Bug#718577: libc6: Libc6/libm-2.17 almost two times slower than 2.13 in trigonometric calculations
Package: libc6
Version: 2.17-7
Severity: important
Dear Maintainer,
* What led up to the situation?
I tried to upgrade my debian testing 64 bits,
based on a 3.2.0-amd64 kernel (and libc6/libm-2.13.so)
to use the current testing/jessie debian
based on a 3.9.8-amd64 kernel (and libc6/libm-2.17.so).
Before any modification, here is the configuration
for libm.so.6 & cie:
# uname -a
Linux XXX 3.9-1-amd64 #1 SMP Debian 3.9.8-1 x86_64 GNU/Linux
# cat /etc/debian_version
jessie/sid
# cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-3.9-1-amd64 root=UUID=... ro quiet
# grep name /proc/cpuinfo
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
model name : Intel(R) Xeon(R) CPU E5520 @ 2.27GHz
# ls -l /lib/x86_64-linux-gnu/libm.so.6
lrwxrwxrwx 1 root root 34 août 2 11:39 /lib/x86_64-linux-gnu/libm.so.6 -> /lib/x86_64-linux-gnu/libm-2.17.so
# file /lib/x86_64-linux-gnu/libm-2.17.so
/lib/x86_64-linux-gnu/libm-2.17.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), BuildID[sha1]=45ca5df04f1a1ae3f9ef707c6fa167b35649971e, for GNU/Linux 2.6.32, stripped
And one of my programs suddenly appears to be very slow in this new
Debian version. I tried on a second machine
(with another processor (intel i7 ws i5) & hardware (HP vs DELL)) and
I observed exactly the same problem.
* What exactly did you do (or not do) that was effective (or
ineffective)?
I searched a while and finaly retrieved the old version of libm
(libm-2.13.so), because my program is intensively using libm and
obtained the following results when switching between the two versions
of libm (running with jessie) :
# gcc loopmodel1.c -o loopmodel1 -O2 -Wall -Wextra -lm
# strip loopmodel1
# rm /lib/x86_64-linux-gnu/libm.so.6
# ln -s /lib/x86_64-linux-gnu/libm-2.13.so /lib/x86_64-linux-gnu/libm.so.6
# file /lib/x86_64-linux-gnu/libm-2.13.so
/lib/x86_64-linux-gnu/libm-2.13.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), BuildID[sha1]=6db0096207f3ccc0ea5f39304c7153becb0b821a, for GNU/Linux 2.6.26, stripped
# /usr/bin/time -v ./loopmodel1
Command being timed: "./loopmodel1"
User time (seconds): 297.27
System time (seconds): 0.00
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 4:58.12
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 496
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 173
Voluntary context switches: 1
Involuntary context switches: 25199
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
# rm /lib/x86_64-linux-gnu/libm.so.6
# ln -s /lib/x86_64-linux-gnu/libm-2.17.so /lib/x86_64-linux-gnu/libm.so.6
# /usr/bin/time -v ./loopmodel1
Command being timed: "./loopmodel1"
User time (seconds): 633.36
System time (seconds): 0.00
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 10:35.16
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 540
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 183
Voluntary context switches: 1
Involuntary context switches: 53690
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
* What was the outcome of this action?
Using the old-testing version of libm (libm-2.13.so) on the same OS
without recompiling makes my program run almost two times faster than
using the current testing version of libm (libm-2.17.so).
* What outcome did you expect instead?
At least same speed using the two versions of libm !
I cannot provide the "original program" (confidential) but I can
provide a simplified/modified program, using exclusively trigonometric
functions and a loop to provide an example of such "problem".
Firstly I suspected a problem between i386 and amd64 versions of
libm-2.17.so because on one of the computers I had to use the command
"dpkg --add-architecture i386" to install some 32bits libraries which
are required by a 3D visualization program. But this is not installed
on the second computer!
I have also posted this question in Debian forums with no conclusive
answer.
Do you know if there is a bug in libm-2.17.so ?
Anybody else observes this result ?
Thank you in advance,
BB
-- System Information:
Debian Release: jessie/sid
APT prefers testing
APT policy: (500, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
Kernel: Linux 3.9-1-amd64 (SMP w/8 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
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