Reading /proc/stat is slooow

[Andreas Schwab <schwab@suse.de>]

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At some point after 2.6.13 reading /proc/stat has become too slow.  This
is most visible with xosview, which spends up to 80% in kernel space.  I
have attached a simple program that you can run with "strace -r" to see
that the second read of /proc/stat which eventually returns EOF takes too
much time.  If you remove the loop around read the same amount of time is
now spent in close.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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#include <unistd.h>
#include <fcntl.h>

static char buf[8192];

int
main (int argc, char **argv)
{
  const char *file = argc > 1 ? argv[1] : "/proc/stat";
  while (1)
    {
      int fd = open (file, O_RDONLY);
      while (read (fd, buf, 8192) > 0);
      close (fd);
      usleep (300000);
    }
}