Re: Memory is fragmented even though everything is deallocated

From:
gao_bolin@voila.fr
Newsgroups:
microsoft.public.vc.language
Date:
10 Apr 2007 08:46:06 -0700
Message-ID:
<1176219966.474975.14240@q75g2000hsh.googlegroups.com>
I am running Windows XP indeed. I am not familiar with windbg and
haven't been able to check where this memory lies. However, calling

HeapDestroy(GetProcessHeap())

after alloc_dealloc does not change the drop in max allocatable size,
so I guess it does not solve the problem. Am I missing something?
Plus, if I destroy the process heap, will I be able to allocate on the
heap at all afterwards?

However, if I do create my own heap and replace malloc and free by
HeapAlloc and HeapFree, and destroy this private heap, then, I do not
have any drop in max allocatable size. However, this supposes one is
free to change the code of the function presenting the problem, so
this is not the ultimate solution to my problem.

B.

On Apr 6, 10:44 pm, "Ivan Brugiolo [MSFT]"
<ivanb...@online.microsoft.com> wrote:

It depends where that memory lives.
If you look at the output of `!address -RegionUsageHeap`
in a cdb|ntsd|windbg debugger session, you can see where/how the
region of the address space are `owned` by the C-Runtime heap.
Assuming the regions are owned by heap-segments of a given heap,
and assuming you are not running Vista, then, the regions are kept
till the heap is destroyed. You shoud call HeapDestroy() to reclaim those
regions.

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<gao_bo...@voila.fr> wrote in message

news:1175891442.223120.281860@y66g2000hsf.googlegroups.com...

Okay, so actually I can reproduce the problem on other computers, it's
just a matter of allocating even more. The code is below. When running
it on my laptop I get the following output:

Max malloc: 1560Mb
Max malloc: 1392Mb

The funny thing is that if I remove the deallocation I get the same
result. My guess is that deallocated memory is not released by the
operating system that keeps it for further allocations (and I guess
for small allocations only, since the huge chunk of memory allocated
by max_malloc (code below) is affected by this pool). If that
interpretation is correct, is there a way to tell Windows to release
(part of) this memory?

B.

<code>

#include <iostream>
#include <utility>
#include <vector>

using namespace std;

pair<void *, size_t> max_malloc()
{
 const size_t mega = 1024*1024;
 void * p = NULL;
 size_t i;

 for (i = 2048; i > 0; --i)
 {
   p = malloc(i*mega);
   if (p) break;
 }
 return make_pair(p,i);
}

void print_max_malloc()
{
 pair<void*, size_t> res = max_malloc();
 cout << "Max malloc: " << res.second << "Mb" << endl;
 free(res.first);
}

void alloc_dealloc()
{
 const size_t N = 22000000;
 vector<int*> v(N, 0);
 size_t i;
 for (i = 0; i < N; ++i)
 {
   v[i] = new int;
 }
 for (i = 0; i < N; ++i)
 {
   delete v[i];
   v[i] = 0;
 }
}

int main(int argc, char* argv[])
{
 print_max_malloc();
 alloc_dealloc();
 print_max_malloc();
 return 0;
}

</code>

On Mar 31, 3:09 pm, gao_bo...@voila.fr wrote:

Probably you caused an additional DLL to be loaded into your address
space.
Use Dependency Walker's profiling mode for example.


I didn't see anything like that, plus the allocation/deallocation part
is quite simple to need an additional DLL. Plus, as I said, the drop
in the size of the biggest chunk depends on the number of allocated
ints, which probably means that it isn't related to a fixed-factor
like a DLL.

Allocating 2GB isn't feasible, because you have only 2GB user address
space
(on 32-bit, unless you boot with /3GB option) and DLLs are loaded in
the
middle of that, so no block that large is available.


I am booting with the /3GB option indeed. Btw I checked that this is
not the pb, and it is not: without this option I start from 1.5GB down
to 1.3GB.

B.

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