There's currently a pending SRU for glibc on Groovy, which I based my
debdiff on. Although that SRU has already cleared the 7 day grace
period, there are still a few pending bugs for verification which we
would need to clear before being able to upload a fix for this one.
Since Groovy is going EOL in a couple of weeks, I don't think we should
spend too much effort for that release. Marking as "won't fix"
accordingly.
** Changed in: glibc (Ubuntu Groovy)
Status: In Progress => Won't Fix
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https://bugs.launchpad.net/bugs/1928508
Title:
Performance regression on memcpy() calls for AMD Zen
Status in glibc package in Ubuntu:
Fix Released
Status in glibc source package in Focal:
In Progress
Status in glibc source package in Groovy:
Won't Fix
Bug description:
[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 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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