Re: [Qemu-devel] [PATCH v2] rcu: reduce more than 7MB heap memory by malloc_trim()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Nov 23, 2017 at 02:41:16PM +0800, Yang Zhong wrote:
> Since there are some issues in memory alloc/free machenism
> in glibc for little chunk memory, if Qemu frequently
> alloc/free little chunk memory, the glibc doesn't alloc
> little chunk memory from free list of glibc and still
> allocate from OS, which make the heap size bigger and bigger.
> 
> This patch introduce malloc_trim(), which will free heap memory.
> 
> Below are test results from smaps file.
> (1)without patch
> 55f0783e1000-55f07992a000 rw-p 00000000 00:00 0  [heap]
> Size:              21796 kB
> Rss:               14260 kB
> Pss:               14260 kB
> 
> (2)with patch
> 55cc5fadf000-55cc61008000 rw-p 00000000 00:00 0  [heap]
> Size:              21668 kB
> Rss:                6940 kB
> Pss:                6940 kB
> 
> Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> ---
>  configure  | 4 ++++
>  util/rcu.c | 6 ++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/configure b/configure
> index 0e856bb..5b463d4 100755
> --- a/configure
> +++ b/configure
> @@ -6012,6 +6012,10 @@ if test "$opengl" = "yes" ; then
>    fi
>  fi
>  
> +if test "$tcmalloc" = "yes" || test "$jemalloc" = "yes" ; then
> +  echo "CONFIG_NONGLIBMALLOC=y" >> $config_host_mak

malloc(3) is provided by glibc, not glib, so the name
CONFIG_NONGLIBMALLOC is confusing.

I suggest calling it CONFIG_MALLOC_TRIM instead:

  # Even if malloc_trim() is available, these non-libc memory allocators
  # do not support it.
  if test "$tcmalloc" = "yes" || test "$jemalloc" = "yes" ; then
      if test "$malloc_trim" = "yes" ; then
          echo "Disabling malloc_trim with non-libc memory allocator"
      fi
      malloc_trim="no"
  fi

  if test "$malloc_trim" != "no" ; then
      cat > $TMPC << EOF
  #include <malloc.h>
  int main(void) { malloc_trim(0); return 0; }
  EOF
      if compile_prog "" "" ; then
          malloc_trim="yes"
      else
          malloc_trim="no"
      fi
  fi

  ...

  if test "$malloc_trim" = "yes" ; then
      echo "CONFIG_MALLOC_TRIM=y" >> $config_host_mak
  fi

Then the code in rcu.c just has to #ifdef CONFIG_MALLOC_TRIM and there's
no need for Linux-specific checks.  If other operating systems support
malloc_trim() then QEMU will use it by default.

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