From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail6.bemta8.messagelabs.com (mail6.bemta8.messagelabs.com [216.82.243.55]) by kanga.kvack.org (Postfix) with ESMTP id A41526B0069 for ; Wed, 2 Nov 2011 16:17:10 -0400 (EDT) Date: Wed, 2 Nov 2011 21:17:07 +0100 From: Jan Kara Subject: Re: Latency writing to an mlocked ext4 mapping Message-ID: <20111102201707.GD31575@quack.suse.cz> References: <20111025122618.GA8072@quack.suse.cz> <20111031231031.GD10107@quack.suse.cz> <20111101230320.GH18701@quack.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Jan Kara , Andreas Dilger , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-ext4@vger.kernel.org" On Tue 01-11-11 18:51:04, Andy Lutomirski wrote: > On Tue, Nov 1, 2011 at 4:10 PM, Andy Lutomirski wrote: > > On Tue, Nov 1, 2011 at 4:03 PM, Jan Kara wrote: > >> Avoiding IO during a minor fault would be a decent thing which might be > >> worth pursuing. As you properly noted "stable pages during writeback" > >> requirement is one obstacle which won't be that trivial to avoid though... > > > > There's an easy solution that would be good enough for me: add a mount > > option to turn off stable pages. > > > > Is the other problem just a race, perhaps? __block_page_mkwrite calls > > __block_write_begin (which calls get_block, which I think is where the > > latency comes from) *before* wait_on_page_writeback, which means that > > there might not be any space allocated yet. > > I think I'm right (other than calling it a race). If I change my code to do: > > - map the file (with MCL_FUTURE set) > - fallocate > - dirty all pages > - fsync > - dirty all pages again > > in the non-real-time thread, then a short test that was a mediocre > reproducer seems to work. > > This is annoying, though -- I'm not generating twice as much write I/O > as I used to. Is there any way to force the delalloc code to do its > thing without triggering writeback? I don't think fallocate has this > effect. fallocate() will preallocate blocks on disk backing the mapped page. That should get rid of latency in __block_write_begin(). Extents will still be marked as uninitialized, but conversion from uninitialized to initialized state happens during writeback / IO completion so you should not care much about it. Honza -- Jan Kara SUSE Labs, CR -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org