Re: strange stack limit behavior when allocating more than 2GB mem on 32bit machine

[Joe <longapple@gmail.com>]

Hi Glynn,

Thanks for your explanation. However as you can see, I got 2GB mem and
~10GB swap, totally 12GB.

With ulimit -s 10240(KB), I can allocate 2.5GB, I guess these are in
swap, right?
With ulimit -s unlimited, as you said, kernel reserved 1GB, stack
reserved 2GB, there are still 12-3=9GB left??

Why did malloc failed, instead of allocating this abundant swap space?

Regards,
Joe



On Thu, Aug 20, 2009 at 10:58 PM, Glynn
Clements<glynn@gclements.plus.com> wrote:
>
> Joe wrote:
>
>> Here I encounter something which I can't understand.
>> What I want to do is to allocate ~2.5GB mem, it fails when stack limit
>> is unlimited, but succeeded when stack limit is 10240.
>
>> $ ulimit -s
>> 10240
>> $ ./malloc 2500
>> Malloc succeeded               <======= succeeds when stack limit is 10240
>> $ ulimit -s unlimited
>> $ ./malloc 2500
>> malloc failed: Cannot allocate memory   <======== fails when stack
>> limit is unlimited???
>>
>>
>> BTW, there is no such problem on 64bit machine
>>
>>
>> Could you please give some insight on this?
>
> A 32-bit system has a 4 GiB address space. The top GiB is reserved for
> the kernel. If you set a stack size of unlimited, 2 GiB are reserved
> for the stack and shared libraries, causing shared libraries to be
> mapped at 1GiB and up. This leaves around 860 MiB for the heap.
>
> The result is that there isn't any areay of the address space which is
> large enough for a single 2500 MiB allocation:
>
> glynn@cerise:~ $ ulimit -s 8192
> glynn@cerise:~ $ cat /proc/self/maps
> 08048000-08053000 r-xp 00000000 08:01 3966484    /bin/cat
> 08053000-08054000 r--p 0000a000 08:01 3966484    /bin/cat
> 08054000-08055000 rw-p 0000b000 08:01 3966484    /bin/cat
> 080a4000-080c6000 rw-p 080a4000 00:00 0          [heap]
> b7f67000-b7f68000 rw-p b7f67000 00:00 0
> b7f68000-b80a0000 r-xp 00000000 08:01 9784624    /lib/libc-2.9.so
> b80a0000-b80a2000 r--p 00138000 08:01 9784624    /lib/libc-2.9.so
> b80a2000-b80a3000 rw-p 0013a000 08:01 9784624    /lib/libc-2.9.so
> b80a3000-b80a6000 rw-p b80a3000 00:00 0
> b80bd000-b80be000 rw-p b80bd000 00:00 0
> b80be000-b80bf000 r-xp b80be000 00:00 0          [vdso]
> b80bf000-b80db000 r-xp 00000000 08:01 9785919    /lib/ld-2.9.so
> b80db000-b80dc000 r--p 0001b000 08:01 9785919    /lib/ld-2.9.so
> b80dc000-b80dd000 rw-p 0001c000 08:01 9785919    /lib/ld-2.9.so
> bf81e000-bf833000 rw-p bffeb000 00:00 0          [stack]
>
> glynn@cerise:~ $ ulimit -s unlimited
> glynn@cerise:~ $ cat /proc/self/maps
> 08048000-08053000 r-xp 00000000 08:01 3966484    /bin/cat
> 08053000-08054000 r--p 0000a000 08:01 3966484    /bin/cat
> 08054000-08055000 rw-p 0000b000 08:01 3966484    /bin/cat
> 0a016000-0a038000 rw-p 0a016000 00:00 0          [heap]
> 40000000-4001c000 r-xp 00000000 08:01 9785919    /lib/ld-2.9.so
> 4001c000-4001d000 r--p 0001b000 08:01 9785919    /lib/ld-2.9.so
> 4001d000-4001e000 rw-p 0001c000 08:01 9785919    /lib/ld-2.9.so
> 4001e000-4001f000 r-xp 4001e000 00:00 0          [vdso]
> 4001f000-40020000 rw-p 4001f000 00:00 0
> 40037000-4016f000 r-xp 00000000 08:01 9784624    /lib/libc-2.9.so
> 4016f000-40171000 r--p 00138000 08:01 9784624    /lib/libc-2.9.so
> 40171000-40172000 rw-p 0013a000 08:01 9784624    /lib/libc-2.9.so
> 40172000-40176000 rw-p 40172000 00:00 0
> bfb35000-bfb4a000 rw-p bffeb000 00:00 0          [stack]
>
> If you were to allocate 1900 MiB, the allocation would succeed with
> e.g. "ulimit -s 8192" (the allocation coming from the heap) or with
> "ulimit -s unlimited" (the allocation coming from the stack), but
> would fail with intermediate values, as neither region is large
> enough.
>
> --
> Glynn Clements <glynn@gclements.plus.com>
>
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