From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751126AbWCKF6Y (ORCPT ); Sat, 11 Mar 2006 00:58:24 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750865AbWCKF6X (ORCPT ); Sat, 11 Mar 2006 00:58:23 -0500 Received: from mail28.syd.optusnet.com.au ([211.29.133.169]:11957 "EHLO mail28.syd.optusnet.com.au") by vger.kernel.org with ESMTP id S1750718AbWCKF6X (ORCPT ); Sat, 11 Mar 2006 00:58:23 -0500 From: Con Kolivas To: Mike Galbraith Subject: Re: [PATCH] mm: Implement swap prefetching tweaks Date: Sat, 11 Mar 2006 16:58:10 +1100 User-Agent: KMail/1.9.1 Cc: Andrew Morton , linux-kernel@vger.kernel.org, ck@vds.kolivas.org References: <200603102054.20077.kernel@kolivas.org> <1142055239.7819.47.camel@homer> <200603111650.23727.kernel@kolivas.org> In-Reply-To: <200603111650.23727.kernel@kolivas.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200603111658.10520.kernel@kolivas.org> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Saturday 11 March 2006 16:50, Con Kolivas wrote: > On Saturday 11 March 2006 16:33, Mike Galbraith wrote: > > On Sat, 2006-03-11 at 14:50 +1100, Con Kolivas wrote: > > > On Saturday 11 March 2006 09:35, Andrew Morton wrote: > > > > Con Kolivas wrote: > > > > > + /* > > > > > + * get_page_state is super expensive so we only perform it every > > > > > + * SWAP_CLUSTER_MAX prefetched_pages. > > > > > > > > nr_running() is similarly expensive btw. > > > > > > Yes which is why I do it just as infrequently as get_page_state. > > > > > > > > * We also test if we're the only > > > > > + * task running anywhere. We want to have as little impact on all > > > > > + * resources (cpu, disk, bus etc). As this iterates over every > > > > > cpu + * we measure this infrequently. > > > > > + */ > > > > > + if (!(sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)) { > > > > > + unsigned long cpuload = nr_running(); > > > > > + > > > > > + if (cpuload > 1) > > > > > + goto out; > > > > > > > > Sorry, this is just wrong. If swap prefetch is useful then it's also > > > > useful if some task happens to be sitting over in the corner > > > > calculating pi. > > > > > > > > What's the actual problem here? Someone's 3d game went blippy? Why? > > > > How much? Are we missing a cond_resched()? > > > > > > No, it's pretty easy to reproduce, kprefetchd sits there in > > > uninterruptible sleep with one cpu on SMP pegged at 100% iowait due to > > > it. This tends to have noticeable effects everywhere on HT or SMP. On > > > UP the yielding helped it but even then it still causes blips. How > > > much? Well to be honest it's noticeable a shipload. Running a game, any > > > game, that uses 100% (and most fancy games do) causes stuttering on > > > audio, pauses and so on. This is evident on linux native games, games > > > under emulators or qemu and so on. That iowait really hurts, and > > > tweaking just priority doesn't help it in any way. > > > > That doesn't really make sense to me. If a task can trigger audio > > dropout and stalls by sleeping, we have a serious problem. In your > > SMP/HT case, I'd start crawling over the load balancing code. I can't > > see how trivial CPU with non-saturated IO can cause dropout in the UP > > case either. Am I missing something? > > Clearly you, me and everyone else is missing something. I see it with each > task bound to one cpu with cpu affinity so it's not a balancing issue. Try > it yourself if you can instead of not believing me. Get a big dd reader > (virtually no cpu and all io wait sleep) on one cpu and try and play a game > on the other cpu. It dies rectally. I happen to have a tool to instrument this as you're probably aware (interbench). Here is an old log I found of this.: --- Benchmarking simulated cpu of Gaming in the presence of simulated --- Load Latency +/- SD (ms) Max Latency % Desired CPU None 0 +/- 0 0 100 Write 36.5 +/- 103 966 73.3 Read 17.2 +/- 22.9 244 85.3 Note the max latency being massive and desired cpu dropping. This is on a HT machine. Cheers, Con