更新glibc后,std :: tan()非常慢

Question

更新glibc后,std :: tan()非常慢

her*_*apa 14 c++ linux performance gcc glibc

我有一个C++程序,它调用很多trig函数.它已经运行了一年多了.我最近安装了gcc-4.8,并且同样去了,更新了glibc.这导致我的程序减慢了几乎x1000倍.使用gdb我发现减速的原因是调用std :: tan().当参数是pi或pi/2时,该函数需要很长时间才能返回.

这是一个MWE,如果没有优化编译就会重现问题(真实程序在有和没有-O2标志的情况下都有相同的问题).

#include <cmath>

int main() {
    double pi = 3.141592653589793;
    double approxPi = 3.14159;
    double ret = 0.;

    for(int i = 0; i < 100000; ++i) ret = std::tan(pi); //Very slow
    for(int i = 0; i < 100000; ++i) ret = std::tan(approxPi); //Not slow
}

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这是来自gdb的示例回溯(使用Ctrl + c随机中断程序后获得).从调用tan开始,回溯在MWE和我的真实程序中是相同的.

#0  0x00007ffff7b1d048 in __mul (p=32, z=0x7fffffffc740, y=0x7fffffffcb30, x=0x7fffffffc890) at ../sysdeps/ieee754/dbl-64/mpa.c:458
#1  __mul (x=0x7fffffffc890, y=0x7fffffffcb30, z=0x7fffffffc740, p=32) at ../sysdeps/ieee754/dbl-64/mpa.c:443
#2  0x00007ffff7b1e348 in cc32 (p=32, y=0x7fffffffc4a0, x=0x7fffffffbf60) at ../sysdeps/ieee754/dbl-64/sincos32.c:111
#3  __c32 (x=<optimized out>, y=0x7fffffffcf50, z=0x7fffffffd0a0, p=32) at ../sysdeps/ieee754/dbl-64/sincos32.c:128
#4  0x00007ffff7b1e170 in __mptan (x=<optimized out>, mpy=0x7fffffffd690, p=32) at ../sysdeps/ieee754/dbl-64/mptan.c:57
#5  0x00007ffff7b45b46 in tanMp (x=<optimized out>) at ../sysdeps/ieee754/dbl-64/s_tan.c:503
#6  __tan_avx (x=<optimized out>) at ../sysdeps/ieee754/dbl-64/s_tan.c:488
#7  0x00000000004005b8 in main ()

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我尝试在四个不同的系统上运行代码(MWE和真实程序).其中两个是集群,我运行我的代码.两个是我的笔记本电脑.MWE在其中一个集群和一台笔记本电脑上运行时没有问题.我检查了每个系统使用哪个版本的libm.so.6,以防相关.以下列表显示了系统描述(取自cat /etc/*-release),CPU是32位还是64位,MWE是否运行缓慢,最后是运行/lib/libc.so.6和输出cat /proc/cpuinfo.

SUSE Linux Enterprise Server 11(x86_64),64位,使用libm-2.11.1.so(MWE很快)

GNU C Library stable release version 2.11.1 (20100118), by Roland McGrath et al.
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Configured for x86_64-suse-linux.
Compiled by GNU CC version 4.3.4 [gcc-4_3-branch revision 152973].
Compiled on a Linux 2.6.32 system on 2012-04-12.
Available extensions:
        crypt add-on version 2.1 by Michael Glad and others
        GNU Libidn by Simon Josefsson
        Native POSIX Threads Library by Ulrich Drepper et al
        BIND-8.2.3-T5B
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 63
model name      : Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
stepping        : 2
microcode       : 53
cpu MHz         : 1200.000
cache size      : 30720 KB
physical id     : 0
siblings        : 24
core id         : 0
cpu cores       : 12
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 15
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid fsgsbase bmi1 avx2 smep bmi2 erms invpcid
bogomips        : 5000.05
clflush size    : 64
cache_alignment : 64
address sizes   : 46 bits physical, 48 bits virtual
power management:

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CentOS版本6.7(最终版),64位,使用libm-2.12.so(MWE很慢)

GNU C Library stable release version 2.12, by Roland McGrath et al.
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.4.7 20120313 (Red Hat 4.4.7-16).
Compiled on a Linux 2.6.32 system on 2015-09-22.
Available extensions:
        The C stubs add-on version 2.1.2.
        crypt add-on version 2.1 by Michael Glad and others
        GNU Libidn by Simon Josefsson
        Native POSIX Threads Library by Ulrich Drepper et al
        BIND-8.2.3-T5B
        RT using linux kernel aio
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Xeon(R) CPU           E5507  @ 2.27GHz
stepping        : 5
cpu MHz         : 1596.000
cache size      : 4096 KB
physical id     : 0
siblings        : 4
core id         : 0
cpu cores       : 4
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 11
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm dca sse4_1 sse4_2 popcnt lahf_lm tpr_shadow vnmi flexpriority ept vpid
bogomips        : 4533.16
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management:

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Ubuntu精确(12.04.5 LTS),64位,使用libm-2.15.so(我的第一台笔记本电脑,MWE很慢)

GNU C Library (Ubuntu EGLIBC 2.15-0ubuntu10.15) stable release version 2.15, by Roland McGrath et al.
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.6.3.
Compiled on a Linux 3.2.79 system on 2016-05-26.
Available extensions:
    crypt add-on version 2.1 by Michael Glad and others
    GNU Libidn by Simon Josefsson
    Native POSIX Threads Library by Ulrich Drepper et al
    BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.debian.org/Bugs/>.

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 42
model name  : Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
stepping    : 7
microcode   : 0x1a
cpu MHz     : 800.000
cache size  : 4096 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid
bogomips    : 5387.59
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

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Ubuntu精确(12.04.5 LTS),32位,使用libm-2.15.so(我的第二台笔记本电脑,MWE很快)

GNU C Library (Ubuntu EGLIBC 2.15-0ubuntu10.12) stable release version 2.15, by Roland McGrath et al.
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.6.3.
Compiled on a Linux 3.2.68 system on 2015-03-26.
Available extensions:
    crypt add-on version 2.1 by Michael Glad and others
    GNU Libidn by Simon Josefsson
    Native POSIX Threads Library by Ulrich Drepper et al
    BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.debian.org/Bugs/>.

processor    : 0
vendor_id    : GenuineIntel
cpu family    : 6
model        : 15
model name    : Intel(R) Core(TM)2 Duo CPU     T5800  @ 2.00GHz
stepping    : 13
microcode    : 0xa3
cpu MHz        : 800.000
cache size    : 2048 KB
physical id    : 0
siblings    : 2
core id        : 0
cpu cores    : 2
apicid        : 0
initial apicid    : 0
fdiv_bug    : no
hlt_bug        : no
f00f_bug    : no
coma_bug    : no
fpu        : yes
fpu_exception    : yes
cpuid level    : 10
wp        : yes
flags        : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon pebs bts aperfmperf pni dtes64 monitor ds_cpl est tm2 ssse3 cx16 xtpr pdcm lahf_lm dtherm
bogomips    : 3989.79
clflush size    : 64
cache_alignment    : 64
address sizes    : 36 bits physical, 48 bits virtual
power management:

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我希望我能够提供足够的背景信息.这些是我的问题.

为什么std :: tan()转慢？
有没有办法将其恢复到正常速度？

我非常喜欢不需要安装/替换一堆库的解决方案.这可能适用于我的笔记本电脑,但我没有集群节点上的必要权限.

更新#1: 我删除了关于将常量传递给棕褐色的观察结果,正如Sam Varshavchik所解释的那样.我将运行的输出添加/lib/libc.so.6到我的系统列表中.还增加了第四个系统.至于时机,这里的运行的输出time ./mwe与pi环路(approxPi注释掉).

real    0m11.483s
user    0m11.465s
sys 0m0.004s

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这是approxPi循环(pi注释掉).

real    0m0.011s
user    0m0.008s
sys 0m0.000s

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更新#2: 对于每个系统,添加CPU是32位还是64位以及cat /proc/cpuinfo第一个核心的输出.

Answer 1

Dun*_*nes 1

超越函数（例如三角函数和指数）的准确性始终是一个问题¹。

为什么某些三角函数调用比其他函数慢

对于三角函数的许多参数，有一个快速近似可以为大多数参数产生高度准确的结果。然而，对于某些论点，近似值可能是完全错误的。因此，需要采用更精确的方法，但这些方法需要更长的时间（正如您所注意到的）。

为什么新库现在可能会变慢

长期以来，英特尔对其浮点版本三角函数的准确性做出了误导性的声明，称它们比实际情况要准确得多²。以至于 glibc 过去只是sin(double)作为fsin(float)^{3 的}包装器。您可能已升级到已纠正此错误的 glibc 版本。我不能代表 AMD 的 libm，但它很可能仍然依赖于三角函数^4、5的浮点版本的准确性声明。

该怎么办

如果您想要速度并且不太在意准确性，那么请使用 tan ( ftan) 的浮点版本。否则，如果您需要准确性，那么您将不得不使用较慢的方法。您能做的最好的事情就是缓存tan(pi)的结果tan(pi/2)，并在您认为可能需要时使用预先计算的值。

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