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Re: [Linux] malloc: small size - small pointer?

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On 2011-09-13, Alex Vinokur <alex.vinokur@gmail.com> wrote:
>
>
> Hi,
>
>
>
> Linux 2.6.18-238.1.1.el5 #1 SMP Tue Jan 4 13:32:19 EST 2011 x86_64
> x86_64 x86_64 GNU/Linux
>
> Intel(R) C++ Intel(R) 64 Compiler XE for applications running on
> Intel(R) 64, Version 12.0.4.191 Build 20110427
>
>
> malloc() returns "small" pointers for small sizes.
> Any suugestions for such behavior of the malloc function.
>
> Are "small" pointers on heap?

The glibc malloc uses sbrk() for "small" allocations, and mmap for large
ones. There are additional complications. Under multiple threads, there
are mmaped arenas, which are used even for "small" allocations.

Furthermore, sbrk() can fail because it runs into a mmapped area.

In that case, allocation continues with a malloced arena.

(I know all this because I debugged a glibc issue a couple of years ago.  I was
working as the "put together all things Linux, kernel and user" guy, and some
developers came to me with the "problem" of malloc not returning all the memory
to the OS in a test case.)

By the way, the mmap threshold is adjustable. Look up the documentation
for the mallopt function.

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Thread

[Linux] malloc: small size - small pointer? Alex Vinokur <alex.vinokur@gmail.com> - 2011-09-13 01:17 -0700
  Re: [Linux] malloc: small size - small pointer? Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-09-13 10:23 +0200
    Re: malloc: small size - small pointer? Alex Vinokur <alex.vinokur@gmail.com> - 2011-09-13 23:21 -0700
    Re: malloc: small size - small pointer? Alex Vinokur <alex.vinokur@gmail.com> - 2011-09-13 23:18 -0700
      Re: malloc: small size - small pointer? Alex Vinokur <alex.vinokur@gmail.com> - 2011-09-14 00:08 -0700
  Re: [Linux] malloc: small size - small pointer? Kaz Kylheku <kaz@kylheku.com> - 2011-12-24 06:56 +0000

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