** Description changed:
[Impact]
On AMD Zen systems, memcpy() calls see a heavy performance regression in
Focal and Groovy, due to the way __x86_non_temporal_threshold is calculated.
Before 'glibc-2.33~455', cache values were calculated taking into
consideration the number of hardware threads in the CPU. On AMD Ryzen
and EPYC systems, this can be counter-productive if the number of
threads is high enough for the last-level caches to "overrun" each other
and cause cache line flushes. The solution is to reduce the allocated
size for these non_temporal stores, removing the number of threads from
the equation.
[Test Plan]
Attached to this bug is a short C program that exercises memcpy() calls in
buffers of variable length. This has been obtained from a similar bug report
for Red Hat, and is publicly available at [0].
This test program was compiled with gcc 10.2.0, using the following flags:
$ gcc -mtune=generic -march=x86_64 -g -03 test_memcpy.c -o test_memcpy64
Tests were performed with the following criteria:
- use 32Mb buffers ("./test_memcpy64 32")
- benchmark with the hyperfine tool [1], as it calculates relevant statistics
automatically
- benchmark with at least 10 runs in the same environment, to minimize
variance
- measure on AMD Zen (3700X) and on Intel Xeon (E5-2683), to ensure we don't
penalize one x86 vendor in favor of the other
Below is a comparison between two Focal containers, leveraging LXD to
make use of different libc versions on the same host:
$ hyperfine -n libc-2.31-0ubuntu9.2 'lxc exec focal ./test_memcpy64 32' -n
libc-patched 'lxc exec focal-patched ./test_memcpy64 32'
Benchmark #1: libc-2.31-0ubuntu9.2
Time (mean ± σ): 2.723 s ± 0.013 s [User: 4.7 ms, System: 5.1 ms]
Range (min … max): 2.693 s … 2.735 s 10 runs
Benchmark #2: libc-patched
Time (mean ± σ): 1.522 s ± 0.004 s [User: 3.9 ms, System: 5.6 ms]
Range (min … max): 1.515 s … 1.528 s 10 runs
Summary
'libc-patched' ran
1.79 ± 0.01 times faster than 'libc-2.31-0ubuntu9.2'
$ head -n5 /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 23
model : 113
model name : AMD Ryzen 7 3700X 8-Core Processor
[0] https://bugzilla.redhat.com/show_bug.cgi?id=1880670
[1] https://github.com/sharkdp/hyperfine/
[Where problems could occur]
Since we're messing with the cacheinfo for x86 in general, we need to be
careful not to introduce further performance regressions on memory-heavy
workloads. Even though initial results might reveal improvement on AMD Ryzen
and EPYC hardware, we should also validate different configurations (e.g.
Intel, different buffer sizes, etc) to make sure we won't hurt performance in
other non-AMD environments.
[Other Info]
- This has been fixed by the following upstream commit:
+ This issue has been fixed by the following upstream commit:
- d3c57027470b (Reversing calculation of __x86_shared_non_temporal_threshold)
$ git describe --contains d3c57027470b
glibc-2.33~455
$ rmadison glibc -s focal,focal-updates,groovy,groovy-proposed,hirsute
glibc | 2.31-0ubuntu9 | focal | source
glibc | 2.31-0ubuntu9.2 | focal-updates | source
glibc | 2.32-0ubuntu3 | groovy | source
glibc | 2.32-0ubuntu3.2 | groovy-proposed | source
glibc | 2.33-0ubuntu5 | hirsute | source
Affected releases include Ubuntu Focal and Groovy. Bionic is not
affected, and releases starting with Hirsute already ship the upstream
patch to fix this regression.
+
+ glibc exports this specific variable as a tunable, so we could also tweak it
with the GLIBC_TUNABLES env var:
+ $ hyperfine -n clean-env 'lxc exec focal env ./test_memcpy64 32' -n tunables
'lxc exec focal env
GLIBC_TUNABLES=glibc.cpu.x86_non_temporal_threshold=1024*1024*3*4
./test_memcpy64 32'
+ Benchmark #1: clean-env
+ Time (mean ± σ): 2.529 s ± 0.061 s [User: 6.0 ms, System: 4.7 ms]
+ Range (min … max): 2.457 s … 2.615 s 10 runs
+
+ Benchmark #2: tunables
+ Time (mean ± σ): 1.427 s ± 0.030 s [User: 6.5 ms, System: 3.8 ms]
+ Range (min … max): 1.402 s … 1.482 s 10 runs
+
+ Summary
+ 'tunables' ran
+ 1.77 ± 0.06 times faster than 'clean-env'
+
+ This solution is not ideal, but it offers a secondary way of fixing the
+ performance issues. However, the speed gains for memcpy() are noticeable
+ enough that we should strongly consider changing the defaults in the
+ Focal LTS release, so that it performs similarly to Bionic and future
+ Ubuntu releases starting with Hirsute.
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https://bugs.launchpad.net/bugs/1928508
Title:
Performance regression on memcpy() calls for AMD Zen
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