Re: memory allocation and virtual memory increase
Re: memory allocation and virtual memory increase
- Subject: Re: memory allocation and virtual memory increase
- From: Shawn Erickson <email@hidden>
- Date: Tue, 28 Nov 2006 07:54:08 -0800
On Nov 28, 2006, at 6:43 AM, John Stiles wrote:
Bill Bumgarner wrote:
On Nov 27, 2006, at 10:22 PM, Bruce Johnson wrote:
My program contantly allocates (via malloc) chunks of memory to hold
computer generated data. It then writes the data out to disk via
fwrite. Then the memory is then freed.
As Shawn indicated, it certainly sounds like your application has
a memory leak. The other, extremely remote, explanation is that
you have a seriously pathological allocaiton/deallocation pattern
that is fragmenting the heap so badly that you are exhausted
addresses without actually exhausting memory. Could happen, but
*extremely* unlikely -- safer to assume that it really is a memory
leak.
Leaks can lie because memory isn't always zeroed across
allocations. Try setting the MALLOC_SCRIBBLE (and
MALLOC_PRE_SCRIBBLE, as it catches other bugs) environment
variable. At a very minor cost in allocation performance, it'll
vastly reduce the number of false negatives ignored by leaks.
Now, though, a second question is raised:
Why is your app "constantly allocat[ing] .... writing ... then the
memory is freed"?
This sounds like a fairly standard situation in which you would
typically want to allocate a handful of buffers that are filled
with data, emptied of said data, and then recycled.
I'm not sure I completely agree with your recommendation of
recycling buffers here. In my experience, unless an app is doing
hundreds of allocations and frees per second, there isn't a strong
need for pooling buffers. Malloc basically /is/ a buffer pooling
system, and it's darn fast on OS X, so unless you have special
needs and malloc isn't cutting it for you, why reinvent that wheel?
Well if they are large buffers (OP didn't say) then reallocating them
can fragment the processes virtual memory space as the large buffers
intermix with other allocations. It could result in not having a
large enough contiguous virtual memory range to reallocate one of the
buffers after some cycles.
Of course I don't think that is happening (not easy to do... unless
dealing with rather large buffers and/or much memory allocation in
between)... I think he is hitting some type of memory leak or some
other type of crash in his code that he is incorrectly linking with
having a large virtual memory space allocated.
-Shawn
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