From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [PATCH -mm] mm: more likely reclaim MADV_SEQUENTIAL mappings Date: Tue, 22 Jul 2008 13:49:28 +1000 References: <87y73x4w6y.fsf@saeurebad.de> <20080721230405.6cfde9bd@bree.surriel.com> <200807221343.40017.nickpiggin@yahoo.com.au> In-Reply-To: <200807221343.40017.nickpiggin@yahoo.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200807221349.28641.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: Rik van Riel Cc: Andrew Morton , KOSAKI Motohiro , Johannes Weiner , Peter Zijlstra , Nossum , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Tuesday 22 July 2008 13:43, Nick Piggin wrote: > On Tuesday 22 July 2008 13:04, Rik van Riel wrote: > > On Tue, 22 Jul 2008 12:54:28 +1000 > > > But we are not doing nothing because we already know and have coded > > > for the fact that the mapping will be accessed once, sequentially. > > > Now that we have gone this far, we should actually do it properly and > > > 1. unmap after use, 2. POSIX_FADV_DONTNEED after use. This will give > > > you much better performance and cache behaviour than any automatic > > > detection scheme, and it doesn't introduce any regressions for existing > > > code. > > > > If you run just one instance of the application! > > > > Think about something like an ftp server or a media server, > > where you want to cache the data that is served up many > > times, while evicting the data that got served just once. > > > > The kernel has much better knowledge of what the aggregate > > of all processes in the system are doing than any individual > > process has. > > That's true, but this case isn't really very good anyway. The information > goes away after you drop the mapping anyway. Or did you hope that the > backup program or indexer keeps all those mappings open until all the pages > have filtered through? Or maybe we can add yet more branches into the unmap > path to test for this flag as well? > > I don't think it is a good idea to add random things just because they seem > at first glance like a good idea. BTW. in the backup of a busy fileserver or some case like that, I'd bet that even using FADV_DONTNEED would be much faster than leaving these mappings around to try to drop them due to the decreased churn on the LRUs overall anyway. -- 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