Very poor performance of JNI AttachCurrentThread on Linux

[Andrew Haley]

get_stack_bounds() was rewritten because of a small memory leak.
Instead of simply free()ing the memory to prevent the leak, it was
rewritten to use a byte-by-byte loop around read() :

http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2011-February/001864.html

Unfortunately, the performance impact of this change is tragic.  As
you can imagine, tens of thousands of system calls are made whenever
get_stack_bounds() is called.

Before rewrite: typically 100 microseconds
After rewrite: typically 1500 microseconds

It's impossible for me to tell from the discussion on the mailing list
why such a change was made.  There is a rather elliptical comment

> I'm strictly against reading /proc entry using stdio functions,
> as (a) /proc file could be changed while we are reading it (b) it's not
> fancy as we are buffering kmem.

but this doesn't make any sense.  If the contents of "/proc/self/maps"
really did change while it was being read, reading a byte at a time
wouldn't help at all.  I don't know what the second sentence means.

It would be possible to read "/proc/self/maps" in chunks and scan that,
but my measurements show that it would not be significantly faster
than the original version of get_stack_bounds().

This severe regression is impacting a current large Java deployment.
See https://bugzilla.redhat.com/show_bug.cgi?id=902004

Is there any reason why I should not simply submit a webrev that reverts
to the original code, with suitable use of free() ?  I have attached the
code that I am testing.

Andrew.




static bool
get_stack_bounds(uintptr_t *bottom, uintptr_t *top)
{
  FILE *f = fopen("/proc/self/maps", "r");
  if (f == NULL)
    return false;
  char *str = NULL;
  while (!feof(f)) {
    size_t dummy;
    ssize_t len = getline(&str, &dummy, f);
    if (len == -1) {
      free(str);
      fclose(f);
      return false;
    }

    if (len > 0 && str[len-1] == '\n') {
      str[len-1] = 0;
      len--;
    }

    static const char *stack_str = "[stack]";
    if (len > (ssize_t)strlen(stack_str)
	&& (strcmp(str + len - strlen(stack_str), stack_str) == 0)) {
      if (sscanf(str, "%" SCNxPTR "-%" SCNxPTR, bottom, top) == 2) {
        uintptr_t sp = (uintptr_t)__builtin_frame_address(0);
        if (sp >= *bottom && sp <= *top) {
          free(str);
          fclose(f);
          return true;
        }
      }
    }
  }
  free(str);
  fclose(f);
  return false;
}