From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8219FC432C2 for ; Wed, 25 Sep 2019 12:09:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5758D222BE for ; Wed, 25 Sep 2019 12:09:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404790AbfIYMJC (ORCPT ); Wed, 25 Sep 2019 08:09:02 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52962 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2404622AbfIYMJC (ORCPT ); Wed, 25 Sep 2019 08:09:02 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 01006877A64; Wed, 25 Sep 2019 12:09:02 +0000 (UTC) Received: from bfoster (dhcp-41-2.bos.redhat.com [10.18.41.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8A8D961559; Wed, 25 Sep 2019 12:09:01 +0000 (UTC) Date: Wed, 25 Sep 2019 08:08:59 -0400 From: Brian Foster To: Dave Chinner Cc: "Darrick J. Wong" , linux-xfs@vger.kernel.org Subject: Re: [PATCH 1/2] xfs: Lower CIL flush limit for large logs Message-ID: <20190925120859.GC21991@bfoster> References: <20190909015159.19662-1-david@fromorbit.com> <20190909015159.19662-2-david@fromorbit.com> <20190916163325.GZ2229799@magnolia> <20190924222901.GI16973@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190924222901.GI16973@dread.disaster.area> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.69]); Wed, 25 Sep 2019 12:09:02 +0000 (UTC) Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Wed, Sep 25, 2019 at 08:29:01AM +1000, Dave Chinner wrote: > On Mon, Sep 16, 2019 at 09:33:25AM -0700, Darrick J. Wong wrote: > > On Mon, Sep 09, 2019 at 11:51:58AM +1000, Dave Chinner wrote: > > > From: Dave Chinner > > > > > > The current CIL size aggregation limit is 1/8th the log size. This > > > means for large logs we might be aggregating at least 250MB of dirty objects > > > in memory before the CIL is flushed to the journal. With CIL shadow > > > buffers sitting around, this means the CIL is often consuming >500MB > > > of temporary memory that is all allocated under GFP_NOFS conditions. > > > > > > Flushing the CIL can take some time to do if there is other IO > > > ongoing, and can introduce substantial log force latency by itself. > > > It also pins the memory until the objects are in the AIL and can be > > > written back and reclaimed by shrinkers. Hence this threshold also > > > tends to determine the minimum amount of memory XFS can operate in > > > under heavy modification without triggering the OOM killer. > > > > > > Modify the CIL space limit to prevent such huge amounts of pinned > > > metadata from aggregating. We can have 2MB of log IO in flight at > > > once, so limit aggregation to 16x this size. This threshold was > > > chosen as it little impact on performance (on 16-way fsmark) or log > > > traffic but pins a lot less memory on large logs especially under > > > heavy memory pressure. An aggregation limit of 8x had 5-10% > > > performance degradation and a 50% increase in log throughput for > > > the same workload, so clearly that was too small for highly > > > concurrent workloads on large logs. > > > > It would be nice to capture at least some of this reasoning in the > > already lengthy comment preceeding the #define.... > > A lot of it is already there, but I will revise it. > > > > > > This was found via trace analysis of AIL behaviour. e.g. insertion > > > from a single CIL flush: > > > > > > xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 type XFS_LI_INODE flags IN_AIL > > > > > > $ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l > > > 1721823 > > > $ > > > > > > So there were 1.7 million objects inserted into the AIL from this > > > CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which > > > was the end of the trace (i.e. it hadn't finished). Clearly a major > > > problem. > > > > > > Signed-off-by: Dave Chinner > > > --- > > > fs/xfs/xfs_log_priv.h | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h > > > index b880c23cb6e4..187a43ffeaf7 100644 > > > --- a/fs/xfs/xfs_log_priv.h > > > +++ b/fs/xfs/xfs_log_priv.h > > > @@ -329,7 +329,8 @@ struct xfs_cil { > > > * enforced to ensure we stay within our maximum checkpoint size bounds. > > > * threshold, yet give us plenty of space for aggregation on large logs. > > > > ...also, does XLOG_CIL_SPACE_LIMIT correspond to "a lower threshold at > > which background pushing is attempted", or "a separate, higher bound"? > > I think it's the first (????) but ... I don't know. The name made me > > think it was the second, but the single use of the symbol suggests the > > first. :) > > See, the comment here talks about two limits, because that was how > the initial implementation worked - the background CIL push was not > async, so there was some juggling done to prevent new commits from > blocking on background pushes in progress unless the size was > actually growing to large. This patch pretty much describes the > whole issue here: > > https://lore.kernel.org/linux-xfs/1285552073-14663-2-git-send-email-david@fromorbit.com/ > > That's in commit 80168676ebfe ("xfs: force background CIL push under > sustained load") which went into 2.6.38 or so. The cause of the > problem in that case was concurrent transaction commit load causing > lock contention and preventing a background push from getting the > context lock to do the actual push. > More related to the next patch, but what prevents a similar but generally unbound concurrent workload from exceeding the new hard limit once transactions start to block post commit? Brian > The hard limit in the CIL code was dropped when the background push > was converted to run asynchronously to use a work queue in 2012 as > it allowed the locking to be changed (down_write_trylock -> > down_write) to turn it into a transaction commit barrier while the > contexts are switched over. That was done in 2012 via commit > 4c2d542f2e78 ("xfs: Do background CIL flushes via a workqueue") and > so we haven't actually capped CIL checkpoint sizes since 2012. > > Essentially, the comment you point out documents the two limits from > the original code, and this commit is restoring that behaviour for > background CIL pushes.... > > I'll do some work to update it all. > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com