From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:41632)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1UOCoY-000779-Q3
	for qemu-devel@nongnu.org; Fri, 05 Apr 2013 16:00:48 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1UOCoX-0003xc-1I
	for qemu-devel@nongnu.org; Fri, 05 Apr 2013 16:00:46 -0400
Received: from mx1.redhat.com ([209.132.183.28]:29207)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1UOCoW-0003xJ-Oa
	for qemu-devel@nongnu.org; Fri, 05 Apr 2013 16:00:44 -0400
Message-ID: <515F2D47.2000002@redhat.com>
Date: Fri, 05 Apr 2013 22:00:07 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1364291919-19563-1-git-send-email-pl@kamp.de>
	<1364291919-19563-7-git-send-email-pl@kamp.de>
	<20130405192327.GG2351@dhcp-200-207.str.redhat.com>
In-Reply-To: <20130405192327.GG2351@dhcp-200-207.str.redhat.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCHv5 06/10] migration: search for zero instead
 of dup pages
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, quintela@redhat.com, Stefan Hajnoczi <stefanha@gmail.com>, Peter Lieven <pl@kamp.de>, qemu-devel@nongnu.org, Orit Wasserman <owasserm@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