Bug 154603

Description of problem:

Using flock() over NFS under the 2.6.x SMP kernels causes bizarre system-wide
performance degradation. Non-SMP kernels and 2.4.x kernels do not exhibit this
problem. This is what I see:

I was debugging a performance degradation of our compute-intensive application
after upgrading from RHL8.0 to FC3. On our dual-cpu (Athlon and P4) machines,
when running two of these applications at the same time, the machine feels very
sluggish (interactive response is slow, NFS mounts sometimes timeout), "top"
reports 50% "user" and 50% "system" CPU usage (oprofile, vmstat and
"/usr/bin/time" show the same), "strace" shows that both applications do not
make any system calls (they just compute). What?!? No system calls but 50%
"system" CPU usage?!?

I traced the problem to the use of flock() over NFS early in the application. As
we know, flock() over NFS "does not work", but this application worked fine
until the 2.6.x kernels in FC2 and FC3. (The locking code in this application
probably predates Linux) (The locking code in the application is useless and I
am presently removing it).

The problem is with closing the lockfile before releasing the lock. "man flock"
says that it should "just release the lock" and I guess it works in the 2.4 and
the non-SMP 2.6 kernels.

I reduced our application to this example that exhibits the performance problem:

// flock.cc
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>

int main(int argc,char*argv[])
{
  char *file = argv[1];

  int fd = open(file,O_RDWR);
  if (fd == -1) { perror("open");  return -1; }
  if (flock(fd, LOCK_EX | LOCK_NB )) { perror("flock"); return -1; }

  //flock(fd, LOCK_UN); // add this time to fix the problem
  close(fd); // or remove this line to fix the problem

  while (1) { /* compute stuff */ }
}

Compile and run on an SMP machine:
[olchansk@tw04 flock]$ g++     flock.cc   -o flock
[olchansk@tw04 flock]$ touch /tmp/aaa
[olchansk@tw04 flock]$ ./flock /tmp/aaa <---- use local file
In "vmstat" observe 50% "user", 50% idle and 0% "system" cpu usage:
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 2  0 1074880 1261436   6868  46624    0    0     0     0 1121   178 50  0 50  0


Now run:
[olchansk@tw04 flock]$ ./flock /some/nfs/mounted/file <--- NFS mounted
In "vmstat" observe high "system" cpu usage:
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 4  0 1074880 1261364   6932  46628    0    0     0     0 1217   162 50 37 13  0
 1  0 1074880 1261364   6940  46620    0    0     0    16 1158   279 50 26 24  0
Also try typing shell commands, observe sluggish interactive response.
Inspect the example source code: there are no system calls, nothing to cause
"system" cpu usage or slow down the machine.

Remove the "close" or add the "flock-unlock" statement, recompile, rerun,
observe "system" cpu usage goes back to zero, interactive response returns to
same as for "local file locking".

Version-Release number of selected component (if applicable):

SMP FC2, FC3 are affected. Anything older and non-SMP FC2, FC3 are fine (no
performance degradation). AMD64, Athlon and Pentium4 machines all behave the
same way.

[olchansk@bench flock]$ uname -a
Linux bench.triumf.ca 2.6.10-1.770_FC3smp #1 SMP Thu Feb 24 18:36:43 EST 2005
x86_64 x86_64 x86_64 GNU/Linux
[olchansk@bench flock]$ rpm -q glibc
glibc-2.3.5-0.fc3.1
[olchansk@bench flock]$ rpm -q gcc
gcc-3.4.2-6.fc3

K.O.