documentation: move benchmark description to separate file

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 137 insertions, 0 deletions

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+
+ 1) Parallel Compression Benchmark
+ *********************************
+
+ 1.1) How was the Benchmark Performed?
+
+ An optimized build of squashfs-tools-ng was compiled and installed to a tmpfs:
+
+  $ mkdir /dev/shm/temp
+  $ ln -s /dev/shm/temp out
+  $ ./autogen.sh
+  $ ./configure CFLAGS="-O3 -Ofast -march=native -mtune=native" \
+                LDFLAGS="-O3 -Ofast" --prefix=$(pwd)/out
+  $ make -j install
+  $ cd out
+
+ A SquashFS image to be tested was unpacked in this directory:
+
+  $ ./bin/sqfs2tar <IMAGE> > test.tar
+
+ And then repacked as follows:
+
+  $ time ./bin/tar2sqfs -j <NUM_CPU> -c <COMPRESSOR> -f test.sqfs < test.tar
+
+
+ Out of 4 runs, the worst wall-clock time ("real") was used for comparison.
+
+
+ For the serial reference version, configure was re-run with the option
+ --without-pthread, the tools re-compiled and re-installed.
+
+
+ 1.2) What Image was Tested?
+
+ A Debian image extracted from the Debian 10.2 LiveDVD for AMD64 with XFCE
+ was used.
+
+ The input size and resulting output sizes turned out to be as follows:
+
+  - As uncompressed tarball:           ~6.5GiB (7,008,118,272)
+  - As LZ4 compressed SquashFS image:  ~3.1GiB (3,381,751,808)
+  - As LZO compressed SquashFS image:  ~2.5GiB (2,732,015,616)
+  - As zstd compressed SquashFS image: ~2.1GiB (2,295,017,472)
+  - As gzip compressed SquashFS image: ~2.3GiB (2,471,276,544)
+  - As lzma compressed SquashFS image: ~2.0GiB (2,102,169,600)
+  - As XZ compressed SquashFS image:   ~2.0GiB (2,098,466,816)
+
+
+ The Debian image is expected to contain realistic input data for a Linux
+ file system and also provide enough data for an interesting benchmark.
+
+
+ 1.3) What Test System was used?
+
+  AMD Ryzen 7 3700X
+  32GiB DDR4 RAM
+  Fedora 31
+
+
+ 1.4) What software version was used?
+
+ squashfs-tools-ng v0.9
+
+ TODO: update data and write the *exact* commit hash here.
+
+
+ 1.5) Results
+
+ The raw timing results are as follows:
+
+ Jobs    XZ          lzma        gzip        LZO         LZ4      zstd
+ serial  17m39.613s  16m10.710s   9m56.606s  13m22.337s  12.159s  9m33.600s
+      1  17m38.050s  15m49.753s   9m46.948s  13m06.705s  11.908s  9m23.445s
+      2   9m26.712s   8m24.706s   5m08.152s   6m53.872s   7.395s  5m 1.734s
+      3   6m29.733s   5m47.422s   3m33.235s   4m44.407s   6.069s  3m30.708s
+      4   5m02.993s   4m30.361s   2m43.447s   3m39.825s   5.864s  2m44.418s
+      5   4m07.959s   3m40.860s   2m13.454s   2m59.395s   5.749s  2m16.745s
+      6   3m30.514s   3m07.816s   1m53.641s   2m32.461s   5.926s  1m57.607s
+      7   3m04.009s   2m43.765s   1m39.742s   2m12.536s   6.281s  1m43.734s
+      8   2m45.050s   2m26.996s   1m28.776s   1m58.253s   6.395s  1m34.500s
+      9   2m34.993s   2m18.868s   1m21.668s   1m50.461s   6.890s  1m29.820s
+     10   2m27.399s   2m11.214s   1m15.461s   1m44.060s   7.225s  1m26.176s
+     11   2m20.068s   2m04.592s   1m10.286s   1m37.749s   7.557s  1m22.566s
+     12   2m13.131s   1m58.710s   1m05.957s   1m32.596s   8.127s  1m18.883s
+     13   2m07.472s   1m53.481s   1m02.041s   1m27.982s   8.704s  1m16.218s
+     14   2m02.365s   1m48.773s   1m00.337s   1m24.444s   9.494s  1m14.175s
+     15   1m58.298s   1m45.079s     58.348s   1m21.445s  10.192s  1m12.134s
+     16   1m55.940s   1m42.176s     56.615s   1m19.030s  10.964s  1m11.049s
+
+ The file "benchmark.ods" contains those values, values derived from this and
+ charts depicting the results.
+
+
+ 1.6) Discussion
+
+ Most obviously, the results indicate that LZ4, unlike the other compressors,
+ is clearly I/O bound and not CPU bound and doesn't benefit from parallelization
+ beyond 2-4 worker threads and even that benefit is marginal with efficiency
+ plummetting immediately.
+
+
+ The other compressors are clearly CPU bound. Speedup increases linearly until
+ about 8 cores, but with a slope < 1, as evident by efficiency linearly
+ decreasing and reaching 80% for 8 cores.
+
+ A reason for this sub-linear scaling may be the choke point introduced by the
+ creation of fragment blocks, that *requires* a synchronization. To test this
+ theory, a second benchmark should be performed with fragment block generation
+ completely disabled. This requires a new flag to be added to tar2sqfs (and
+ also gensquashfs).
+
+
+ Using more than 8 jobs causes a much slower increase in speedup and efficency
+ declines even faster. This is probably due to the fact that the test system
+ only has 8 physical cores and beyond that, SMT has to be used.
+
+
+ It should also be noted that the thread pool compressor with only a single
+ thread turns out to be *slightly* faster than the serial reference
+ implementation. A possible explanation for this might be that the fragment
+ blocks are actually assembled in the main thread, in parallel to the worker
+ that can still continue with other data blocks. Because of this decoupling
+ there is in fact some degree of parallelism, even if only one worker thread
+ is used.
+
+
+ As a side effect, this benchmark also produces some insights into the
+ compression ratio and throughput of the supported compressors. Indicating that
+ for the Debian live image, XZ clearly provides the highest data density, while
+ LZ4 is clearly the fastest compressor available.
+
+ The throughput of the zstd compressor is comparable to gzip, while the
+ resulting compression ratio is closer to LZMA.
+
+ Repeating the benchmark without tail-end-packing and with fragments completely
+ disabled would also show the effectiveness of tail-end-packing and fragment
+ packing as a side effect.