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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 05/04/2013 21:23, Kevin Wolf ha scritto:
>> > 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.
> 
> The problem seems to be that the RAM of a guest is in fact _not_ zeroed
> during initialisation. It hits my test case reliably because I'm running
> with MALLOC_PERTURB_. Now I'm wondering if in practice this happens only
> under such test conditions, or if real guests could be affected as well
> and we should make sure to get zeroed memory for RAM.

I think we should MADV_DONTNEED it.

Paolo