From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:38750) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SWnpC-0008Vn-L2 for qemu-devel@nongnu.org; Tue, 22 May 2012 08:04:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1SWnp7-0006Ci-JS for qemu-devel@nongnu.org; Tue, 22 May 2012 08:04:26 -0400 Received: from mx1.redhat.com ([209.132.183.28]:22321) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1SWnp7-0006CB-BJ for qemu-devel@nongnu.org; Tue, 22 May 2012 08:04:21 -0400 Date: Tue, 22 May 2012 15:03:59 +0300 From: "Michael S. Tsirkin" Message-ID: <20120522120358.GA3504@redhat.com> References: <4FB9F89A.90702@redhat.com> <20120521083132.GI4674@redhat.com> <4FBABF2D.2020200@codemonkey.ws> <1337639166.2779.117.camel@pasglop> <4FBAC22A.5010708@codemonkey.ws> <20120521224436.GL17031@redhat.com> <1337661278.2779.166.camel@pasglop> <1337671059.2779.188.camel@pasglop> <20120522111441.GC3032@redhat.com> <1337686901.3038.12.camel@pasglop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1337686901.3038.12.camel@pasglop> Subject: Re: [Qemu-devel] [PATCH] Add a memory barrier to DMA functions List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Benjamin Herrenschmidt Cc: Paolo Bonzini , Rusty Russell , qemu-devel@nongnu.org, Anthony Liguori , David Gibson On Tue, May 22, 2012 at 09:41:41PM +1000, Benjamin Herrenschmidt wrote: > On Tue, 2012-05-22 at 14:14 +0300, Michael S. Tsirkin wrote: > > > The baseline is the laptop without kvm talking to the server. The > > > TCP_STREAM test results are: > > > > It's not a good test. The thing most affecting throughput results is > > how > > much CPU does you guest get. So as a minumum you need to measure CPU > > utilization on the host and divide by that. > > The simple fact that we don't reach the baseline while in qemu seems to > be a reasonably good indication that we tend to be CPU bound already so > it's not -that- relevant. It would be if we were saturating the network. > > But yes, I can try to do more tests tomorrow, it would be nice if you > could contribute a proper test protocol (or even test on some machines) > since you seem to be familiar with those measurements (and I have a very > limited access to x86 gear ... basically just my laptop). > > Cheers, > Ben. I have a deja vu. Amos sent perf results when you argued about exactly the same issue in guest virtio. Delta was small but measureable. At the moment I have no free time or free hardware to redo the same work all over again. It's a well known fact that actual memory barrier is slow on x86 CPUs. You can't see it with network on your laptop? Write a microbenchmark. -- MST