From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 31 Jul 2008 17:29:53 +0400 From: Evgeniy Polyakov Subject: Re: [patch v3] splice: fix race with page invalidation Message-ID: <20080731132953.GB1120@2ka.mipt.ru> References: <20080731001131.GA30900@shareable.org> <20080731004214.GA32207@shareable.org> <20080731061201.GA7156@shareable.org> <20080731102612.GA29766@2ka.mipt.ru> <20080731123350.GB16481@shareable.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080731123350.GB16481@shareable.org> Sender: owner-linux-mm@kvack.org Return-Path: To: Jamie Lokier Cc: Linus Torvalds , Miklos Szeredi , jens.axboe@oracle.com, akpm@linux-foundation.org, nickpiggin@yahoo.com.au, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org List-ID: On Thu, Jul 31, 2008 at 01:33:50PM +0100, Jamie Lokier (jamie@shareable.org) wrote: > This is why marking the pages COW would be better. Automatic! > There's no need for a notification, merely letting go of the page > references - yes, the hardware / TCP acks already do that, no locking > or anything! :-) The last reference is nothing special, it just means > the next file write/truncate sees the count is 1 and doesn't need to > COW the page. It depends... COW can DoS the system: consider attacker who sends a page, writes there, sends again and so on in lots of threads. Depending on link capacity eventually COW will eat the whole RAM. > > There was a linux aio_sendfile() too. Google still knows about its > > numbers, graphs and so on... :) > > I vaguely remember it's performance didn't seem that good. Benchmark of the 100 1MB files transfer (files are in VFS already) using sync sendfile() against aio_sendfile_path() shows about 10MB/sec performance win (78 MB/s vs 66-72 MB/s over 1 Gb network, sendfile sending server is one-way AMD Athlong 64 3500+) for aio_sendfile_path(). So, it was really better that sync sendfile :) > One of the problems is you don't really want AIO all the time, just > when a process would block because the data isn't in cache. You > really don't want to be sending *all* ops to worker threads, even > kernel threads. And you preferably don't want the AIO interface > overhead for ops satisfied from cache. That's how all AIO should work of course. We are getting into a bit of offtopic, but aio_sendfile() worked that way as long as syslets, although the former did allocate some structures before trying to send the data. > Syslets got some of the way there, and maybe that's why they were > faster than AIO for some things. There are user-space hacks which are > a bit like syslets. (Bind two processes to the same CPU, process 1 > wakes process 2 just before 1 does a syscall, and puts 2 back to sleep > if 2 didn't wake and do an atomic op to prove it's awake). I haven't > tested their performance, it could suck. Looks scary :) Thread allocation in userspace is rather costly operations compared to syslet threads in kernelspace. But depending on IO pattern this may or may not be a noticeble factor... It requires testing and numbers. -- Evgeniy Polyakov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org