Re: [Qemu-devel] [PATCH v3] rcu: reduce more than 7MB heap memory by malloc_trim()

[Shannon Zhao <zhaoshenglong@huawei.com>]

Hi,

On 2017/11/24 14:30, 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  | 29 +++++++++++++++++++++++++++++
>  util/rcu.c |  6 ++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/configure b/configure
> index 0c6e757..6292ab0 100755
> --- a/configure
> +++ b/configure
> @@ -426,6 +426,7 @@ vxhs=""
>  supported_cpu="no"
>  supported_os="no"
>  bogus_os="no"
> +malloc_trim="yes"
>  
>  # parse CC options first
>  for opt do
> @@ -3857,6 +3858,30 @@ if test "$tcmalloc" = "yes" && test "$jemalloc" = "yes" ; then
>      exit 1
>  fi
>  
> +# 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 "$malloc_trim" = "yes" ; then
> +        echo "Disabling malloc_trim with non-libc memory allocator"
> +    fi
> +    malloc_trim="no"
> +fi
> +
> +#######################################
> +# malloc_trim
> +
> +if test "$malloc_trim" != "no" ; then
> +    cat > $TMPC << EOF
> +#include <malloc.h>
> +int main(void) { malloc_trim(0); return 0; }
> +EOF
> +    if compile_prog "" "" ; then
> +        malloc_trim="yes"
> +    else
> +        malloc_trim="no"
> +    fi
> +fi
> +
>  ##########################################
>  # tcmalloc probe
>  
> @@ -6012,6 +6037,10 @@ if test "$opengl" = "yes" ; then
>    fi
>  fi
>  
> +if test "$malloc_trim" = "yes" ; then
> +  echo "CONFIG_MALLOC_TRIM=y" >> $config_host_mak
> +fi
> +
>  if test "$avx2_opt" = "yes" ; then
>    echo "CONFIG_AVX2_OPT=y" >> $config_host_mak
>  fi
> diff --git a/util/rcu.c b/util/rcu.c
> index ca5a63e..f403b77 100644
> --- a/util/rcu.c
> +++ b/util/rcu.c
> @@ -32,6 +32,9 @@
>  #include "qemu/atomic.h"
>  #include "qemu/thread.h"
>  #include "qemu/main-loop.h"
> +#if defined(CONFIG_MALLOC_TRIM)
> +#include <malloc.h>
> +#endif
>  
>  /*
>   * Global grace period counter.  Bit 0 is always one in rcu_gp_ctr.
> @@ -272,6 +275,9 @@ static void *call_rcu_thread(void *opaque)
>              node->func(node);
>          }
>          qemu_mutex_unlock_iothread();
> +#if defined(CONFIG_MALLOC_TRIM)
> +        malloc_trim(4 * 1024 * 1024);
> +#endif
>      }
>      abort();
>  }
> 

Looks like this patch introduces a performance regression. With this
patch the time of booting a VM with 60 scsi disks on ARM64 is increased
by 200+ seconds.

Thanks,
-- 
Shannon