From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed Date: Sun, 05 Oct 2008 07:23:09 +0200 Message-ID: <48E84F3D.4070301@redhat.com> References: <1223071531-31817-1-git-send-email-ryanh@us.ibm.com> <1223071531-31817-5-git-send-email-ryanh@us.ibm.com> <200810040017.09081.paul@codesourcery.com> <48E6AC36.3060404@codemonkey.ws> <48E73ECD.9080309@redhat.com> <20081004135749.pphehrhuw9w4gwsc@imap.linux.ibm.com> <20081004214700.GH31395@us.ibm.com> <48E7ECC1.90008@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Ryan Harper , qemu-devel@nongnu.org, Paul Brook , Anthony Liguori , kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from mx2.redhat.com ([66.187.237.31]:44552 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751451AbYJEFbB (ORCPT ); Sun, 5 Oct 2008 01:31:01 -0400 In-Reply-To: <48E7ECC1.90008@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: Anthony Liguori wrote: > > The second concern is that if a guest can allocate arbitrary amounts > of memory, it could generate a swap storm. Unfortunately, AFAIK, > Linux is not yet to a point where it can deal with swap fairness. > Hopefully this is a limitation that the IO controller folks are taking > into account. Even if Linux was 100% fair (or rather 100% controlled -- we don't want fairness here) in dealing with memory overcommit, we don't want swapping just because a guest issued large requests. Breaking up large requests will reduce efficiency (I believe negligibly is the limit is 1MB or above), swapping will be much worse. And again, there's no real reason to allocate memory here, we should be doing I/O directly to the guest's scatter/gather buffers. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.