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src/storage/benchmarks/README.md

Fuchsia Filesystem Benchmarks

Benchmarks

There are 12 benchmarks that get run for every filesystem. The currently supported filesystems are Fxfs, F2fs, Memfs, and Minfs.

IO Benchmarks

The IO benchmarks are all of the combinations of read/write, sequential/random, and warm/cold. Every read/write call uses an 8KiB buffer and each operation is performed 1024 times spread across an 8MiB file. The benchmarks measure how long each read/write operation takes.

  • Read: makes pread calls to the file.
    • Cold: the file is written, the filesystem is remounted, then we measure the time to read the file.
    • Warm: same as cold but without remounting.
  • Write: makes pwrite call to the file.
    • Cold: we create a new file, then measure the time to write to it.
    • Warm: we create a new file, write to it (to populate it), then measure the time to write to it a second time.
  • Sequential: the reads/writes are performed sequentially from the start of the file to the end of the file.
  • Random: the reads/writes are performed randomly across the entire file. Every part of the file is accessed exactly once.
  • Fsync: the fsync is performed for every write.

WalkDirectoryTree Benchmarks

The WalkDirectoryTree benchmarks measure how long it takes to walk a directory tree with POSIX readdir calls. The directory tree consists of 62 directories and 189 files and is traversed 20 times by the benchmarks. The “cold” variant of the benchmarks remounts the filesystem between each traversal and the “warm” variant does not.

OpenFile Benchmarks

The OpenFile benchmark measures how long it takes for a filesystem to open a file.

The OpenDeeplyNestedFile benchmark expands on the OpenFile benchmark by placing the file several directories deep and then opening it from the root of the filesystem. When compared to the OpenFile benchmark, the OpenDeeplyNestedFile captures how long it takes the filesystem to internally traverse directories.

StatPath Benchmark

The StatPath benchmark measure how long it takes to call stat on a path to a file.

GitStatus Benchmark

The GitStatus benchmark mimics the filesystem usage pattern of running git status. The benchmark contains 2 phases:

  • Phase 1: Calling fstatat on all of the files in the index to see if any of them have changed. All of the fstatat calls happen relative to the top level directory.
  • Phase 2: Walking the directory tree in a recursive DFS order to see if any new files were added.

Blob Benchmarks

The PageInBlob benchmarks measure page fault times for mmap'ed blobs.

  • PageInBlobSequentialUncompressed creates an incompressible blob and pages it in by sequentially accessing each page.
  • PageInBlobSequentialCompressed creates a compressible blob and pages it in by sequentially accessing each page.
  • PageInBlobRandomCompressed creates a compressible blob and randomly accesses 60% of the pages in a way similar to executing an executable. Only 60% of pages are accessed to try to mimic an executable starting.

The blob writing benchmarks measure how long it takes to write blobs. This is important for both fast updates in production and development workflows.

  • WriteBlob writes a single realistically compressible blob to a blob filesystem.
  • WriteRealisticBlobs creates several realistically compressible blobs with varying sizes and concurrently writes 2 blobs to a blob filesystem. This ideally mimics how pkg-cache writes blobs. The benchmark measure how long it takes to write all of the blobs.

“Cold” Benchmarks

At the beginning of most benchmarks is a setup phase that creates files within the filesystem. Simply closing all handles to those files doesn‘t guarantee that the filesystem will immediately clear all caches related to those files. If the caches aren’t cleared then the benchmark may only ever hit cached (warm) data. “Cold” (uncached) read benchmarks remount the Fuchsia filesystem before doing their read operations. Remounting the filesystem guarantees that all data related the file that isn't normally cached gets dropped.

Memfs and Cold Writes

When cold writing to memfs, the kernel needs to allocate pages for the VMO backing the file as the pages are used. This causes cold writes to be slower than warm writes which have the pages already allocated.

Framework

The Fuchsia Filesystem Benchmarks use a custom framework for timing filesystem operations. Filesystems hold state external to the read or write operations being benchmarked which can lead to drastically different timings between consecutive operations. For other performance tests, we want to treat the initial one or more iterations as warm-up iterations and drop their timings. (For example, for some IPC performance tests, the initial iteration doesn‘t complete until a subprocess has finished starting up, making it much slower than the later iterations.) These storage tests differ in that we don’t want to drop the initial iterations' timings.

Ex. On the first read operation to a file in Minfs, Minfs reads the entire file into memory and each subsequent read is served from memory. The warm-up phase of fuchsia-criterion would hide the extremely slow read call.

Running the Benchmarks

  1. Include //src/storage/benchmarks in fx set.
  2. Run fx test fuchsia-pkg://fuchsia.com/storage-benchmarks#meta/storage-benchmarks.cm
  3. If you are running on an emulator, provide an external block device to run the tests on:
touch /tmp/blk.bin
truncate -s 256M /tmp/blk.bin
fdisk /tmp/blk.bin  # Press 'g' to create a GPT partition table, and then 'w' to save
fx qemu -kN  -- -drive file=/tmp/blk.bin,index=0,media=disk,cache=directsync

The set of benchmarks and filesystems can filtered with the --filter flag.