From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753708Ab2GWNjG (ORCPT ); Mon, 23 Jul 2012 09:39:06 -0400 Received: from cantor2.suse.de ([195.135.220.15]:58631 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753395Ab2GWNiy (ORCPT ); Mon, 23 Jul 2012 09:38:54 -0400 From: Mel Gorman To: Stable Cc: Linux-MM , LKML , Mel Gorman Subject: [PATCH 08/34] vmscan: reduce wind up shrinker->nr when shrinker can't do work Date: Mon, 23 Jul 2012 14:38:21 +0100 Message-Id: <1343050727-3045-9-git-send-email-mgorman@suse.de> X-Mailer: git-send-email 1.7.9.2 In-Reply-To: <1343050727-3045-1-git-send-email-mgorman@suse.de> References: <1343050727-3045-1-git-send-email-mgorman@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Chinner commit 3567b59aa80ac4417002bf58e35dce5c777d4164 upstream. Stable note: Not tracked in Bugzilla. This patch reduces excessive reclaim of slab objects reducing the amount of information that has to be brought back in from disk. The third and fourth paragram in the series describes the impact. When a shrinker returns -1 to shrink_slab() to indicate it cannot do any work given the current memory reclaim requirements, it adds the entire total_scan count to shrinker->nr. The idea behind this is that when the shrinker is next called and can do work, it will do the work of the previously aborted shrinker call as well. However, if a filesystem is doing lots of allocation with GFP_NOFS set, then we get many, many more aborts from the shrinkers than we do successful calls. The result is that shrinker->nr winds up to it's maximum permissible value (twice the current cache size) and then when the next shrinker call that can do work is issued, it has enough scan count built up to free the entire cache twice over. This manifests itself in the cache going from full to empty in a matter of seconds, even when only a small part of the cache is needed to be emptied to free sufficient memory. Under metadata intensive workloads on ext4 and XFS, I'm seeing the VFS caches increase memory consumption up to 75% of memory (no page cache pressure) over a period of 30-60s, and then the shrinker empties them down to zero in the space of 2-3s. This cycle repeats over and over again, with the shrinker completely trashing the inode and dentry caches every minute or so the workload continues. This behaviour was made obvious by the shrink_slab tracepoints added earlier in the series, and made worse by the patch that corrected the concurrent accounting of shrinker->nr. To avoid this problem, stop repeated small increments of the total scan value from winding shrinker->nr up to a value that can cause the entire cache to be freed. We still need to allow it to wind up, so use the delta as the "large scan" threshold check - if the delta is more than a quarter of the entire cache size, then it is a large scan and allowed to cause lots of windup because we are clearly needing to free lots of memory. If it isn't a large scan then limit the total scan to half the size of the cache so that windup never increases to consume the whole cache. Reducing the total scan limit further does not allow enough wind-up to maintain the current levels of performance, whilst a higher threshold does not prevent the windup from freeing the entire cache under sustained workloads. Signed-off-by: Dave Chinner Signed-off-by: Al Viro Signed-off-by: Mel Gorman --- mm/vmscan.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/mm/vmscan.c b/mm/vmscan.c index 31b551e..8ca1cd5 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -277,6 +277,21 @@ unsigned long shrink_slab(struct shrink_control *shrink, } /* + * We need to avoid excessive windup on filesystem shrinkers + * due to large numbers of GFP_NOFS allocations causing the + * shrinkers to return -1 all the time. This results in a large + * nr being built up so when a shrink that can do some work + * comes along it empties the entire cache due to nr >>> + * max_pass. This is bad for sustaining a working set in + * memory. + * + * Hence only allow the shrinker to scan the entire cache when + * a large delta change is calculated directly. + */ + if (delta < max_pass / 4) + total_scan = min(total_scan, max_pass / 2); + + /* * Avoid risking looping forever due to too large nr value: * never try to free more than twice the estimate number of * freeable entries. -- 1.7.9.2 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx192.postini.com [74.125.245.192]) by kanga.kvack.org (Postfix) with SMTP id 3B6BF6B0073 for ; Mon, 23 Jul 2012 09:38:55 -0400 (EDT) From: Mel Gorman Subject: [PATCH 08/34] vmscan: reduce wind up shrinker->nr when shrinker can't do work Date: Mon, 23 Jul 2012 14:38:21 +0100 Message-Id: <1343050727-3045-9-git-send-email-mgorman@suse.de> In-Reply-To: <1343050727-3045-1-git-send-email-mgorman@suse.de> References: <1343050727-3045-1-git-send-email-mgorman@suse.de> Sender: owner-linux-mm@kvack.org List-ID: To: Stable Cc: Linux-MM , LKML , Mel Gorman From: Dave Chinner commit 3567b59aa80ac4417002bf58e35dce5c777d4164 upstream. Stable note: Not tracked in Bugzilla. This patch reduces excessive reclaim of slab objects reducing the amount of information that has to be brought back in from disk. The third and fourth paragram in the series describes the impact. When a shrinker returns -1 to shrink_slab() to indicate it cannot do any work given the current memory reclaim requirements, it adds the entire total_scan count to shrinker->nr. The idea behind this is that when the shrinker is next called and can do work, it will do the work of the previously aborted shrinker call as well. However, if a filesystem is doing lots of allocation with GFP_NOFS set, then we get many, many more aborts from the shrinkers than we do successful calls. The result is that shrinker->nr winds up to it's maximum permissible value (twice the current cache size) and then when the next shrinker call that can do work is issued, it has enough scan count built up to free the entire cache twice over. This manifests itself in the cache going from full to empty in a matter of seconds, even when only a small part of the cache is needed to be emptied to free sufficient memory. Under metadata intensive workloads on ext4 and XFS, I'm seeing the VFS caches increase memory consumption up to 75% of memory (no page cache pressure) over a period of 30-60s, and then the shrinker empties them down to zero in the space of 2-3s. This cycle repeats over and over again, with the shrinker completely trashing the inode and dentry caches every minute or so the workload continues. This behaviour was made obvious by the shrink_slab tracepoints added earlier in the series, and made worse by the patch that corrected the concurrent accounting of shrinker->nr. To avoid this problem, stop repeated small increments of the total scan value from winding shrinker->nr up to a value that can cause the entire cache to be freed. We still need to allow it to wind up, so use the delta as the "large scan" threshold check - if the delta is more than a quarter of the entire cache size, then it is a large scan and allowed to cause lots of windup because we are clearly needing to free lots of memory. If it isn't a large scan then limit the total scan to half the size of the cache so that windup never increases to consume the whole cache. Reducing the total scan limit further does not allow enough wind-up to maintain the current levels of performance, whilst a higher threshold does not prevent the windup from freeing the entire cache under sustained workloads. Signed-off-by: Dave Chinner Signed-off-by: Al Viro Signed-off-by: Mel Gorman --- mm/vmscan.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/mm/vmscan.c b/mm/vmscan.c index 31b551e..8ca1cd5 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -277,6 +277,21 @@ unsigned long shrink_slab(struct shrink_control *shrink, } /* + * We need to avoid excessive windup on filesystem shrinkers + * due to large numbers of GFP_NOFS allocations causing the + * shrinkers to return -1 all the time. This results in a large + * nr being built up so when a shrink that can do some work + * comes along it empties the entire cache due to nr >>> + * max_pass. This is bad for sustaining a working set in + * memory. + * + * Hence only allow the shrinker to scan the entire cache when + * a large delta change is calculated directly. + */ + if (delta < max_pass / 4) + total_scan = min(total_scan, max_pass / 2); + + /* * Avoid risking looping forever due to too large nr value: * never try to free more than twice the estimate number of * freeable entries. -- 1.7.9.2 -- 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