Re: [PATCH V2] Btrfs: introduce ticketed enospc infrastructure

From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Josef Bacik <jbacik@fb.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH V2] Btrfs: introduce ticketed enospc infrastructure
Date: Wed, 18 May 2016 07:24:46 -0400	[thread overview]
Message-ID: <1ff146f2-c8e6-1f84-bb03-1f73cc716177@gmail.com> (raw)
In-Reply-To: <1463506255-15918-1-git-send-email-jbacik@fb.com>

On 2016-05-17 13:30, Josef Bacik wrote:
> Our enospc flushing sucks.  It is born from a time where we were early
> enospc'ing constantly because multiple threads would race in for the same
> reservation and randomly starve other ones out.  So I came up with this solution
> to block any other reservations from happening while one guy tried to flush
> stuff to satisfy his reservation.  This gives us pretty good correctness, but
> completely crap latency.
>
> The solution I've come up with is ticketed reservations.  Basically we try to
> make our reservation, and if we can't we put a ticket on a list in order and
> kick off an async flusher thread.  This async flusher thread does the same old
> flushing we always did, just asynchronously.  As space is freed and added back
> to the space_info it checks and sees if we have any tickets that need
> satisfying, and adds space to the tickets and wakes up anything we've satisfied.
>
> Once the flusher thread stops making progress it wakes up all the current
> tickets and tells them to take a hike.
>
> There is a priority list for things that can't flush, since the async flusher
> could do anything we need to avoid deadlocks.  These guys get priority for
> having their reservation made, and will still do manual flushing themselves in
> case the async flusher isn't running.
>
> This patch gives us significantly better latencies.  Thanks,
>
> Signed-off-by: Josef Bacik <jbacik@fb.com>
I've had this running on my test system (which is _finally_ working 
again) for about 16 hours now, nothing is breaking, and a number of the 
tests are actually completing marginally faster, so you can add:

Tested-by: Austin S. Hemmelgarn <ahferroin7@gmail.com>