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Date: Mon, 8 Apr 2013 10:49:14 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCHv5 06/10] migration: search for zero instead
 of dup pages
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To: Peter Lieven <pl@kamp.de>
Cc: Anthony Liguori <aliguori@us.ibm.com>, quintela@redhat.com, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Orit Wasserman <owasserm@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>

Am 08.04.2013 um 10:33 hat Peter Lieven geschrieben:
> 
> Am 05.04.2013 um 21:23 schrieb Kevin Wolf <kwolf@redhat.com>:
> 
> > Am 26.03.2013 um 10:58 hat Peter Lieven geschrieben:
> >> virtually all dup pages are zero pages. remove
> >> the special is_dup_page() function and use the
> >> optimized buffer_find_nonzero_offset() function
> >> instead.
> >> 
> >> here buffer_find_nonzero_offset() is used directly
> >> to avoid the unnecssary additional checks in
> >> buffer_is_zero().
> >> 
> >> raw performace gain checking 1 GByte zeroed memory
> >> over is_dup_page() is approx. 10-12% with SSE2
> >> and 8-10% with unsigned long arithmedtic.
> >> 
> >> Signed-off-by: Peter Lieven <pl@kamp.de>
> >> Reviewed-by: Orit Wasserman <owasserm@redhat.com>
> >> Reviewed-by: Eric Blake <eblake@redhat.com>
> > 
> > Okay, so I bisected again and this is the second patch that is involved
> > in the slowness of qemu-iotests case 007.
> > 
> 
> Can you try if the following solves your issue:
> 
> diff --git a/exec.c b/exec.c
> index 786987a..54baa4a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1071,6 +1071,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>              memory_try_enable_merging(new_block->host, size);
>          }
>      }
> +    qemu_madvise(new_block->host, size, QEMU_MADV_DONTNEED);
>      new_block->length = size;
>  
>      /* Keep the list sorted from biggest to smallest block.  */

It does. But perhaps Paolo's suggestion of using mmap() to allocate the
memory would be better. I'm not sure how MADV_DONTNEED behaves on
non-Linux.

Kevin