From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=47117 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OHJD4-0003W0-I5 for qemu-devel@nongnu.org; Wed, 26 May 2010 12:11:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OHJD3-0006os-Dm for qemu-devel@nongnu.org; Wed, 26 May 2010 12:11:58 -0400 Received: from mail-vw0-f45.google.com ([209.85.212.45]:33648) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OHJD3-0006oi-BA for qemu-devel@nongnu.org; Wed, 26 May 2010 12:11:57 -0400 Received: by vws6 with SMTP id 6so1158938vws.4 for ; Wed, 26 May 2010 09:11:56 -0700 (PDT) Message-ID: <4BFD4849.6050108@codemonkey.ws> Date: Wed, 26 May 2010 11:11:53 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive References: <1274091292-4812-1-git-send-email-agraf@suse.de> <4BF14CE9.5040907@suse.de> <4BF15DC8.8080104@codemonkey.ws> <201005171723.15675.paul@codesourcery.com> <4BF16E22.6090400@codemonkey.ws> <4BFC0FF6.1080005@suse.de> <20100525210113.GE1402@hall.aurel32.net> <4BFC79E8.1070700@codemonkey.ws> <20100526085248.GC21913@volta.aurel32.net> <4BFD269E.9090503@codemonkey.ws> <4BFD2C57.4050606@aurel32.net> <4BFD2DF7.6020806@codemonkey.ws> <4BFD40F3.7020406@aurel32.net> In-Reply-To: <4BFD40F3.7020406@aurel32.net> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Aurelien Jarno Cc: kwolf@redhat.com, Paul Brook , qemu-devel@nongnu.org, Alexander Graf , hch@lst.de On 05/26/2010 10:40 AM, Aurelien Jarno wrote: >> I highly doubt that this is even visible on benchmarks without using >> KVM. The improvement on a microbenchmark was relatively small and the >> cost from TCG would almost certainly dwarf it. >> > It is something clearly visible. Before fsync() was not used, and it > happens this syscall can be very expensive (ie a few seconds, especially > with some other i/o load on the system) on ext3 with not so old kernels. > A google search for "firefox fsync" will give you a few pointers. > I'm well aware. I've asked multiple times for benchmark data with ext3 because of this issue. So far, it hasn't been noticable. >> Also, remember before KVM, we had single threaded IO and posix-aio >> (which is still single threaded). If KVM never happened, block >> performance would be far, far worse than it is today with cache=writeback. >> >> > io thread is not enable by default in QEMU. > But the thread pool based aio implementation is always enabled regardless of io thread. I was going to build and run 0.9.1 to do a benchmark of TCG comparing that to the latest git, but I don't have an easy way to get a version of gcc-3. I've forgotten how annoying that used to be :-) I would be very surprised if TCG + IDE had any observable performance difference pre/post the ide flush implementation. I'm almost inclined to suggestion that flush controls should be a qdev property verses a drive property since that's really where the change happened. Regards, Anthony Liguori