Gb | 41 | 1.4Gb | 55 | 1.3Gb | 35 | 1.7Gb
[Ds] | 46 | 1.2Gb | 47 | 1.3Gb | 37 | 1.6Gb | 36 | 2.2Gb | [Ds] | 6 | 1.2Gb | 5 | 1.3Gb | 4 | 1.6Gb | 4 | 2.1Gb | 135 | 1.4Gb | 119 | 1.7Gb | 159 | 1.8Gb | 132 | 2.0Gb
27998 x 25000 | [s] | 152 | 1.2Gb | 122 | 1.3Gb | 90 | 1.7Gb | 63 | 2.3Gb | 1000 x 25000 | [s] | 6 | 1.2Gb | 6 | 1.3Gb | 7 | 1.9Gb | 7 | 2.1Gb | 84 | 1.2Gb | 76 | 1.5Gb | 94 | 1.7Gb | 76 | 2.0Gb
[D] | 94 | 1.3Gb | 98 | 1.5Gb | 114 | 1.7Gb | 99 | 2.2Gb | [D] | 7 | 1.3Gb | 4 | 1.3Gb | 5 | 1.5Gb | 4 | 1.8Gb | 94 | 1.4Gb | 78 | 1.5Gb | 81 | 1.7Gb | 130 | 1.5Gb
[Ds] | 91 | 1.3Gb | 89 | 1.4Gb | 88 | 1.9Gb | 75 | 2.3Gb | [Ds] | 11 | 1.3Gb | 8 | 1.4Gb | 8 | 1.8Gb | 9 | 2.3Gb | 241 | 1.5Gb | 221 | 2.0Gb | 285 | 2.0Gb | 244 | 2.6Gb
27998 x 50000 | [s] | 328 | 1.2Gb | 238 | 1.5Gb | 178 | 2.0Gb | 140 | 2.5Gb | 1000 x 50000 | [s] | 13 | 1.2Gb | 15 | 1.4Gb | 13 | 1.8Gb | 12 | 2.5Gb | 165 | 1.2Gb | 156 | 1.4Gb | 193 | 1.8Gb | 176 | 2.2Gb
[D] | 183 | 1.4Gb | 198 | 1.5Gb | 219 | 1.7Gb | 209 | 2.4Gb | [D] | 17 | 1.3Gb | 13 | 1.3Gb | 9 | 1.5Gb | 9 | 1.6Gb | 241 | 1.4Gb | 215 | 1.5Gb | 189 | 1.6Gb | 167 | 2.2Gb
[Ds] | 183 | 1.2Gb | 174 | 1.5Gb | 167 | 1.9Gb | 165 | 2.5Gb | [Ds] | 27 | 1.2Gb | 20 | 1.5Gb | 16 | 1.7Gb | 15 | 2.4Gb | 535 | 1.6Gb | 474 | 1.9Gb | 620 | 1.9Gb | 472 | 3.6Gb
27998 x 100000 | [s] | 642 | 1.2Gb | 458 | 1.6Gb | 365 | 2.0Gb | 280 | 2.9Gb | 1000 x 100000 | [s] | 23 | 1.3Gb | 25 | 1.5Gb | 25 | 1.8Gb | 23 | 2.7Gb | 304 | 1.3Gb | 268 | 1.5Gb | 305 | 1.7Gb | 332 | 2.6Gb
[D] | 344 | 1.4Gb | 392 | 1.6Gb | 439 | 1.7Gb | 417 | 2.3Gb | [D] | 34 | 1.3Gb | 25 | 1.3Gb | 18 | 1.6Gb | 17 | 1.9Gb | 520 | 1.5Gb | 424 | 1.6Gb | 260 | 1.7Gb | 310 | 2.2Gb
[Ds] | 354 | 1.2Gb | 340 | 1.5Gb | 323 | 2.0Gb | 334 | 3.0Gb | [Ds] | 52 | 1.3Gb | 40 | 1.5Gb | 34 | 1.9Gb | 30 | 3.1Gb | 1057 | 1.7Gb | 1016 | 2.0Gb | 1080 | 2.2Gb | 944 | 3.9Gb
27998 x 200000 | [s] | 1284 | 1.3Gb | 985 | 1.6Gb | 738 | 2.0Gb | 610 | 3.7Gb | 1000 x 200000 | [s] | 46 | 1.3Gb | 47 | 1.6Gb | 52 | 2.0Gb | 46 | 3.4Gb | 585 | 1.3Gb | 535 | 1.6Gb | 703 | 1.8Gb | 665 | 2.9Gb
[D] | 694 | 1.4Gb | 796 | 1.9Gb | 886 | 1.7Gb | 855 | 3.0Gb | [D] | 78 | 1.3Gb | 62 | 1.8Gb | 49 | 1.6Gb | 36 | 2.8Gb | 1588 | 1.6Gb | 1088 | 2.1Gb | 892 | 1.8Gb | 676 | 3.0Gb
[Ds] | 775 | 1.3Gb | 680 | 1.6Gb | 642 | 2.4Gb | 677 | 3.6Gb | [Ds] | 113 | 1.4Gb | 93 | 1.6Gb | 74 | 2.2Gb | 62 | 3.4Gb | 2756 | 2.0Gb | 2327 | 2.1Gb | 2525 | 2.6Gb | 2103 | 3.8Gb
|  | 1. NORMALIZATION & selection of 2000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 81 | 1.1Gb | 58 | 1.3Gb | 45 | 1.6Gb | 34 | 2.0Gb | 2000 x 12500 | [s] | 4 | 1.1Gb | 4 | 1.3Gb | 4 | 1.5Gb | 5 | 1.8Gb | 76 | 1.1Gb | 55 | 1.3Gb | 70 | 1.4Gb | 121 | 1.7Gb
[D] | 55 | 1.3Gb | 50 | 1.4Gb | 56 | 1.7Gb | 48 | 2.3Gb | [D] | 5 | 1.3Gb | 3 | 1.3Gb | 4 | 1.6Gb | 3 | 1.8Gb | 91 | 1.4Gb | 84 | 1.5Gb | 86 | 1.6Gb | 75 | 2.0Gb
[Ds] | 54 | 1.1Gb | 49 | 1.3Gb | 45 | 1.6Gb | 38 | 2.2Gb | [Ds] | 9 | 1.1Gb | 6 | 1.4Gb | 6 | 1.7Gb | 6 | 2.2Gb | 198 | 1.5Gb | 183 | 1.8Gb | 233 | 1.9Gb | 224 | 2.5Gb
27998 x 25000 | [s] | 162 | 1.2Gb | 127 | 1.3Gb | 95 | 1.6Gb | 75 | 2.2Gb | 2000 x 25000 | [s] | 8 | 1.2Gb | 9 | 1.3Gb | 9 | 1.7Gb | 8 | 2.1Gb | 128 | 1.2Gb | 127 | 1.3Gb | 127 | 1.5Gb | 155 | 2.0Gb
[D] | 103 | 1.3Gb | 102 | 1.5Gb | 117 | 1.6Gb | 105 | 2.1Gb | [D] | 13 | 1.3Gb | 9 | 1.4Gb | 7 | 1.5Gb | 6 | 1.6Gb | 217 | 1.4Gb | 186 | 1.5Gb | 184 | 1.6Gb | 150 | 2.1Gb
[Ds] | 102 | 1.3Gb | 94 | 1.4Gb | 95 | 1.7Gb | 91 | 2.6Gb | [Ds] | 18 | 1.3Gb | 15 | 1.5Gb | 11 | 1.5Gb | 11 | 2.4Gb | 401 | 1.6Gb | 398 | 1.8Gb | 426 | 1.8Gb | 435 | 3.1Gb
27998 x 50000 | [s] | 341 | 1.1Gb | 250 | 1.3Gb | 181 | 1.8Gb | 146 | 2.7Gb | 2000 x 50000 | [s] | 18 | 1.1Gb | 18 | 1.3Gb | 18 | 1.7Gb | 17 | 2.7Gb | 238 | 1.1Gb | 205 | 1.4Gb | 256 | 1.6Gb | 229 | 2.3Gb
[D] | 194 | 1.4Gb | 204 | 1.5Gb | 226 | 1.7Gb | 214 | 2.2Gb | [D] | 27 | 1.4Gb | 18 | 1.3Gb | 13 | 1.4Gb | 13 | 1.8Gb | 358 | 1.4Gb | 341 | 1.5Gb | 403 | 1.3Gb | 360 | 2.0Gb
[Ds] | 210 | 1.2Gb | 183 | 1.5Gb | 176 | 2.0Gb | 172 | 2.5Gb | [Ds] | 40 | 1.2Gb | 28 | 1.5Gb | 22 | 1.9Gb | 21 | 2.6Gb | 701 | 1.7Gb | 653 | 2.0Gb | 732 | 2.3Gb | 684 | 3.4Gb
27998 x 100000 | [s] | 684 | 1.2Gb | 471 | 1.4Gb | 374 | 1.9Gb | 292 | 2.9Gb | 2000 x 100000 | [s] | 33 | 1.2Gb | 30 | 1.4Gb | 32 | 1.9Gb | 29 | 2.7Gb | 434 | 1.2Gb | 351 | 1.4Gb | 496 | 1.7Gb | 410 | 2.5Gb
[D] | 376 | 1.4Gb | 407 | 1.5Gb | 452 | 1.7Gb | 433 | 2.3Gb | [D] | 61 | 1.4Gb | 44 | 1.4Gb | 33 | 1.6Gb | 25 | 2.0Gb | 990 | 1.5Gb | 788 | 1.6Gb | 656 | 1.9Gb | 859 | 1.4Gb
[Ds] | 405 | 1.2Gb | 359 | 1.5Gb | 341 | 2.1Gb | 349 | 2.8Gb | [Ds] | 88 | 1.3Gb | 68 | 1.5Gb | 51 | 2.1Gb | 44 | 2.7Gb | 1621 | 1.8Gb | 1425 | 2.0Gb | 1591 | 2.4Gb | 1451 | 3.4Gb
27998 x 200000 | [s] | 1373 | 1.3Gb | 1033 | 1.6Gb | 774 | 1.9Gb | 643 | 2.9Gb | 2000 x 200000 | [s] | 62 | 1.4Gb | 62 | 1.6Gb | 66 | 1.9Gb | 69 | 2.8Gb | 787 | 1.4Gb | 692 | 1.6Gb | 929 | 1.7Gb | 944 | 2.4Gb
[D] | 762 | 1.4Gb | 820 | 1.9Gb | 925 | 1.7Gb | 930 | 3.3Gb | [D] | 138 | 1.3Gb | 102 | 1.9Gb | 75 | 1.5Gb | 57 | 2.5Gb | 2487 | 1.6Gb | 1725 | 2.1Gb | 1361 | 1.8Gb | 1285 | 2.2Gb
[Ds] | 860 | 1.3Gb | 718 | 1.8Gb | 686 | 2.3Gb | 748 | 3.4Gb | [Ds] | 185 | 1.4Gb | 141 | 1.8Gb | 119 | 2.1Gb | 92 | 2.9Gb | 3745 | 1.9Gb | 3048 | 2.1Gb | 3196 | 2.9Gb | 3187 | 3.3Gb
Formats: [s] TENxMatrix (sparse) — [D] HDF5Matrix (dense) — [Ds] HDF5Matrix as sparse. For each operation, the best time across the four different block sizes is displayed in bold. In addition, if it’s also the best time across the three formats ([s],[D], and [Ds]), then we box it (only for Normalization and PCA). The “max. mem. used” is the max RSS (Resident Set Size) value obtained by running `ps u -p <PID>` every second while performing a given operation.

## Timings for Supermicro SuperServer 1029GQ-TRT

Specs for Supermicro SuperServer 1029GQ-TRT
OS | Linux Ubuntu 22.04 | Disk performance | Output of `sudo hdparm -Tt <device>`: `Timing cached reads: 20592 MB in 1.99 seconds = 10361.41 MB/sec Timing buffered disk reads: 1440 MB in 3.00 seconds = 479.66 MB/sec`
RAM | 384GB
Disk | 1.3TB ATA Disk

Table 2: Timings for Supermicro SuperServer 1029GQ-TRT
Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | Normalized Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
nrow x ncol (# genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | nrow x ncol (# sel. genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used
|  | 1. NORMALIZATION & selection of 1000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 129 | 1.0Gb | 96 | 1.2Gb | 69 | 1.5Gb | 56 | 2.0Gb | 1000 x 12500 | [s] | 5 | 1.0Gb | 5 | 1.2Gb | 6 | 1.4Gb | 6 | 1.6Gb | 85 | 1.1Gb | 77 | 1.2Gb | 91 | 1.4Gb | 171 | 1.5Gb
[D] | 79 | 1.2Gb | 83 | 1.3Gb | 83 | 1.7Gb | 75 | 2.2Gb | [D] | 5 | 1.2Gb | 4 | 1.2Gb | 3 | 1.0Gb | 3 | 1.6Gb | 75 | 1.3Gb | 67 | 1.3Gb | 77 | 1.5Gb | 60 | 1.6Gb
[Ds] | 71 | 1.1Gb | 74 | 1.3Gb | 59 | 1.5Gb | 59 | 2.1Gb | [Ds] | 9 | 1.1Gb | 7 | 1.3Gb | 7 | 1.6Gb | 8 | 1.6Gb | 239 | 1.3Gb | 239 | 1.5Gb | 276 | 1.7Gb | 253 | 1.8Gb
27998 x 25000 | [s] | 255 | 1.1Gb | 186 | 1.2Gb | 152 | 1.6Gb | 112 | 1.9Gb | 1000 x 25000 | [s] | 11 | 1.1Gb | 12 | 1.3Gb | 11 | 1.5Gb | 12 | 1.9Gb | 167 | 1.1Gb | 168 | 1.3Gb | 166 | 1.5Gb | 158 | 1.8Gb
[D] | 149 | 1.2Gb | 159 | 1.4Gb | 181 | 1.6Gb | 160 | 2.1Gb | [D] | 11 | 1.2Gb | 6 | 1.3Gb | 7 | 1.4Gb | 7 | 1.3Gb | 161 | 1.3Gb | 122 | 1.3Gb | 142 | 1.6Gb | 215 | 1.4Gb
[Ds] | 141 | 1.1Gb | 144 | 1.2Gb | 127 | 1.9Gb | 115 | 2.1Gb | [Ds] | 21 | 1.1Gb | 14 | 1.3Gb | 13 | 1.6Gb | 13 | 2.1Gb | 432 | 1.5Gb | 425 | 1.7Gb | 514 | 1.8Gb | 451 | 2.4Gb
27998 x 50000 | [s] | 496 | 1.1Gb | 377 | 1.4Gb | 301 | 1.7Gb | 241 | 2.4Gb | 1000 x 50000 | [s] | 22 | 1.1Gb | 21 | 1.4Gb | 22 | 1.7Gb | 21 | 2.6Gb | 311 | 1.1Gb | 286 | 1.4Gb | 367 | 1.7Gb | 334 | 2.3Gb
[D] | 282 | 1.3Gb | 331 | 1.4Gb | 347 | 1.6Gb | 329 | 2.2Gb | [D] | 24 | 1.3Gb | 20 | 1.3Gb | 14 | 1.3Gb | 14 | 1.6Gb | 376 | 1.3Gb | 360 | 1.4Gb | 298 | 1.2Gb | 282 | 2.1Gb
[Ds] | 275 | 1.1Gb | 280 | 1.3Gb | 251 | 1.8Gb | 255 | 2.6Gb | [Ds] | 39 | 1.1Gb | 30 | 1.4Gb | 26 | 1.7Gb | 25 | 2.3Gb | 934 | 1.5Gb | 894 | 1.8Gb | 1087 | 1.8Gb | 897 | 3.5Gb
27998 x 100000 | [s] | 1015 | 1.1Gb | 721 | 1.4Gb | 615 | 1.9Gb | 456 | 3.1Gb | 1000 x 100000 | [s] | 44 | 1.1Gb | 42 | 1.4Gb | 43 | 1.7Gb | 38 | 2.8Gb | 620 | 1.1Gb | 569 | 1.4Gb | 662 | 1.7Gb | 624 | 2.7Gb
[D] | 558 | 1.3Gb | 630 | 1.4Gb | 707 | 1.6Gb | 688 | 2.2Gb | [D] | 51 | 1.2Gb | 35 | 1.3Gb | 28 | 1.3Gb | 27 | 1.9Gb | 797 | 1.4Gb | 570 | 1.5Gb | 595 | 1.6Gb | 544 | 2.1Gb
[Ds] | 557 | 1.2Gb | 534 | 1.5Gb | 493 | 2.0Gb | 507 | 3.1Gb | [Ds] | 79 | 1.2Gb | 68 | 1.4Gb | 59 | 1.7Gb | 49 | 2.9Gb | 1901 | 1.5Gb | 1906 | 2.0Gb | 1989 | 2.3Gb | 1778 | 3.6Gb
27998 x 200000 | [s] | 2163 | 1.2Gb | 1579 | 1.4Gb | 1204 | 2.1Gb | 1014 | 3.3Gb | 1000 x 200000 | [s] | 77 | 1.2Gb | 92 | 1.4Gb | 88 | 2.1Gb | 81 | 3.1Gb | 1131 | 1.3Gb | 1102 | 1.4Gb | 1218 | 1.9Gb | 1133 | 3.0Gb
[D] | 1132 | 1.3Gb | 1294 | 1.6Gb | 1437 | 1.6Gb | 1368 | 3.2Gb | [D] | 114 | 1.3Gb | 93 | 1.4Gb | 75 | 1.4Gb | 57 | 2.9Gb | 2379 | 1.4Gb | 1698 | 1.7Gb | 1489 | 1.7Gb | 1133 | 2.9Gb
[Ds] | 1120 | 1.2Gb | 1082 | 1.5Gb | 989 | 2.3Gb | 1003 | 3.7Gb | [Ds] | 172 | 1.3Gb | 149 | 1.5Gb | 123 | 2.1Gb | 99 | 3.1Gb | 4643 | 1.8Gb | 4203 | 1.9Gb | 4064 | 2.5Gb | 3697 | 3.6Gb
|  | 1. NORMALIZATION & selection of 2000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 139 | 1.0Gb | 100 | 1.2Gb | 78 | 1.6Gb | 59 | 2.0Gb | 2000 x 12500 | [s] | 7 | 1.1Gb | 8 | 1.2Gb | 9 | 1.5Gb | 8 | 1.7Gb | 141 | 1.1Gb | 122 | 1.2Gb | 155 | 1.4Gb | 120 | 1.8Gb
[D] | 85 | 1.2Gb | 85 | 1.4Gb | 89 | 1.7Gb | 77 | 2.2Gb | [D] | 7 | 1.2Gb | 5 | 1.3Gb | 5 | 1.4Gb | 5 | 1.8Gb | 147 | 1.3Gb | 133 | 1.1Gb | 143 | 1.5Gb | 126 | 1.9Gb
[Ds] | 83 | 1.1Gb | 81 | 1.3Gb | 70 | 1.7Gb | 63 | 2.1Gb | [Ds] | 15 | 1.1Gb | 10 | 1.3Gb | 9 | 1.6Gb | 11 | 2.1Gb | 351 | 1.5Gb | 437 | 1.5Gb | 432 | 1.8Gb | 361 | 2.3Gb
27998 x 25000 | [s] | 266 | 1.1Gb | 193 | 1.3Gb | 159 | 1.5Gb | 130 | 2.0Gb | 2000 x 25000 | [s] | 14 | 1.1Gb | 15 | 1.3Gb | 19 | 1.5Gb | 13 | 2.1Gb | 269 | 1.1Gb | 252 | 1.3Gb | 284 | 1.5Gb | 273 | 1.8Gb
[D] | 162 | 1.2Gb | 166 | 1.4Gb | 188 | 1.6Gb | 174 | 2.0Gb | [D] | 19 | 1.2Gb | 14 | 1.3Gb | 11 | 1.3Gb | 11 | 1.6Gb | 336 | 1.3Gb | 299 | 1.4Gb | 284 | 1.2Gb | 452 | 1.3Gb
[Ds] | 163 | 1.1Gb | 155 | 1.3Gb | 138 | 1.7Gb | 139 | 2.3Gb | [Ds] | 33 | 1.1Gb | 24 | 1.3Gb | 19 | 1.5Gb | 18 | 2.1Gb | 752 | 1.4Gb | 705 | 1.7Gb | 731 | 1.9Gb | 702 | 3.3Gb
27998 x 50000 | [s] | 530 | 1.1Gb | 383 | 1.3Gb | 315 | 1.5Gb | 258 | 2.3Gb | 2000 x 50000 | [s] | 27 | 1.1Gb | 31 | 1.2Gb | 31 | 1.6Gb | 29 | 2.2Gb | 430 | 1.1Gb | 403 | 1.3Gb | 431 | 1.6Gb | 500 | 2.0Gb
[D] | 307 | 1.3Gb | 343 | 1.4Gb | 362 | 1.6Gb | 340 | 2.2Gb | [D] | 39 | 1.2Gb | 28 | 1.2Gb | 21 | 1.4Gb | 20 | 1.9Gb | 568 | 1.3Gb | 565 | 1.4Gb | 682 | 1.1Gb | 452 | 2.1Gb
[Ds] | 316 | 1.1Gb | 293 | 1.5Gb | 277 | 1.8Gb | 279 | 2.5Gb | [Ds] | 62 | 1.2Gb | 48 | 1.4Gb | 38 | 1.8Gb | 38 | 2.5Gb | 1235 | 1.5Gb | 1149 | 1.9Gb | 1317 | 2.1Gb | 1291 | 3.3Gb
27998 x 100000 | [s] | 1075 | 1.1Gb | 756 | 1.3Gb | 636 | 1.8Gb | 484 | 2.9Gb | 2000 x 100000 | [s] | 55 | 1.1Gb | 55 | 1.3Gb | 55 | 1.8Gb | 56 | 2.7Gb | 827 | 1.2Gb | 729 | 1.4Gb | 782 | 1.8Gb | 822 | 2.4Gb
[D] | 609 | 1.3Gb | 663 | 1.4Gb | 730 | 1.6Gb | 720 | 2.2Gb | [D] | 92 | 1.2Gb | 67 | 1.3Gb | 51 | 1.6Gb | 42 | 2.1Gb | 1534 | 1.4Gb | 1322 | 1.5Gb | 996 | 1.7Gb | 914 | 2.1Gb
[Ds] | 627 | 1.2Gb | 570 | 1.5Gb | 533 | 2.0Gb | 550 | 2.8Gb | [Ds] | 131 | 1.2Gb | 106 | 1.5Gb | 83 | 1.9Gb | 74 | 2.8Gb | 2821 | 1.6Gb | 2635 | 1.9Gb | 2907 | 2.3Gb | 2663 | 3.3Gb
27998 x 200000 | [s] | 2245 | 1.2Gb | 1599 | 1.4Gb | 1271 | 1.9Gb | 1069 | 3.0Gb | 2000 x 200000 | [s] | 107 | 1.2Gb | 111 | 1.4Gb | 117 | 1.9Gb | 110 | 3.0Gb | 1578 | 1.2Gb | 1494 | 1.4Gb | 1642 | 1.8Gb | 1718 | 2.6Gb
[D] | 1240 | 1.3Gb | 1345 | 1.6Gb | 1501 | 1.6Gb | 1429 | 3.2Gb | [D] | 199 | 1.2Gb | 151 | 1.5Gb | 113 | 1.5Gb | 84 | 2.6Gb | 3766 | 1.4Gb | 2798 | 1.7Gb | 2316 | 1.7Gb | 1836 | 2.7Gb
[Ds] | 1266 | 1.2Gb | 1142 | 1.6Gb | 1207 | 2.0Gb | 1102 | 3.0Gb | [Ds] | 288 | 1.2Gb | 219 | 1.6Gb | 191 | 1.8Gb | 148 | 2.6Gb | 6365 | 1.7Gb | 5369 | 2.0Gb | 5751 | 2.5Gb | 5848 | 3.2Gb
Formats: [s] TENxMatrix (sparse) — [D] HDF5Matrix (dense) — [Ds] HDF5Matrix as sparse. For each operation, the best time across the four different block sizes is displayed in bold. In addition, if it’s also the best time across the three formats ([s],[D], and [Ds]), then we box it (only for Normalization and PCA). The “max. mem. used” is the max RSS (Resident Set Size) value obtained by running `ps u -p <PID>` every second while performing a given operation.

## Timings for Apple Silicon Mac Pro

Specs for Apple Silicon Mac Pro (Apple M2 Ultra)
OS | macOS 13.7.1 | Disk performance | N/A
RAM | 192GB
Disk | 2TB SSD

Table 3: Timings for Apple Silicon Mac Pro
Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | Normalized Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
nrow x ncol (# genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | nrow x ncol (# sel. genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used
|  | 1. NORMALIZATION & selection of 1000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 66 | 1.4Gb | 48 | 2.2Gb | 35 | 2.5Gb | 28 | 3.2Gb | 1000 x 12500 | [s] | 3 | 1.6Gb | 2 | 2.2Gb | 4 | 2.5Gb | 3 | 3.4Gb | 40 | 1.7Gb | 29 | 2.0Gb | 29 | 2.7Gb | 29 | 3.4Gb
[D] | 41 | 1.6Gb | 36 | 2.1Gb | 35 | 2.5Gb | 31 | 3.6Gb | [D] | 3 | 1.7Gb | 2 | 2.1Gb | 2 | 2.5Gb | 2 | 3.6Gb | 38 | 1.9Gb | 34 | 2.2Gb | 30 | 2.8Gb | 23 | 3.7Gb
[Ds] | 36 | 1.6Gb | 35 | 2.1Gb | 30 | 2.5Gb | 29 | 3.0Gb | [Ds] | 5 | 1.6Gb | 3 | 2.2Gb | 4 | 2.6Gb | 4 | 3.1Gb | 104 | 1.9Gb | 101 | 2.4Gb | 125 | 3.2Gb | 104 | 3.6Gb
27998 x 25000 | [s] | 138 | 1.8Gb | 102 | 2.3Gb | 76 | 3.1Gb | 54 | 3.6Gb | 1000 x 25000 | [s] | 5 | 1.8Gb | 5 | 2.5Gb | 6 | 3.1Gb | 6 | 3.7Gb | 61 | 1.8Gb | 60 | 2.5Gb | 76 | 3.1Gb | 62 | 4.0Gb
[D] | 73 | 1.9Gb | 70 | 2.4Gb | 73 | 2.4Gb | 65 | 3.8Gb | [D] | 6 | 1.7Gb | 3 | 2.4Gb | 4 | 2.4Gb | 3 | 3.8Gb | 74 | 1.8Gb | 53 | 2.4Gb | 54 | 2.4Gb | 50 | 4.0Gb
[Ds] | 71 | 1.6Gb | 71 | 2.1Gb | 68 | 3.2Gb | 58 | 4.1Gb | [Ds] | 10 | 1.6Gb | 8 | 2.2Gb | 7 | 3.3Gb | 7 | 4.3Gb | 204 | 2.1Gb | 207 | 2.6Gb | 218 | 3.6Gb | 178 | 4.8Gb
27998 x 50000 | [s] | 257 | 1.8Gb | 199 | 2.7Gb | 150 | 3.2Gb | 116 | 5.0Gb | 1000 x 50000 | [s] | 11 | 1.9Gb | 12 | 2.7Gb | 12 | 3.2Gb | 11 | 4.7Gb | 128 | 2.1Gb | 124 | 2.7Gb | 162 | 3.1Gb | 147 | 4.9Gb
[D] | 144 | 1.8Gb | 140 | 2.9Gb | 144 | 2.3Gb | 139 | 3.5Gb | [D] | 13 | 1.9Gb | 11 | 2.9Gb | 7 | 2.4Gb | 6 | 3.5Gb | 189 | 2.0Gb | 157 | 2.7Gb | 140 | 2.5Gb | 123 | 3.7Gb
[Ds] | 144 | 1.7Gb | 138 | 2.2Gb | 132 | 3.2Gb | 129 | 5.5Gb | [Ds] | 21 | 1.9Gb | 16 | 2.2Gb | 14 | 3.4Gb | 13 | 5.9Gb | 418 | 2.6Gb | 400 | 3.2Gb | 479 | 4.1Gb | 365 | 7.1Gb
27998 x 100000 | [s] | 575 | 1.9Gb | 398 | 2.9Gb | 308 | 3.5Gb | 237 | 4.9Gb | 1000 x 100000 | [s] | 22 | 1.9Gb | 22 | 3.0Gb | 22 | 3.5Gb | 21 | 4.6Gb | 256 | 1.9Gb | 237 | 3.0Gb | 249 | 3.5Gb | 260 | 5.1Gb
[D] | 281 | 2.2Gb | 276 | 2.5Gb | 295 | 3.0Gb | 289 | 3.8Gb | [D] | 28 | 2.0Gb | 20 | 2.5Gb | 14 | 3.0Gb | 14 | 3.9Gb | 407 | 2.1Gb | 297 | 2.6Gb | 345 | 3.0Gb | 223 | 3.9Gb
[Ds] | 285 | 2.0Gb | 269 | 2.2Gb | 259 | 4.1Gb | 255 | 6.0Gb | [Ds] | 43 | 2.1Gb | 33 | 2.7Gb | 27 | 3.9Gb | 25 | 6.3Gb | 887 | 3.0Gb | 935 | 3.9Gb | 910 | 5.0Gb | 730 | 7.0Gb
27998 x 200000 | [s] | 1122 | 2.6Gb | 834 | 3.3Gb | 631 | 4.1Gb | 514 | 7.6Gb | 1000 x 200000 | [s] | 41 | 2.6Gb | 41 | 3.4Gb | 44 | 4.1Gb | 41 | 7.5Gb | 466 | 2.7Gb | 495 | 3.4Gb | 655 | 4.2Gb | 547 | 7.4Gb
[D] | 561 | 2.2Gb | 554 | 3.3Gb | 588 | 2.7Gb | 595 | 4.4Gb | [D] | 62 | 2.2Gb | 48 | 3.3Gb | 37 | 2.8Gb | 28 | 4.6Gb | 1244 | 2.5Gb | 809 | 3.4Gb | 652 | 2.9Gb | 453 | 4.7Gb
[Ds] | 583 | 2.0Gb | 529 | 2.8Gb | 501 | 4.2Gb | 525 | 7.5Gb | [Ds] | 91 | 2.2Gb | 78 | 2.9Gb | 62 | 4.2Gb | 52 | 7.2Gb | 2221 | 3.2Gb | 1977 | 4.2Gb | 1992 | 5.3Gb | 1718 | 7.9Gb
|  | 1. NORMALIZATION & selection of 2000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 68 | 1.5Gb | 50 | 1.9Gb | 39 | 2.3Gb | 29 | 3.3Gb | 2000 x 12500 | [s] | 4 | 1.7Gb | 4 | 2.0Gb | 3 | 2.3Gb | 4 | 3.3Gb | 58 | 1.7Gb | 54 | 2.1Gb | 48 | 2.2Gb | 52 | 3.7Gb
[D] | 44 | 1.8Gb | 37 | 2.1Gb | 38 | 2.5Gb | 32 | 3.8Gb | [D] | 4 | 1.8Gb | 3 | 2.2Gb | 2 | 2.5Gb | 2 | 3.8Gb | 72 | 2.1Gb | 55 | 2.3Gb | 56 | 2.6Gb | 51 | 3.8Gb
[Ds] | 42 | 1.6Gb | 39 | 2.1Gb | 35 | 2.5Gb | 31 | 3.3Gb | [Ds] | 7 | 1.6Gb | 5 | 2.1Gb | 5 | 2.5Gb | 5 | 3.4Gb | 149 | 2.0Gb | 151 | 2.8Gb | 166 | 3.3Gb | 159 | 4.4Gb
27998 x 25000 | [s] | 148 | 1.6Gb | 104 | 2.0Gb | 80 | 2.7Gb | 62 | 4.0Gb | 2000 x 25000 | [s] | 8 | 1.8Gb | 7 | 2.1Gb | 7 | 2.7Gb | 7 | 4.2Gb | 119 | 1.8Gb | 94 | 2.3Gb | 107 | 2.7Gb | 127 | 4.5Gb
[D] | 80 | 2.1Gb | 72 | 2.4Gb | 76 | 2.5Gb | 71 | 3.8Gb | [D] | 10 | 2.1Gb | 7 | 2.6Gb | 5 | 2.5Gb | 5 | 3.8Gb | 174 | 2.3Gb | 138 | 2.7Gb | 108 | 2.5Gb | 107 | 4.4Gb
[Ds] | 82 | 1.6Gb | 77 | 2.1Gb | 74 | 3.1Gb | 69 | 5.1Gb | [Ds] | 16 | 1.7Gb | 13 | 2.3Gb | 9 | 3.1Gb | 9 | 5.2Gb | 327 | 2.5Gb | 368 | 3.0Gb | 336 | 3.6Gb | 334 | 6.2Gb
27998 x 50000 | [s] | 272 | 1.8Gb | 208 | 2.2Gb | 155 | 3.4Gb | 122 | 4.8Gb | 2000 x 50000 | [s] | 14 | 2.0Gb | 14 | 2.4Gb | 13 | 3.1Gb | 14 | 4.8Gb | 175 | 2.2Gb | 158 | 2.3Gb | 242 | 3.1Gb | 172 | 5.0Gb
[D] | 157 | 2.2Gb | 144 | 3.0Gb | 149 | 2.3Gb | 144 | 3.6Gb | [D] | 22 | 2.2Gb | 14 | 2.9Gb | 10 | 2.3Gb | 9 | 3.7Gb | 296 | 2.3Gb | 240 | 3.0Gb | 157 | 2.3Gb | 178 | 3.7Gb
[Ds] | 163 | 1.8Gb | 147 | 2.2Gb | 141 | 3.5Gb | 137 | 5.4Gb | [Ds] | 33 | 1.9Gb | 25 | 2.4Gb | 20 | 3.4Gb | 17 | 5.4Gb | 563 | 2.8Gb | 561 | 3.2Gb | 591 | 4.2Gb | 526 | 7.0Gb
27998 x 100000 | [s] | 596 | 1.9Gb | 414 | 2.5Gb | 318 | 3.6Gb | 247 | 5.6Gb | 2000 x 100000 | [s] | 30 | 2.1Gb | 27 | 2.5Gb | 28 | 3.8Gb | 28 | 5.5Gb | 355 | 2.6Gb | 299 | 2.5Gb | 385 | 3.8Gb | 341 | 5.1Gb
[D] | 309 | 2.3Gb | 284 | 2.6Gb | 305 | 2.9Gb | 299 | 3.7Gb | [D] | 52 | 2.4Gb | 35 | 2.7Gb | 27 | 2.9Gb | 18 | 3.7Gb | 771 | 2.4Gb | 621 | 3.0Gb | 525 | 3.1Gb | 676 | 3.9Gb
[Ds] | 323 | 1.9Gb | 282 | 2.6Gb | 276 | 4.0Gb | 269 | 6.4Gb | [Ds] | 73 | 1.9Gb | 54 | 2.7Gb | 43 | 4.1Gb | 34 | 6.3Gb | 1337 | 3.0Gb | 1241 | 3.8Gb | 1297 | 5.2Gb | 1120 | 6.8Gb
27998 x 200000 | [s] | 1200 | 2.6Gb | 876 | 2.8Gb | 661 | 4.0Gb | 527 | 7.1Gb | 2000 x 200000 | [s] | 55 | 2.6Gb | 55 | 2.8Gb | 55 | 4.1Gb | 58 | 7.1Gb | 662 | 3.0Gb | 570 | 3.0Gb | 584 | 4.2Gb | 775 | 7.0Gb
[D] | 612 | 2.6Gb | 569 | 3.1Gb | 611 | 2.8Gb | 617 | 4.1Gb | [D] | 112 | 2.6Gb | 80 | 3.1Gb | 56 | 2.8Gb | 38 | 4.1Gb | 1924 | 2.7Gb | 1318 | 3.3Gb | 1020 | 3.1Gb | 960 | 4.1Gb
[Ds] | 658 | 1.8Gb | 560 | 3.0Gb | 537 | 4.3Gb | 556 | 7.1Gb | [Ds] | 149 | 2.2Gb | 117 | 3.2Gb | 94 | 4.2Gb | 74 | 6.9Gb | 3107 | 3.4Gb | 2693 | 4.2Gb | 2571 | 5.8Gb | 2528 | 8.2Gb
Formats: [s] TENxMatrix (sparse) — [D] HDF5Matrix (dense) — [Ds] HDF5Matrix as sparse. For each operation, the best time across the four different block sizes is displayed in bold. In addition, if it’s also the best time across the three formats ([s],[D], and [Ds]), then we box it (only for Normalization and PCA). The “max. mem. used” is the max RSS (Resident Set Size) value obtained by running `ps u -p <PID>` every second while performing a given operation. “max. mem. used” values > 4Gb are displayed in light red.

## Timings for Intel Mac Pro

Specs for Intel Mac Pro (24-Core Intel Xeon W)
OS | macOS 12.7.6 | Disk performance | N/A
RAM | 96GB
Disk | 1TB SSD

Table 4: Timings for Intel Mac Pro
Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | Normalized Test Dataset | F o r m a t | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb | block size = 16 Mb | block size = 40 Mb | block size = 100 Mb | block size = 250 Mb
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
nrow x ncol (# genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | nrow x ncol (# sel. genes x # cells) | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used | time in sec. | max. mem. used
|  | 1. NORMALIZATION & selection of 1000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 106 | 2.0Gb | 76 | 2.3Gb | 58 | 2.6Gb | 45 | 3.0Gb | 1000 x 12500 | [s] | 5 | 1.9Gb | 6 | 2.3Gb | 6 | 2.7Gb | 5 | 3.1Gb | 73 | 2.2Gb | 58 | 2.6Gb | 58 | 3.0Gb | 76 | 3.7Gb
[D] | 77 | 2.4Gb | 63 | 2.4Gb | 64 | 2.6Gb | 56 | 3.7Gb | [D] | 4 | 2.5Gb | 3 | 2.5Gb | 3 | 2.6Gb | 2 | 3.7Gb | 84 | 2.8Gb | 65 | 2.7Gb | 59 | 3.0Gb | 44 | 4.0Gb
[Ds] | 62 | 2.1Gb | 58 | 2.7Gb | 48 | 2.8Gb | 47 | 3.3Gb | [Ds] | 7 | 2.2Gb | 5 | 2.8Gb | 6 | 2.8Gb | 6 | 3.6Gb | 194 | 4.1Gb | 200 | 3.8Gb | 240 | 3.9Gb | 202 | 4.7Gb
27998 x 25000 | [s] | 219 | 2.2Gb | 167 | 2.6Gb | 129 | 3.5Gb | 91 | 3.8Gb | 1000 x 25000 | [s] | 9 | 2.2Gb | 10 | 2.5Gb | 11 | 3.5Gb | 11 | 3.8Gb | 124 | 2.6Gb | 119 | 2.9Gb | 150 | 3.8Gb | 126 | 4.8Gb
[D] | 137 | 2.7Gb | 125 | 2.7Gb | 132 | 2.6Gb | 115 | 3.9Gb | [D] | 9 | 2.7Gb | 5 | 2.7Gb | 6 | 2.7Gb | 5 | 3.9Gb | 155 | 3.3Gb | 103 | 2.8Gb | 107 | 2.8Gb | 94 | 4.3Gb
[Ds] | 123 | 2.7Gb | 110 | 3.0Gb | 110 | 4.1Gb | 96 | 4.3Gb | [Ds] | 16 | 2.8Gb | 11 | 3.0Gb | 11 | 4.2Gb | 12 | 4.4Gb | 397 | 4.4Gb | 373 | 4.3Gb | 427 | 5.2Gb | 361 | 6.1Gb
27998 x 50000 | [s] | 448 | 2.7Gb | 336 | 3.1Gb | 255 | 4.0Gb | 189 | 5.4Gb | 1000 x 50000 | [s] | 18 | 2.6Gb | 18 | 3.1Gb | 23 | 3.9Gb | 19 | 5.3Gb | 245 | 2.8Gb | 227 | 3.2Gb | 257 | 4.4Gb | 288 | 5.9Gb
[D] | 268 | 2.9Gb | 248 | 3.4Gb | 260 | 2.5Gb | 240 | 4.8Gb | [D] | 19 | 3.0Gb | 16 | 3.4Gb | 10 | 2.6Gb | 9 | 4.8Gb | 402 | 3.3Gb | 314 | 3.5Gb | 258 | 2.9Gb | 216 | 5.2Gb
[Ds] | 237 | 3.2Gb | 222 | 3.9Gb | 213 | 4.4Gb | 206 | 6.1Gb | [Ds] | 34 | 3.4Gb | 27 | 3.9Gb | 24 | 4.6Gb | 22 | 6.5Gb | 836 | 4.7Gb | 810 | 5.2Gb | 936 | 6.2Gb | 701 | 8.5Gb
27998 x 100000 | [s] | 964 | 2.9Gb | 635 | 3.5Gb | 523 | 4.1Gb | 372 | 5.7Gb | 1000 x 100000 | [s] | 37 | 2.9Gb | 37 | 3.4Gb | 41 | 4.0Gb | 38 | 5.6Gb | 461 | 3.3Gb | 417 | 3.7Gb | 490 | 4.7Gb | 534 | 6.3Gb
[D] | 515 | 3.8Gb | 491 | 2.7Gb | 528 | 3.3Gb | 495 | 5.1Gb | [D] | 44 | 3.8Gb | 27 | 2.8Gb | 20 | 3.3Gb | 19 | 5.2Gb | 836 | 4.0Gb | 567 | 3.2Gb | 663 | 3.6Gb | 400 | 5.6Gb
[Ds] | 485 | 3.6Gb | 542 | 5.4Gb | 448 | 4.7Gb | 435 | 6.7Gb | [Ds] | 70 | 3.7Gb | 81 | 5.5Gb | 51 | 4.9Gb | 42 | 6.9Gb | 1737 | 5.3Gb | 1993 | 6.6Gb | 1865 | 6.7Gb | 1497 | 8.4Gb
27998 x 200000 | [s] | 1947 | 3.4Gb | 1461 | 4.2Gb | 1062 | 5.2Gb | 840 | 7.9Gb | 1000 x 200000 | [s] | 75 | 3.5Gb | 77 | 4.2Gb | 87 | 5.1Gb | 79 | 7.5Gb | 912 | 3.7Gb | 913 | 4.3Gb | 1082 | 5.7Gb | 1005 | 8.1Gb
[D] | 1013 | 3.7Gb | 982 | 3.4Gb | 1059 | 2.9Gb | 1019 | 5.4Gb | [D] | 88 | 3.8Gb | 67 | 3.6Gb | 51 | 3.0Gb | 43 | 5.5Gb | 2372 | 4.7Gb | 1477 | 4.4Gb | 1219 | 3.8Gb | 795 | 5.9Gb
[Ds] | 990 | 4.2Gb | 947 | 5.1Gb | 805 | 7.0Gb | 843 | 8.8Gb | [Ds] | 140 | 4.3Gb | 127 | 5.1Gb | 101 | 6.8Gb | 88 | 8.7Gb | 4205 | 5.8Gb | 3974 | 7.3Gb | 3654 | 8.2Gb | 3303 | 10.2Gb
|  | 1. NORMALIZATION & selection of 2000 most variable genes |  |  | 2. ON-DISK REALIZATION of the normalized dataset | 3. PCA of the normalized dataset
27998 x 12500 | [s] | 114 | 2.0Gb | 79 | 2.0Gb | 65 | 2.7Gb | 47 | 3.1Gb | 2000 x 12500 | [s] | 7 | 2.2Gb | 7 | 2.0Gb | 6 | 2.7Gb | 7 | 3.2Gb | 121 | 2.9Gb | 109 | 2.1Gb | 101 | 2.9Gb | 112 | 3.8Gb
[D] | 82 | 2.3Gb | 65 | 2.4Gb | 69 | 2.6Gb | 57 | 3.9Gb | [D] | 6 | 2.4Gb | 3 | 2.4Gb | 4 | 2.6Gb | 4 | 3.9Gb | 149 | 3.5Gb | 96 | 2.6Gb | 111 | 2.9Gb | 94 | 4.0Gb
[Ds] | 70 | 2.4Gb | 64 | 2.3Gb | 58 | 3.4Gb | 51 | 3.6Gb | [Ds] | 10 | 2.4Gb | 9 | 2.4Gb | 8 | 3.4Gb | 9 | 3.7Gb | 308 | 4.1Gb | 303 | 4.2Gb | 330 | 4.6Gb | 321 | 5.6Gb
27998 x 25000 | [s] | 225 | 2.4Gb | 181 | 2.3Gb | 132 | 3.2Gb | 106 | 4.4Gb | 2000 x 25000 | [s] | 14 | 2.7Gb | 14 | 2.4Gb | 13 | 3.3Gb | 13 | 4.5Gb | 218 | 3.9Gb | 199 | 2.4Gb | 212 | 3.6Gb | 273 | 5.3Gb
[D] | 147 | 3.0Gb | 131 | 2.6Gb | 137 | 2.6Gb | 125 | 4.9Gb | [D] | 16 | 3.0Gb | 12 | 2.9Gb | 9 | 2.7Gb | 7 | 4.9Gb | 377 | 3.3Gb | 283 | 3.3Gb | 209 | 3.0Gb | 191 | 5.3Gb
[Ds] | 137 | 2.6Gb | 123 | 2.8Gb | 123 | 4.0Gb | 117 | 5.0Gb | [Ds] | 26 | 2.8Gb | 21 | 3.1Gb | 17 | 3.9Gb | 15 | 5.3Gb | 649 | 4.5Gb | 634 | 4.7Gb | 652 | 5.7Gb | 623 | 7.3Gb
27998 x 50000 | [s] | 448 | 2.7Gb | 344 | 2.9Gb | 265 | 3.9Gb | 203 | 5.7Gb | 2000 x 50000 | [s] | 27 | 3.1Gb | 24 | 2.9Gb | 25 | 3.9Gb | 28 | 5.5Gb | 357 | 4.6Gb | 300 | 3.0Gb | 436 | 3.8Gb | 399 | 6.4Gb
[D] | 289 | 3.3Gb | 257 | 3.3Gb | 270 | 2.5Gb | 249 | 4.8Gb | [D] | 35 | 3.3Gb | 21 | 3.3Gb | 15 | 2.5Gb | 14 | 4.9Gb | 637 | 3.6Gb | 488 | 3.5Gb | 311 | 2.6Gb | 339 | 5.2Gb
[Ds] | 270 | 3.2Gb | 245 | 3.2Gb | 227 | 4.5Gb | 224 | 6.0Gb | [Ds] | 54 | 3.4Gb | 39 | 3.2Gb | 36 | 4.5Gb | 33 | 5.8Gb | 1105 | 4.9Gb | 1091 | 5.2Gb | 1124 | 6.0Gb | 1011 | 8.0Gb
27998 x 100000 | [s] | 1148 | 2.6Gb | 745 | 3.0Gb | 588 | 4.3Gb | 420 | 6.2Gb | 2000 x 100000 | [s] | 60 | 3.1Gb | 54 | 3.0Gb | 55 | 4.2Gb | 48 | 6.3Gb | 849 | 4.2Gb | 650 | 3.1Gb | 675 | 4.4Gb | 807 | 7.0Gb
[D] | 557 | 4.1Gb | 508 | 2.7Gb | 547 | 3.3Gb | 512 | 4.9Gb | [D] | 84 | 4.1Gb | 51 | 3.0Gb | 43 | 3.4Gb | 27 | 5.0Gb | 1616 | 4.2Gb | 1160 | 3.9Gb | 977 | 3.9Gb | 1205 | 5.5Gb
[Ds] | 544 | 3.6Gb | 474 | 4.4Gb | 448 | 5.5Gb | 435 | 7.4Gb | [Ds] | 110 | 3.8Gb | 92 | 4.3Gb | 80 | 5.3Gb | 58 | 7.1Gb | 2567 | 5.3Gb | 2327 | 6.0Gb | 2331 | 7.3Gb | 2130 | 9.2Gb
27998 x 200000 | [s] | 2135 | 4.3Gb | 1570 | 4.4Gb | 1136 | 5.5Gb | 869 | 8.5Gb | 2000 x 200000 | [s] | 121 | 4.8Gb | 113 | 4.4Gb | 107 | 5.5Gb | 114 | 8.2Gb | 1408 | 6.2Gb | 1236 | 4.5Gb | 1532 | 5.7Gb | 1761 | 9.3Gb
[D] | 1109 | 5.0Gb | 1005 | 3.4Gb | 1105 | 2.9Gb | 1043 | 5.2Gb | [D] | 152 | 5.0Gb | 111 | 3.5Gb | 83 | 3.0Gb | 58 | 5.3Gb | 3937 | 5.3Gb | 2364 | 4.3Gb | 1792 | 3.9Gb | 1328 | 5.7Gb
[Ds] | 1124 | 4.7Gb | 952 | 5.9Gb | 882 | 6.7Gb | 911 | 8.5Gb | [Ds] | 231 | 4.9Gb | 191 | 6.1Gb | 168 | 6.7Gb | 119 | 8.4Gb | 5913 | 7.1Gb | 4940 | 7.7Gb | 4822 | 8.6Gb | 4874 | 10.6Gb
Formats: [s] TENxMatrix (sparse) — [D] HDF5Matrix (dense) — [Ds] HDF5Matrix as sparse. For each operation, the best time across the four different block sizes is displayed in bold. In addition, if it’s also the best time across the three formats ([s],[D], and [Ds]), then we box it (only for Normalization and PCA). The “max. mem. used” is the max RSS (Resident Set Size) value obtained by running `ps u -p <PID>` every second while performing a given operation. “max. mem. used” values > 4Gb are displayed in light red.

# Discussion

The “[Ds] HDF5Matrix as sparse” representation didn’t live up to its promise so we leave it alone for now. Note that the code for loading blocks of data from the dense HDF5 file to memory does not currently take full advantage of the SVT_SparseArray representation, a new efficient data structure for multidimensional sparse data implemented in the SparseArray package that overcomes some of the limitations of the dgCMatrix representation from the Matrix package. This will need to be addressed.

## TENxMatrix (sparse) vs HDF5Matrix (dense)

### Normalization

There’s no obvious best choice between the “[s] TENxMatrix (sparse)” and “[D] HDF5Matrix (dense)” representations. More precisely, for normalization, the former tends to give the best times when using bigger blocks (e.g. 250 Mb), whereas the latter tends to give the best times when using smaller blocks (e.g. 16 Mb).

Therefore, on a machine with enough memory to support big block sizes, one will get the best results with the [s] representation and blocks of 250 Mb or more. However, on a machine with not enough memory to support such big blocks, one should instead use the [D] representation with blocks of 16 Mb.

[TODO: Add some plots to help vizualize the above observations.]

### PCA

For PCA, choosing the “[s] TENxMatrix (sparse)” representation and using small block sizes (40 Mb) tends to give the best performance.

[TODO: Add some plots to help vizualize this observation.]

### Hybrid approach

Note that, on a machine where using blocks of 250 Mb or more for normalization is not an option, one should use the following hybrid approach:

- Start with the “[D] HDF5Matrix (dense)” representation.

- Perform normalization, using very small blocks (16 Mb).

- Switch to the “[s] TENxMatrix (sparse)” format when writing the normalized dataset to disk, that is, use `writeTENxMatrix()` instead of `writeHDF5Array()` for on-disk realization of the intermediate dataset.

- Perform PCA on the object returned by the on-disk realization step (`writeTENxMatrix()`), using small blocks (40 Mb).

### A note about memory usage

The machines running macOS use between 2x and 3x more memory than the machines running Linux for the same task using the same block size.

Overall, on Linux, and for a given choice of block size, memory usage doesn’t seem to be affected too much by the number of cells in the dataset, that is, all operations seem to perform at almost constant memory.

However, the “[D] HDF5Matrix (dense)” representation appears to be better than the “[s] TENxMatrix (sparse)” representation at keeping memory usage (mostly) flat as the number of cells in the dataset increases. This is even more accentuated on macOS where, somehow counter intuitively, using the dense representation manages to keep memory usage at a reasonable level (and more or less capped with respect to the number of cells), while using the sparse representation fails to do that.

## Main takeaways

1. Using the TENxMatrix representation (sparse format), one can perform normalization and PCA of the bigger dataset (200,000 cells) on an average consumer-grade laptop like the DELL XPS 15 laptop (model 9520) in less than 25 minutes and using less than 4Gb of memory, as shown in the table below. For comparison, normalization and PCA on an in-memory representation of the same dataset (e.g. on a SparseMatrix object) takes less then 4 minutes. However, that consumes about 18.5Gb of memory! This will be the subject of an upcoming vignette in the SparseArray package.

Comparing times across machines
Machine | NORMALIZATION time block size = 250 Mb | REALIZATION time block size = 250 Mb | PCA time block size = 40 Mb | TOTAL time | Max. mem. used
--- | --- | --- | --- | --- | ---
DELL XPS 15 laptop | 643 | 69 | 692 | 1404 | 2.9Gb
Supermicro SuperServer 1029GQ-TRT | 1069 | 110 | 1494 | 2673 | 3.0Gb
Apple Silicon Mac Pro | 527 | 58 | 570 | 1155 | 7.1Gb
Intel Mac Pro | 869 | 114 | 1236 | 2219 | 8.5Gb
For each machine, we show the normalization, realization, and PCA times (plus total time) obtained on the 27998 x 200000 dataset, using the “[s] TENxMatrix (sparse)” format, and selecting the 2000 most variable genes during the normalization step. All times are in seconds.

2. Normalization and PCA are roughly linear in time with respect to the number of cells in the dataset, regardless of representation (sparse or dense) or block size.

3. Block size matters. When using the TENxMatrix representation (sparse format), the bigger the blocks the faster normalization will be (at the cost of increased memory usage). On the other hand, it seems that PCA prefers small blocks, at least with our naive `simple_PCA()` implementation.

4. Disk performance is important (not surprisingly) as attested by the lower performance of the Supermicro SuperServer 1029GQ-TRT machine, likely due to its slower disk.

# Session information

    sessionInfo()

    ## R version 4.6.0 (2026-04-24)
    ## Platform: x86_64-pc-linux-gnu
    ## Running under: Ubuntu 24.04.4 LTS
    ##
    ## Matrix products: default
    ## BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
    ## LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0
    ##
    ## locale:
    ##  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
    ##  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
    ##  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
    ##  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
    ##  [9] LC_ADDRESS=C               LC_TELEPHONE=C
    ## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
    ##
    ## time zone: Etc/UTC
    ## tzcode source: system (glibc)
    ##
    ## attached base packages:
    ## [1] stats4    stats     graphics  grDevices utils     datasets  methods
    ## [8] base
    ##
    ## other attached packages:
    ##  [1] TENxBrainData_1.33.0        SingleCellExperiment_1.35.1
    ##  [3] SummarizedExperiment_1.43.0 Biobase_2.73.1
    ##  [5] GenomicRanges_1.65.0        Seqinfo_1.3.0
    ##  [7] RSpectra_0.16-2             DelayedMatrixStats_1.35.0
    ##  [9] ExperimentHub_3.3.0         AnnotationHub_4.3.0
    ## [11] BiocFileCache_3.3.0         dbplyr_2.5.2
    ## [13] HDF5Array_1.41.0            h5mread_1.5.0
    ## [15] rhdf5_2.57.0                DelayedArray_0.39.2
    ## [17] SparseArray_1.13.2          S4Arrays_1.13.0
    ## [19] IRanges_2.47.1              abind_1.4-8
    ## [21] S4Vectors_0.51.2            MatrixGenerics_1.25.0
    ## [23] matrixStats_1.5.0           BiocGenerics_0.59.3
    ## [25] generics_0.1.4              Matrix_1.7-5
    ## [27] BiocStyle_2.41.0
    ##
    ## loaded via a namespace (and not attached):
    ##  [1] KEGGREST_1.53.0          xfun_0.57                bslib_0.11.0
    ##  [4] httr2_1.2.2              lattice_0.22-9           rhdf5filters_1.25.0
    ##  [7] vctrs_0.7.3              tools_4.6.0              curl_7.1.0
    ## [10] tibble_3.3.1             AnnotationDbi_1.75.0     RSQLite_3.52.0
    ## [13] blob_1.3.0               pkgconfig_2.0.3          sparseMatrixStats_1.25.0
    ## [16] lifecycle_1.0.5          compiler_4.6.0           Biostrings_2.81.2
    ## [19] htmltools_0.5.9          sys_3.4.3                buildtools_1.0.0
    ## [22] sass_0.4.10              yaml_2.3.12              pillar_1.11.1
    ## [25] crayon_1.5.3             jquerylib_0.1.4          cachem_1.1.0
    ## [28] tidyselect_1.2.1         digest_0.6.39            purrr_1.2.2
    ## [31] dplyr_1.2.1              BiocVersion_3.24.0       maketools_1.3.2
    ## [34] fastmap_1.2.0            grid_4.6.0               cli_3.6.6
    ## [37] magrittr_2.0.5           withr_3.0.2              filelock_1.0.3
    ## [40] rappdirs_0.3.4           bit64_4.8.2              rmarkdown_2.31
    ## [43] XVector_0.53.0           httr_1.4.8               bit_4.6.0
    ## [46] png_0.1-9                memoise_2.0.1            evaluate_1.0.5
    ## [49] knitr_1.51               rlang_1.2.0              Rcpp_1.1.1-1.1
    ## [52] glue_1.8.1               DBI_1.3.0                BiocManager_1.30.27
    ## [55] jsonlite_2.0.0           R6_2.6.1                 Rhdf5lib_2.1.0