[Group.of.nepali.translators] [Bug 1663280] Re: Serious performance degradation of math functions
This bug was fixed in the package glibc - 2.23-0ubuntu11
---
glibc (2.23-0ubuntu11) xenial; urgency=medium
* debian/patches/ubuntu/xsave-part1.diff and
debian/patches/ubuntu/xsave-part2.diff: Fix a serious performance
regression when mixing SSE and AVX code on certain processors.
The patches are from the upstream 2.23 stable branch. (LP: #1663280)
-- Daniel Axtens Thu, 04 Oct 2018
10:29:55 +1000
** Changed in: glibc (Ubuntu Xenial)
Status: Fix Committed => Fix Released
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https://bugs.launchpad.net/bugs/1663280
Title:
Serious performance degradation of math functions
Status in GLibC:
Fix Released
Status in glibc package in Ubuntu:
Fix Released
Status in glibc source package in Xenial:
Fix Released
Status in glibc source package in Zesty:
Won't Fix
Status in glibc package in Fedora:
Fix Released
Bug description:
SRU Justification
=
[Impact]
* Severe performance hit on many maths-heavy workloads. For example,
a user reports linpack performance of 13 Gflops on Trusty and Bionic
and 3.9 Gflops on Xenial.
* Because the impact is so large (>3x) and Xenial is supported until
2021, the fix should be backported.
* The fix avoids an AVX-SSE transition penalty. It stops
_dl_runtime_resolve() from using AVX-256 instructions which touch the
upper halves of various registers. This change means that the
processor does not need to save and restore them.
[Test Case]
Firstly, you need a suitable Intel machine. Users report that Sandy
Bridge, Ivy Bridge, Haswell, and Broadwell CPUs are affected, and I
have been able to reproduce it on a Skylake CPU using a suitable Azure
VM.
Create the following C file, exp.c:
#include
#include
int main () {
double a, b;
for (a = b = 0.0; b < 2.0; b += 0.0005) a += exp(b);
printf("%f\n", a);
return 0;
}
$ gcc -O3 -march=x86-64 -o exp exp.c -lm
With the current version of glibc:
$ time ./exp
...
real0m1.349s
user0m1.349s
$ time LD_BIND_NOW=1 ./exp
...
real0m0.625s
user0m0.621s
Observe that LD_BIND_NOW makes a big difference as it avoids the call
to _dl_runtime_resolve.
With the proposed update:
$ time ./exp
...
real0m0.625s
user0m0.621s
$ time LD_BIND_NOW=1 ./exp
...
real0m0.631s
user0m0.631s
Observe that the normal case is faster, and LD_BIND_NOW makes a
negligible difference.
[Regression Potential]
glibc is the nightmare case for regressions as could affect pretty much
anything, and this patch touches a key part (dynamic libraries).
We can be fairly confident in the fix generally - it's in the glibc in
Bionic, Debian and some RPM-based distros. The backport is based on
the patches in the release/2.23/master branch in the upstream glibc
repository, and the backport was straightforward.
Obviously that doesn't remove all risk. There is also a fair bit of
Ubuntu-specific patching in glibc so other distros are of limited
value for ruling out bugs. So I have done the following testing, and
I'm happy to do more as required. All testing has been done:
- on an Azure VM (affected by the change), with proposed package
- on a local VM (not affected by the change), with proposed package
* Boot with the upgraded libc6.
* Watch a youtube video in Firefox over VNC.
* Build some C code (debuild of zlib).
* Test Java by installing and running Eclipse.
Autopkgtest also passes.
[Original Description]
Bug [0] has been introduced in Glibc 2.23 [1] and fixed in Glibc 2.25
[2]. All Ubuntu versions starting from 16.04 are affected because they
use either Glibc 2.23 or 2.24. Bug introduces serious (2x-4x)
performance degradation of math functions (pow, exp/exp2/exp10,
log/log2/log10, sin/cos/sincos/tan, asin/acos/atan/atan2,
sinh/cosh/tanh, asinh/acosh/atanh) provided by libm. Bug can be
reproduced on any AVX-capable x86-64 machine.
@strikov: According to a quite reliable source [5] all AMD CPUs and
latest Intel CPUs (Skylake and Knights Landing) don't suffer from
AVX/SSE transition penalty. It means that the scope of this bug
becomes smaller and includes only the following generations of Intel's
CPUs: Sandy Bridge, Ivy Bridge, Haswell, and Broadwell. Scope still
remains quite large though.
@strikov: Ubuntu 16.10/17.04 which use Glibc 2.24 may recieve the fix
from upstream 2.24 branch (as Marcel pointed out, fix has been
backported to 2.24 branch where Fedora took it successfully) if such
synchronization will take place. Ubuntu 16.04 (the main target of this
bug) uses Glibc 2.23 which hasn't been patched upstream and will
suffer from performance degradation until we fix it manually.
This bug is all about AVX-SSE
[Group.of.nepali.translators] [Bug 1663280] Re: Serious performance degradation of math functions
zesty is EOL. artful+ are fix released. xenial is the only currently affected supported series. ** Also affects: glibc (Ubuntu Xenial) Importance: Undecided Status: New ** Changed in: glibc (Ubuntu Zesty) Status: Triaged => Won't Fix ** Changed in: glibc (Ubuntu) Status: Triaged => Fix Released -- You received this bug notification because you are a member of नेपाली भाषा समायोजकहरुको समूह, which is subscribed to Xenial. Matching subscriptions: Ubuntu 16.04 Bugs https://bugs.launchpad.net/bugs/1663280 Title: Serious performance degradation of math functions Status in GLibC: Fix Released Status in glibc package in Ubuntu: Fix Released Status in glibc source package in Xenial: New Status in glibc source package in Zesty: Won't Fix Status in glibc package in Fedora: Fix Released Bug description: Bug [0] has been introduced in Glibc 2.23 [1] and fixed in Glibc 2.25 [2]. All Ubuntu versions starting from 16.04 are affected because they use either Glibc 2.23 or 2.24. Bug introduces serious (2x-4x) performance degradation of math functions (pow, exp/exp2/exp10, log/log2/log10, sin/cos/sincos/tan, asin/acos/atan/atan2, sinh/cosh/tanh, asinh/acosh/atanh) provided by libm. Bug can be reproduced on any AVX-capable x86-64 machine. @strikov: According to a quite reliable source [5] all AMD CPUs and latest Intel CPUs (Skylake and Knights Landing) don't suffer from AVX/SSE transition penalty. It means that the scope of this bug becomes smaller and includes only the following generations of Intel's CPUs: Sandy Bridge, Ivy Bridge, Haswell, and Broadwell. Scope still remains quite large though. @strikov: Ubuntu 16.10/17.04 which use Glibc 2.24 may recieve the fix from upstream 2.24 branch (as Marcel pointed out, fix has been backported to 2.24 branch where Fedora took it successfully) if such synchronization will take place. Ubuntu 16.04 (the main target of this bug) uses Glibc 2.23 which hasn't been patched upstream and will suffer from performance degradation until we fix it manually. This bug is all about AVX-SSE transition penalty [3]. 256-bit YMM registers used by AVX-256 instructions extend 128-bit registers used by SSE (XMM0 is a low half of YMM0 and so on). Every time CPU executes SSE instruction after AVX-256 instruction it has to store upper half of the YMM register to the internal buffer and then restore it when execution returns back to AVX instructions. Store/restore is required because old-fashioned SSE knows nothing about the upper halves of its registers and may damage them. This store/restore operation is time consuming (several tens of clock cycles for each operation). To deal with this issue, Intel introduced AVX-128 instructions which operate on the same 128-bit XMM register as SSE but take into account upper halves of YMM registers. Hence, no store/restore required. Practically speaking, AVX-128 instructions is a new smart form of SSE instructions which can be used together with full-size AVX-256 instructions without any penalty. Intel recommends to use AVX-128 instructions instead of SSE instructions wherever possible. To sum things up, it's okay to mix SSE with AVX-128 and AVX-128 with AVX-256. Mixing AVX-128 with AVX-256 is allowed because both types of instructions are aware of 256-bit YMM registers. Mixing SSE with AVX-128 is okay because CPU can guarantee that the upper halves of YMM registers don't contain any meaningful data (how one can put it there without using AVX-256 instructions) and avoid doing store/restore operation (why to care about random trash in the upper halves of the YMM registers). It's not okay to mix SSE with AVX-256 due to the transition penalty. Scalar floating-point instructions used by routines mentioned above are implemented as a subset of SSE and AVX-128 instructions. They operate on a small fraction of 128-bit register but still considered SSE/AVX-128 instruction. And they suffer from SSE/AVX transition penalty as well. Glibc inadvertently triggers a chain of AVX/SSE transition penalties due to inappropriate use of AVX-256 instructions inside _dl_runtime_resolve() procedure. By using AVX-256 instructions to push/pop YMM registers [4], Glibc makes CPU think that the upper halves of XMM registers contain meaningful data which needs to be preserved during execution of SSE instructions. With such a 'dirty' flag set every switch between SSE and AVX instructions (AVX-128 or AVX-256) leads to a time consuming store/restore procedure. This 'dirty' flag never gets cleared during the whole program execution which leads to a serious overall slowdown. Fixed implementation [2] of _dl_runtime_resolve() procedure tries to avoid using AVX-256 instructions if possible. Buggy _dl_runtime_resolve() gets called every time when dynamic linker tries to resolve a symbol (any symbol, not just ones mentioned above). It's
