From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 05/20] Introduce put_vector() and get_vector to QEMUFile and qemu_fopen_ops(). Date: Fri, 23 Apr 2010 16:48:28 +0300 Message-ID: <4BD1A52C.1090406@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Yoshiaki Tamura , kvm@vger.kernel.org, qemu-devel@nongnu.org, Anthony Liguori , mtosatti@redhat.com, ohmura.kei@lab.ntt.co.jp, yoshikawa.takuya@oss.ntt.co.jp To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:57735 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757444Ab0DWNsj (ORCPT ); Fri, 23 Apr 2010 09:48:39 -0400 In-Reply-To: <4BD19F12.2020004@linux.vnet.ibm.com> Sender: kvm-owner@vger.kernel.org List-ID: 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.