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From: Dennis Zhou <dennis@kernel.org>
To: David Sterba <dsterba@suse.cz>
Cc: David Sterba <dsterba@suse.com>, Chris Mason <clm@fb.com>,
	Josef Bacik <josef@toxicpanda.com>,
	Omar Sandoval <osandov@osandov.com>,
	kernel-team@fb.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v6 00/22] btrfs: async discard support
Date: Tue, 17 Dec 2019 19:06:00 -0500
Message-ID: <20191218000600.GB2823@dennisz-mbp> (raw)
In-Reply-To: <20191217145541.GE3929@suse.cz>

On Tue, Dec 17, 2019 at 03:55:41PM +0100, David Sterba wrote:
> On Fri, Dec 13, 2019 at 04:22:09PM -0800, Dennis Zhou wrote:
> > Hello,
> > 
> > Dave reported a lockdep issue [1]. I'm a bit surprised as I can't repro
> > it, but it obviously is right. I believe I fixed the issue by moving the
> > fully trimmed check outside of the block_group lock.  I mistakingly
> > thought the btrfs_block_group lock subsumed btrfs_free_space_ctl
> > tree_lock. This clearly isn't the case.
> > 
> > Changes in v6:
> >  - Move the fully trimmed check outside of the block_group lock.
> 
> v6 passed fstests, with some weird test failures that don't seem to be
> related to the patchset.

Yay!

> 
> Meanwhile I did manual test how the discard behaves. The workload was
> a series of linux git checkouts of various release tags (ie. this should
> provide some freed extents and coalesce them eventually to get larger
> chunks to discard), then a simple large file copy, sync, remove, sync.
> 
> The discards going down to the device were followin the maximum default
> size (64M) but I observed that only one range was discarded per 10
> seconds, while the other stats there are many more extents to discard.
> 
> For the large file it took like 5-10 cycles to send all the trimmed
> ranges, the discardable_extents decreased by one each time until it
> reached ... -1. At this point the discardable bytes were -16384, so
> thre's some accounting problem.
> 
> This happened also when I deleted everything from the filesystem and ran
> full balance.
> 

Oh no :(. I've been trying to repro with some limited checking out and
syncing, then subsequently removing everything and calling balance. It
is coming out to be 0 for me. I'll try harder to repro this and fix it.

> Regarding the slow io submission, I tried to increase the iops value,
> default was 10, but 100 and 1000 made no change. Increasing the maximum
> discard request size to 128M worked (when there was such long extent
> ready). I was expecting a burst of like 4 consecutive IOs after a 600MB
> file is deleted.  I did not try to tweak bps_limit because there was
> nothing to limit.
> 

Ah so there's actually a max time between discards set to 10 seconds as
the maximum timeout is calculated over 6 hours. So if we only have 6
extents, we'd discard 1 per hour(ish given it decays), but this is
clamped to 10 seconds.

At least on our servers, we seem to discard at a reasonable rate to
prevent performance penalties during a large number of reads and writes
while maintaining reasonable write amplification performance. Also,
metadata blocks aren't tracked, so on freeing of a whole metadata block
group (minus relocation), we'll trickle discards slightly slower than
expected.


> So this is something to fix but otherwise the patchset seems to be ok
> for adding to misc-next. Due to the timing of the end of the year and
> that we're already at rc2, this will be the main feature in 5.6.

I'll report back if I continue having trouble reproing it.

Thanks v5.6 sounds good to me!
Dennis

  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-14  0:22 Dennis Zhou
2019-12-14  0:22 ` [PATCH 01/22] bitmap: genericize percpu bitmap region iterators Dennis Zhou
2019-12-14  0:22 ` [PATCH 02/22] btrfs: rename DISCARD opt to DISCARD_SYNC Dennis Zhou
2019-12-14  0:22 ` [PATCH 03/22] btrfs: keep track of which extents have been discarded Dennis Zhou
2019-12-14  0:22 ` [PATCH 04/22] btrfs: keep track of cleanliness of the bitmap Dennis Zhou
2019-12-14  0:22 ` [PATCH 05/22] btrfs: add the beginning of async discard, discard workqueue Dennis Zhou
2019-12-14  0:22 ` [PATCH 06/22] btrfs: handle empty block_group removal Dennis Zhou
2019-12-14  0:22 ` [PATCH 07/22] btrfs: discard one region at a time in async discard Dennis Zhou
2019-12-14  0:22 ` [PATCH 08/22] btrfs: add removal calls for sysfs debug/ Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 09/22] btrfs: make UUID/debug have its own kobject Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 10/22] btrfs: add discard sysfs directory Dennis Zhou
2019-12-18 11:45   ` Anand Jain
2019-12-14  0:22 ` [PATCH 11/22] btrfs: track discardable extents for async discard Dennis Zhou
2019-12-14  0:22 ` [PATCH 12/22] btrfs: keep track of discardable_bytes Dennis Zhou
2019-12-14  0:22 ` [PATCH 13/22] btrfs: calculate discard delay based on number of extents Dennis Zhou
2019-12-30 16:50   ` David Sterba
2020-01-02 16:45     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 14/22] btrfs: add bps discard rate limit Dennis Zhou
2019-12-30 17:58   ` David Sterba
2020-01-02 16:46     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 15/22] btrfs: limit max discard size for async discard Dennis Zhou
2019-12-30 18:00   ` David Sterba
2019-12-30 18:08   ` David Sterba
2020-01-02 16:48     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 16/22] btrfs: make max async discard size tunable Dennis Zhou
2019-12-30 18:05   ` David Sterba
2020-01-02 16:50     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 17/22] btrfs: have multiple discard lists Dennis Zhou
2019-12-14  0:22 ` [PATCH 18/22] btrfs: only keep track of data extents for async discard Dennis Zhou
2019-12-30 17:39   ` David Sterba
2020-01-02 16:55     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 19/22] btrfs: keep track of discard reuse stats Dennis Zhou
2019-12-30 17:33   ` David Sterba
2020-01-02 16:57     ` Dennis Zhou
2019-12-14  0:22 ` [PATCH 20/22] btrfs: add async discard header Dennis Zhou
2019-12-14  0:22 ` [PATCH 21/22] btrfs: increase the metadata allowance for the free_space_cache Dennis Zhou
2019-12-14  0:22 ` [PATCH 22/22] btrfs: make smaller extents more likely to go into bitmaps Dennis Zhou
2019-12-17 14:55 ` [PATCH v6 00/22] btrfs: async discard support David Sterba
2019-12-18  0:06   ` Dennis Zhou [this message]
2019-12-19  2:03     ` Dennis Zhou
2019-12-19 20:06       ` David Sterba
2019-12-19 21:22         ` Dennis Zhou
2019-12-19 20:34     ` David Sterba
2019-12-19 21:17       ` Dennis Zhou
2019-12-30 18:13 ` David Sterba
2019-12-30 18:49   ` Dennis Zhou
2020-01-02 13:22     ` David Sterba

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