From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752196Ab2ITNuL (ORCPT ); Thu, 20 Sep 2012 09:50:11 -0400 Received: from acsinet15.oracle.com ([141.146.126.227]:37096 "EHLO acsinet15.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751275Ab2ITNuJ (ORCPT ); Thu, 20 Sep 2012 09:50:09 -0400 Date: Thu, 20 Sep 2012 09:49:53 -0400 From: Konrad Rzeszutek Wilk To: Jan Beulich Cc: David Vrabel , Oliver Chick , "xen-devel@lists.xen.org" , "linux-kernel@vger.kernel.org" Subject: Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk drivers Message-ID: <20120920134951.GC1998@localhost.localdomain> References: <1348051887-21885-1-git-send-email-oliver.chick@citrix.com> <505AF126.2050806@citrix.com> <1348140637.24539.32.camel@oliverchick-Precision-WorkStation-T3400> <505B1EB9020000780009CA6E@nat28.tlf.novell.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <505B1EB9020000780009CA6E@nat28.tlf.novell.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Source-IP: acsinet22.oracle.com [141.146.126.238] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 20, 2012 at 12:48:41PM +0100, Jan Beulich wrote: > >>> On 20.09.12 at 13:30, Oliver Chick wrote: > > The memory overhead, and fallback mode points are related: > > -Firstly, it turns out that the overhead is actually 2.75MB, not 11MB > > per device. I made a mistake (pointed out by Jan) as the maximum number > > of requests that can fit into a single-page ring is 64, not 256. > > -Clearly, this still scales linearly. So the problem of memory footprint > > will occur with more VMs, or block devices. > > -Whilst 2.75MB per device is probably acceptable (?), if we start using > > multipage rings, then we might not want to have > > BLKIF_MAX_PERS_REQUESTS_PER_DEVICE==__RING_SIZE, as this will cause the > > memory overhead to increase. This is why I have implemented the > > 'fallback' mode. With a multipage ring, it seems reasonable to want the > > first $x$ grefs seen by blkback to be treated as persistent, and any > > later ones to be non-persistent. Does that seem sensible? > > From a resource usage pov, perhaps. But this will get the guest > entirely unpredictable performance. Plus I don't think 11Mb of Wouldn't it fall back to the older performance? > _virtual_ space is unacceptable overhead in a 64-bit kernel. If > you really want/need this in a 32-bit one, then perhaps some > other alternatives would be needed (and persistent grants may > not be the right approach there in the first place). > > Jan >