From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from aserp1040.oracle.com ([141.146.126.69]:32711 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751049AbdESSfA (ORCPT ); Fri, 19 May 2017 14:35:00 -0400 Date: Fri, 19 May 2017 11:32:14 -0700 From: Liu Bo To: Nikolay Borisov Cc: linux-btrfs , jeffm@suse.com Subject: Re: btrfs metadata reclaim behavior/performance characteristics Message-ID: <20170519183214.GB10137@lim.localdomain> Reply-To: bo.li.liu@oracle.com References: <20170518144532.GA28854@lim.localdomain> <20170518214724.GA10554@lim.localdomain> <7a698730-6f67-b158-c172-0a74a291277f@suse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <7a698730-6f67-b158-c172-0a74a291277f@suse.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Fri, May 19, 2017 at 12:54:59PM +0300, Nikolay Borisov wrote: > > From: Liu Bo > > > > Subject: [PATCH] Btrfs: skip commit transaction if we don't have enough pinned bytes > > > > We commit transaction in order to reclaim space from pinned bytes because it could process delayed refs, and in may_commit_transaction(), we check first if pinned bytes are enough for the required space, we then check if that plus bytes reserved for delayed insert are enough for the required space. > > > > This changes the code to the above logic. > > > > Signed-off-by: Liu Bo > > --- > > fs/btrfs/extent-tree.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > > index e390451c72e6..bded1ddd1bb6 100644 > > --- a/fs/btrfs/extent-tree.c > > +++ b/fs/btrfs/extent-tree.c > > @@ -4837,7 +4837,7 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info, > > > > spin_lock(&delayed_rsv->lock); > > if (percpu_counter_compare(&space_info->total_bytes_pinned, > > - bytes - delayed_rsv->size) >= 0) { > > + bytes - delayed_rsv->size) < 0) { > > spin_unlock(&delayed_rsv->lock); > > return -ENOSPC; > > } > > > > Your patch does make a very big difference. Here are a couple of runs of > slow-rm: > > > > root@ubuntu-virtual:~# ./slow-rm.sh > Created 837 files before returning error, time taken 3 > Created 920 files before returning error, time taken 3 > Created 949 files before returning error, time taken 3 > Created 930 files before returning error, time taken 3 > Created 1101 files before returning error, time taken 4 > Created 1082 files before returning error, time taken 4 > Created 1608 files before returning error, time taken 5 > Created 1735 files before returning error, time taken 5 > rming took 1 seconds > > root@ubuntu-virtual:~# ./slow-rm.sh > Created 801 files before returning error, time taken 3 > Created 829 files before returning error, time taken 3 > Created 983 files before returning error, time taken 3 > Created 978 files before returning error, time taken 3 > Created 1023 files before returning error, time taken 3 > Created 1126 files before returning error, time taken 3 > Created 1538 files before returning error, time taken 4 > Created 1737 files before returning error, time taken 5 > rming took 2 seconds > > root@ubuntu-virtual:~# ./slow-rm.sh > Created 875 files before returning error, time taken 3 > Created 891 files before returning error, time taken 3 > Created 969 files before returning error, time taken 4 > Created 1002 files before returning error, time taken 4 > Created 1039 files before returning error, time taken 4 > Created 1051 files before returning error, time taken 4 > Created 1191 files before returning error, time taken 4 > Created 2137 files before returning error, time taken 8 > rming took 2 seconds > > So rming is a lot faster, but we create less files all in all and get > ENOSPC earlier. This means that most of the time bytes_pinned is not > enough to satisfy the allocation hence we are hitting the second > percpu_counter comparison. > Right, it's sort of my expected bahavior because all 1K buffered IO ends up being inline extent, it's likely to run out of metadata space very soon. > Also, the reason why the previous links showed 0 for bytes_pinned was > due to me having completely forgotten that bytes_pinned is a percpu > counter, hence my stap script wasn't actually reading it correctly. I see, bytes_pinned in space_info is different from the percpu one, they're updated at different time, but overall the percpu one is the the preciser counter. -liubo