From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1LAobJ-0006vY-HF for qemu-devel@nongnu.org; Thu, 11 Dec 2008 11:41:21 -0500 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1LAobH-0006uw-Sf for qemu-devel@nongnu.org; Thu, 11 Dec 2008 11:41:21 -0500 Received: from [199.232.76.173] (port=34747 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1LAobH-0006ur-Mi for qemu-devel@nongnu.org; Thu, 11 Dec 2008 11:41:19 -0500 Received: from mail-qy0-f20.google.com ([209.85.221.20]:62718) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1LAobH-0004Kj-Bv for qemu-devel@nongnu.org; Thu, 11 Dec 2008 11:41:19 -0500 Received: by qyk13 with SMTP id 13so1602750qyk.10 for ; Thu, 11 Dec 2008 08:41:16 -0800 (PST) Message-ID: <494142A7.2030908@codemonkey.ws> Date: Thu, 11 Dec 2008 10:41:11 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool 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> In-Reply-To: <494130B5.2080800@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gerd Hoffmann Cc: Andrea Arcangeli , qemu-devel@nongnu.org, kvm-devel 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 >