Wednesday, January 11, 2006

Dtrace and glib

Memory leaks have always been difficult problems to deal it. There are many tools available and most of them preloads the probes and thus adds overhead on the execution of software.

During recent encounters with memory leaks, I thought of trying DTrace, the dynamic tracing tool available from Solaris. It operates on the kernel level probes and thus doesn't add overhead by preloading probes during execution.
It took me a while to get the setup ready for a noble cause. ;)

And bingo!!, got a dtrace script and a supporting perl script that ease the job of printing the stack that leaks memory. Both the scripts are available here. The output of the script was quite satisfactory, however, glib uses memory-pools that doesn't actually free the memory, rather, mark it for re-use and similarly, its allocation routines return pointers from the pool, when available otherwise allocates fresh memory and maintain it in the pool.

The DTrace script in the link actually uses the standard syscall probes, which in this case wouldn't be appropriate. So, I came up with the following script for glib:

#!/usr/sbin/dtrace -s

pid$target:libglib-2.0.so.0:g_malloc:entry
{
self->trace = 1;
self->size = arg0;
}
pid$target:libglib-2.0.so.0:g_malloc:return
/self->trace == 1/
{
printf("Ptr=0x%p Size=%d", arg1, self->size);
ustack();
self->trace = 0;
self->size = 0;
}

pid$target:libglib-2.0.so.0:g_realloc:entry
{
self->trace = 1;
self->size = arg1;
self->oldptr = arg0;
}

pid$target:libglib-2.0.so.0:g_realloc:return
/self->trace == 1/
{
printf("Ptr=0x%p Oldptr=0x%p Size=%d", arg1, self->oldptr,
self->size);
ustack();
self->trace = 0;
self->size = 0;
}

pid$target:libglib-2.0.so.0:g_malloc0:entry
/self->trace == 1/
{
self->trace = 1;
self->size = arg1;
}

pid$target:libglib-2.0.so.0:g_malloc0:return
/self->trace == 1/
{
printf("Ptr=0x%p Size=%d", arg1, self->size);
ustack();
self->trace = 0;
self->size = 0;
}

pid$target:libglib-2.0.so.0:g_free:entry
{
printf("Ptr=0x%p ", arg0);
}


The test program to verify the script:

#include 
#include

int
main ()
{
gchar* str = NULL;
int i;

for (i = 0; i < 10; i++) {
str = g_strdup ("0123456789");
printf ("%s\n", str);
g_free (str);
}
return 0;
}

DTrace reported "Zero" memory leaks and I sort of convinced that the script works and can be run on bigger application like Evolution.

Tuesday, January 03, 2006

Travelogue: NCAC - 2005, PSG Tech, Coimbatore.

My travelogue can be reached here.

It was a real fun to talk about Open Source. Just three weeks before this event, the department had inaugurated a lab of Open Source and some of their alumni members from Novell presented talks on Open Source Technologies and distributed OpenSuSE DVDs to the lab and students.