From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:35652) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WnouA-0003NW-Rk for qemu-devel@nongnu.org; Fri, 23 May 2014 08:49:07 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Wnou1-0005m9-E7 for qemu-devel@nongnu.org; Fri, 23 May 2014 08:48:58 -0400 Received: from mail-ee0-x231.google.com ([2a00:1450:4013:c00::231]:43939) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Wnou1-0005m1-7Q for qemu-devel@nongnu.org; Fri, 23 May 2014 08:48:49 -0400 Received: by mail-ee0-f49.google.com with SMTP id e53so3595372eek.22 for ; Fri, 23 May 2014 05:48:48 -0700 (PDT) Date: Fri, 23 May 2014 14:27:43 +0200 From: Stefan Hajnoczi Message-ID: <20140523122743.GH1260@stefanha-thinkpad.redhat.com> References: <20140520214851.1935.57767.malonedeb@gac.canonical.com> <20140520232105.2195.64239.malone@gac.canonical.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20140520232105.2195.64239.malone@gac.canonical.com> Subject: Re: [Qemu-devel] [Bug 1321464] Re: qemu/block/qcow2.c:1942: possible performance problem ? List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Bug 1321464 <1321464@bugs.launchpad.net> Cc: qemu-devel@nongnu.org On Tue, May 20, 2014 at 11:21:05PM -0000, Max Reitz wrote: > I can only speak for qcow2 and qcow, but for those places, I don't think > it is worth fixing. First of all, both are image formats, so the > bottleneck is generally the disk on which the images are stored and not > main memory, so an overeager memset should not cause any problems. > > For both, the relevant piece of code is in qcow2/qcow_write_compressed() > which are rarely used anyway (as far as I know) and even if used, they > have additional overhead due to having to compress data first, so > “fixing” the memset() won't make them noticibly faster. I agree. It won't make a noticable difference and the compressed writes are only done in qemu-img convert, not for running guests. But patches to change this wouldn't hurt either. Stefan