Stupid C memory allocation query

Martin Schöön · *nix forums Guru Joined: 22 Feb 2005 Posts: 2122

Hi Guys,

Simple one, this: I'm driving some 'legacy' SCSI hardware, shall we say,
hardware and to do this we use our own driver.
I've been watching some of our SGIs lately and they're going into swap
when in all honesty they have plenty of RAM. They run no applications
other than our own.

Now, I traced it down to a bunch of free()s that don't seem to have
immediate effect. The actual freeing is delayed until later on in
execution, by which time we've malloc'd more and the machine is in swap.

This little bit of C demonstrates this perfectly. I'm compiling with gcc
3.4, but my feeling is that this caused by IRIX as under Linux this
works fine. I think there's possibly a tunable parameter somewhere. Anyone?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char **argv)
{
unsigned char *bigbuf1, *bigbuf2;
int megs = 40;
int size = 1024 * 1024;

sleep(5);

printf("allocating bigbuf1\n");
bigbuf1 = malloc(megs * size);
if(bigbuf1 == NULL)
{
printf("allocate failed\n");
exit(1);
}

sleep(5);

printf("allocating bigbuf2\n");
bigbuf2 = malloc(megs * size);
if(bigbuf2 == NULL)
{
printf("allocate failed\n");
exit(1);
}

sleep(5);

printf("freeing bigbuf1\n");
free(bigbuf1);

sleep(5);

printf("freeing bigbuf2\n");
free(bigbuf2);

sleep(5);
return 0;
}

Thanks
Simon

Walter Roberson · *nix forums Guru Joined: 19 Feb 2005 Posts: 1300

In article <446c8c64$0$2585$db0fefd9@news.zen.co.uk>,
Simon Burley <me@privacy.net> wrote:

Tony 'Nicoya' Mantler · *nix forums Guru Wannabe Joined: 02 Jul 2005 Posts: 139

In article <446c8c64$0$2585$db0fefd9@news.zen.co.uk>,
Simon Burley <me@privacy.net> wrote:

: This little bit of C demonstrates this perfectly. I'm compiling with gcc
: 3.4, but my feeling is that this caused by IRIX as under Linux this
: works fine. I think there's possibly a tunable parameter somewhere. Anyone?

As Walter said, it's a matter of how malloc() is implemented. Linux (glibc,
really) specifically uses a mixed-style allocator which uses brk()/sbrk() for
small allocations, and mmap() for large allocations. This allows it to return
larger allocations to the system sooner.

A simple brk/sbrk-only allocator will tend to run into memory fragmentation
problems earlier because, as Walter said, there is no way to return the "holes"
for reuse in other processes.

A simple mmap-only allocator will tend to run into MM performance problems due
to the large number of maps and mapping calls, as well as other types of
fragmentation problems due to the large number of unused "tails" at the end of
mapped pages.

You should be able to port the glibc malloc routines to your project, there
shouldn't be much linux-specific code that can't be reproduced in IRIX (though
there will be linux-specific tuning which might need a few tweaks for best
performance).

Of course, it's LGPL, so read up on the licence and be sure that it's compatible
with how you intend to integrate the code.

Cheers - Tony 'Nicoya' Mantler :)

--
Tony 'Nicoya' Mantler - Master of Code-fu
-- nicoya@ubb.ca -- http://www.ubb.ca/ --

Martin Schöön · *nix forums Guru Joined: 22 Feb 2005 Posts: 2122

Walter Roberson wrote:

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