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

[Peter Lieven]

Am 05.04.2013 um 22:00 schrieb Paolo Bonzini <address@hidden>:

> 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 <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.
>> 
>> 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.

i have to correct myself, on Linux it seems that MADV_DONTNEED guarantees that
subsequent reads will return a zero filled page.

If I am right with that we could MADV_DONTNEED the memory on startup and keep
not sending zero pages in bulk stage if we are running under Linux. Am I right 
with that?

Peter