From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1O5JFK-0001qY-FV
	for qemu-devel@nongnu.org; Fri, 23 Apr 2010 09:48:42 -0400
Received: from [140.186.70.92] (port=57817 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1O5JFI-0001oM-DI
	for qemu-devel@nongnu.org; Fri, 23 Apr 2010 09:48:42 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69)
	(envelope-from <avi@redhat.com>) id 1O5JFF-0005bq-C1
	for qemu-devel@nongnu.org; Fri, 23 Apr 2010 09:48:38 -0400
Received: from mx1.redhat.com ([209.132.183.28]:51400)
	by eggs.gnu.org with esmtp (Exim 4.69)
	(envelope-from <avi@redhat.com>) id 1O5JFF-0005bg-1J
	for qemu-devel@nongnu.org; Fri, 23 Apr 2010 09:48:37 -0400
Message-ID: <4BD1A52C.1090406@redhat.com>
Date: Fri, 23 Apr 2010 16:48:28 +0300
From: Avi Kivity <avi@redhat.com>
MIME-Version: 1.0
References: <1271829445-5328-1-git-send-email-tamura.yoshiaki@lab.ntt.co.jp>
	<1271829445-5328-6-git-send-email-tamura.yoshiaki@lab.ntt.co.jp>
	<4BD0A35E.8000205@linux.vnet.ibm.com>
	<4BD11604.3060309@lab.ntt.co.jp>
	<4BD19F12.2020004@linux.vnet.ibm.com>
In-Reply-To: <4BD19F12.2020004@linux.vnet.ibm.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Subject: [Qemu-devel] Re: [RFC PATCH 05/20] Introduce put_vector() and
 get_vector to QEMUFile and qemu_fopen_ops().
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Anthony Liguori <aliguori@linux.vnet.ibm.com>
Cc: ohmura.kei@lab.ntt.co.jp, kvm@vger.kernel.org, qemu-devel@nongnu.org, mtosatti@redhat.com, Anthony Liguori <aliguori@us.ibm.com>, Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>, yoshikawa.takuya@oss.ntt.co.jp

On 04/23/2010 04:22 PM, Anthony Liguori wrote:
>> I currently don't have data, but I'll prepare it.
>> There were two things I wanted to avoid.
>>
>> 1. Pages to be copied to QEMUFile buf through qemu_put_buffer.
>> 2. Calling write() everytime even when we want to send multiple pages 
>> at once.
>>
>> I think 2 may be neglectable.
>> But 1 seems to be problematic if we want make to the latency as small 
>> as possible, no?
>
>
> Copying often has strange CPU characteristics depending on whether the 
> data is already in cache.  It's better to drive these sort of 
> optimizations through performance measurement because changes are not 
> always obvious.

Copying always introduces more cache pollution, so even if the data is 
in the cache, it is worthwhile (not disagreeing with the need to measure).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.