From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60518) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WKRrP-0002JX-Mu for qemu-devel@nongnu.org; Mon, 03 Mar 2014 07:20:52 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WKRrH-0000xK-8z for qemu-devel@nongnu.org; Mon, 03 Mar 2014 07:20:43 -0500 Received: from mx.ipv6.kamp.de ([2a02:248:0:51::16]:40748 helo=mx01.kamp.de) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WKRrG-0000wl-Qw for qemu-devel@nongnu.org; Mon, 03 Mar 2014 07:20:35 -0500 Message-ID: <53147385.2090906@kamp.de> Date: Mon, 03 Mar 2014 13:20:21 +0100 From: Peter Lieven MIME-Version: 1.0 References: <530DBE6C.5030502@kamp.de> <20140226154154.GB20820@stefanha-thinkpad.muc.redhat.com> <530E0FF0.20501@kamp.de> <20140227085711.GC21749@stefanha-thinkpad.redhat.com> <53109E99.3020102@kamp.de> <20140303120349.GA21055@stefanha-thinkpad.redhat.com> In-Reply-To: <20140303120349.GA21055@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] qemu-img convert cache mode for source List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Paolo Bonzini , "qemu-devel@nongnu.org" , Stefan Hajnoczi On 03.03.2014 13:03, Stefan Hajnoczi wrote: > On Fri, Feb 28, 2014 at 03:35:05PM +0100, Peter Lieven wrote: >> On 27.02.2014 09:57, Stefan Hajnoczi wrote: >>> On Wed, Feb 26, 2014 at 05:01:52PM +0100, Peter Lieven wrote: >>>> On 26.02.2014 16:41, Stefan Hajnoczi wrote: >>>>> On Wed, Feb 26, 2014 at 11:14:04AM +0100, Peter Lieven wrote: >>>>>> I was wondering if it would be a good idea to set the O_DIRECT mode for the source >>>>>> files of a qemu-img convert process if the source is a host_device? >>>>>> >>>>>> Currently the backup of a host device is polluting the page cache. >>>>> Points to consider: >>>>> >>>>> 1. O_DIRECT does not work on Linux tmpfs, you get EINVAL when opening >>>>> the file. A fallback is necessary. >>>>> >>>>> 2. O_DIRECT has no readahead so performance could actually decrease. >>>>> The question is, how important is reahead versus polluting page >>>>> cache? >>>>> >>>>> 3. For raw files it would make sense to tell the kernel that access is >>>>> sequential and data will be used only once. Then we can get the best >>>>> of both worlds (avoid polluting page cache but still get readahead). >>>>> This is done using posix_fadvise(2). >>>>> >>>>> The problem is what to do for image formats. An image file can be >>>>> very fragmented so the readahead might not be a win. Does this mean >>>>> that for image formats we should tell the kernel access will be >>>>> random? >>>>> >>>>> Furthermore, maybe it's best to do readahead inside QEMU so that even >>>>> network protocols (nbd, iscsi, etc) can get good performance. They >>>>> act like O_DIRECT is always on. >>>> your comments are regarding qemu-img convert, right? >>>> How would you implement this? A new open flag because >>>> the fadvise had to goto inside the protocol driver. >>>> >>>> I would start with host_devices first and see how it performs there. >>>> >>>> For qemu-img convert I would issue a FADV_DONTNEED after >>>> a write for the bytes that have been written >>>> (i have tested this with Linux and it seems to work quite well). >>>> >>>> Question is, what is the right paramter for reads? Also FADV_DONTNEED? >>> I think so but this should be justified with benchmark results. >> I ran some benchmarks at found that a FADV_DONTNEED issues after >> a read does not hurt regarding to performance. But it avoids buffers >> increasing while I read from a host_device of raw file. > It was mentioned in this thread that a sequential shouldn't promote the > pages anyway - they should be dropped by the kernel if there is memory > pressure. Yes, but this costs cpu time in spikes and the page cache is polluted with data that is definetely not needed. > > So what is the actual performance problem you are trying to solve and > what benchmark output are you getting when you compare with > FADV_DONTNEED against without FADV_DONTNEED? I found the performance to be identical. For the problem see below please. > > I think there's a danger that the discussion will go around in circles. > Please post the performance results that kicked off this whole effort > and let's focus on the data. That way it's much easier to evaluate what > changes to QEMU are a win and which are not necessary. I found that under memory pressure situations the increasing buffers leads to vserver memory being swapped out. This caused trouble especially in overcommit scenarios (where all memory is backed by swap). > >> As for writing it does only work if I issue a fdatasync after each write, but >> this should be equivalent to O_DIRECT. So I would keep the patch >> to support qemu-img convert sources if they are host_device or file. > fdatasync(2) is much more heavy-weight than writing out a pages because > it sends a disk write cache flush command and waits for it to complete. as mentioned before for the write path the FADV_DONTNEED stuff doesn't work. Peter