From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 045CA6B0044 for ; Wed, 7 Jan 2009 11:40:00 -0500 (EST) Date: Wed, 7 Jan 2009 08:39:01 -0800 (PST) From: Linus Torvalds Subject: Re: Increase dirty_ratio and dirty_background_ratio? In-Reply-To: <1231345546.11687.314.camel@twins> Message-ID: References: <20090107154517.GA5565@duck.suse.cz> <1231345546.11687.314.camel@twins> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Peter Zijlstra Cc: Jan Kara , linux-kernel@vger.kernel.org, linux-mm , Nick Piggin List-ID: On Wed, 7 Jan 2009, Peter Zijlstra wrote: > > > So the question is: What kind of workloads are lower limits supposed to > > help? Desktop? Has anybody reported that they actually help? I'm asking > > because we are probably going to increase limits to the old values for > > SLES11 if we don't see serious negative impact on other workloads... > > Adding some CCs. > > The idea was that 40% of the memory is a _lot_ these days, and writeback > times will be huge for those hitting sync or similar. By lowering these > you'd smooth that out a bit. Not just a bit. If you have 4GB of RAM (not at all unusual for even just a regular desktop, never mind a "real" workstation), it's simply crazy to allow 1.5GB of dirty memory. Not unless you have a really wicked RAID system with great write performance that can push it out to disk (with seeking) in just a few seconds. And few people have that. For a server, where throughput matters but latency generally does not, go ahead and raise it. But please don't raise it for anything sane. The only time it makes sense upping that percentage is for some odd special-case benchmark that otherwise can fit the dirty data set in memory, and never syncs it (ie it deletes all the files after generating them). In other words, yes, 40% dirty can make a big difference to benchmarks, but is almost never actually a good idea any more. That said, the _right_ thing to do is to (a) limit dirty by number of bytes (in addition to having a percentage limit). Current -git adds support for that. (b) scale it dynamically by your IO performance. No, current -git does _not_ support this. but just upping the percentage is not a good idea. Linus -- 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