From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool Date: Thu, 11 Dec 2008 10:41:11 -0600 Message-ID: <494142A7.2030908@codemonkey.ws> References: <1228512061-25398-1-git-send-email-aliguori@us.ibm.com> <493E941D.4000608@redhat.com> <493E965E.5050701@us.ibm.com> <20081210164401.GF18814@random.random> <493FFAB6.2000106@codemonkey.ws> <493FFC8E.9080802@redhat.com> <49400F69.8080707@codemonkey.ws> <20081210190810.GG18814@random.random> <20081211131222.GA14908@random.random> <494130B5.2080800@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Andrea Arcangeli , kvm-devel , qemu-devel@nongnu.org To: Gerd Hoffmann Return-path: Received: from mail-qy0-f11.google.com ([209.85.221.11]:50675 "EHLO mail-qy0-f11.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755554AbYLKQlR (ORCPT ); Thu, 11 Dec 2008 11:41:17 -0500 Received: by qyk4 with SMTP id 4so1263093qyk.13 for ; Thu, 11 Dec 2008 08:41:16 -0800 (PST) In-Reply-To: <494130B5.2080800@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Gerd Hoffmann wrote: > Andrea Arcangeli wrote: > >> My current feeling is that this user thread aio thing will never >> satisfy enterprise usage and kernel aio is mandatory in my view. >> > > Well, linux kernel aio has its share of problems too: > > * Anthony mentioned it may block on certain circumstances (forgot > which ones), and you can't figure beforehand to turn off aio then. > > * It can't handle block allocation. Kernel handles that by doing > such writes synchronously via VFS layer (instead of the separate > aio code paths). Leads to horrible performance and bug reports > such as "installs on sparse files are very slow". > > * support for vectored aio isn't that old. IIRC it was added > somewhen around 2.6.20 (newer that current suse/redhat enterprise > versions). Which IMHO means you can't expect it being present > unconditionally. > > >> And we should concentrate on kernel aio and get rid >> of threads when host OS is linux. >> > > Threads will be there anyway for kvm smp. > > >> Has anybody a patch implementing kernel aio that I can plug into the >> dma zerocopy api? I'm not so sure clone aio is worth maintaining >> inside qemu instead of evolving glibc >> > > Well, wait for glibc isn't going to fly. glibc waits for posix, and > posix waits for a reference implementation (which will not be glibc). > > >> and kernel with preadv/pwritev >> > > With that in place you don't need kernel aio any more, then you can > really do it in userspace with threads. But that probably would be > linux-only ^W^W^W > linux-only is okay but we just need a relatively sane fall back. There have been preadv/pwritev patches posted before, they just for some reason never were merged. http://lwn.net/Articles/163603/ > ahem: http://www.daemon-systems.org/man/preadv.2.html > Yeah, dunno if that's all BSDs or just NetBSD. Regards, Anthony Liguori > cheers, > Gerd >