Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751660AbZJWJhB (ORCPT ); Fri, 23 Oct 2009 05:37:01 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751424AbZJWJhA (ORCPT ); Fri, 23 Oct 2009 05:37:00 -0400 Received: from smtp-out.google.com ([216.239.45.13]:41396 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751371AbZJWJg7 (ORCPT ); Fri, 23 Oct 2009 05:36:59 -0400 DomainKey-Signature: a=rsa-sha1; s=beta; d=google.com; c=nofws; q=dns; h=date:from:x-x-sender:to:cc:subject:in-reply-to:message-id: references:user-agent:mime-version:content-type:x-system-of-record; b=knV9Utvdj2xtpSr+5fqTUPIL29olzItI1Bcb7WbShHibqgFOGccHGnOtSg1TC55Mc WbsLx52ty10QYl1HVvdeA== Date: Fri, 23 Oct 2009 02:36:53 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Mel Gorman cc: Frans Pop , Jiri Kosina , Sven Geggus , Karol Lewandowski , Tobias Oetiker , "Rafael J. Wysocki" , David Miller , Reinette Chatre , Kalle Valo , KOSAKI Motohiro , Mohamed Abbas , Jens Axboe , "John W. Linville" , Pekka Enberg , Bartlomiej Zolnierkiewicz , Greg Kroah-Hartman , Stephan von Krawczynski , Kernel Testers List , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "linux-mm@kvack.org" Subject: Re: [PATCH 4/5] page allocator: Pre-emptively wake kswapd when high-order watermarks are hit In-Reply-To: <20091023091334.GV11778@csn.ul.ie> Message-ID: References: <1256221356-26049-1-git-send-email-mel@csn.ul.ie> <1256221356-26049-5-git-send-email-mel@csn.ul.ie> <20091023091334.GV11778@csn.ul.ie> User-Agent: Alpine 2.00 (DEB 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2667 Lines: 56 On Fri, 23 Oct 2009, Mel Gorman wrote: > > Hmm, is this really supposed to be added to __alloc_pages_high_priority()? > > By the patch description I was expecting kswapd to be woken up > > preemptively whenever the preferred zone is below ALLOC_WMARK_LOW and > > we're known to have just allocated at a higher order, not just when > > current was oom killed (when we should already be freeing a _lot_ of > > memory soon) or is doing a higher order allocation during direct reclaim. > > > > It was a somewhat arbitrary choice to have it trigger in the event high > priority allocations were happening frequently. > I don't quite understand, users of PF_MEMALLOC shouldn't be doing these higher order allocations and if ALLOC_NO_WATERMARKS is by way of the oom killer, we should be freeing a substantial amount of memory imminently when it exits that waking up kswapd would be irrelevant. > > If this is moved to the fastpath, why is this wake_all_kswapd() and not > > wakeup_kswapd(preferred_zone, order)? Do we need to kick kswapd in all > > zones even though they may be free just because preferred_zone is now > > below the watermark? > > > > It probably makes no difference as zones are checked for their watermarks > before any real work happens. However, even if this patch makes a difference, > I don't want to see it merged. At best, it is an extremely heavy-handed > hack which is why I asked for it to be tested in isolation. It shouldn't > be necessary at all because sort of pre-emptive waking of kswapd was never > necessary before. > Ahh, that makes a ton more sense: this particular patch is a debugging effort while the first two are candidates for 2.6.32 and -stable. Gotcha. > > Wouldn't it be better to do this on page_zone(page) instead of > > preferred_zone anyway? > > > > No. The preferred_zone is the zone we should be allocating from. If we > failed to allocate from it, it implies the watermarks are not being met > so we want to wake it. > Oops, I'm even more confused now :) I thought the existing wake_all_kswapd() in the slowpath was doing that and that this patch was waking them prematurely because it speculates that a subsequent high order allocation will fail unless memory is reclaimed. I thought we'd want to reclaim from the zone we just did a high order allocation from so that the fastpath could find the memory next time with ALLOC_WMARK_LOW. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/