Re: [Qemu-devel] [PATCHv5 06/10] migration: search for zero instead of dup pages

[Kevin Wolf]

Am 08.04.2013 um 10:33 hat Peter Lieven geschrieben:
> 
> Am 05.04.2013 um 21:23 schrieb Kevin Wolf <address@hidden>:
> 
> > 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 <address@hidden>
> >> Reviewed-by: Orit Wasserman <address@hidden>
> >> Reviewed-by: Eric Blake <address@hidden>
> > 
> > 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