Subject: Re: [PATCHSET v3][RFC] Make background writeback not suck
To: Dave Chinner <david@fromorbit.com>
References: <1459350477-16404-1-git-send-email-axboe@fb.com>
 <20160331082433.GO11812@dastard> <56FD344F.70908@fb.com>
CC: <linux-kernel@vger.kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <linux-block@vger.kernel.org>
From: Jens Axboe <axboe@fb.com>
Message-ID: <56FD4E70.3090203@fb.com>
Date: Thu, 31 Mar 2016 10:21:04 -0600
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On 03/31/2016 08:29 AM, Jens Axboe wrote:
>> What I see in these performance dips is the XFS transaction
>> subsystem stalling *completely* - instead of running at a steady
>> state of around 350,000 transactions/s, there are *zero*
>> transactions running for periods of up to ten seconds.  This
>> co-incides with the CPU usage falling to almost zero as well.
>> AFAICT, the only thing that is running when the filesystem stalls
>> like this is memory reclaim.
>
> I'll take a look at this, stalls should definitely not be occurring. How
> much memory does the box have?

I can't seem to reproduce this at all. On an nvme device, I get a fairly 
steady 60K/sec file creation rate, and we're nowhere near being IO 
bound. So the throttling has no effect at all.

On a raid0 on 4 flash devices, I get something that looks more IO bound, 
for some reason. Still no impact of the throttling, however. But given 
that your setup is this:

	virtio in guest, XFS direct IO -> no-op -> scsi in host.

we do potentially have two throttling points, which we don't want. Is 
both the guest and the host running the new code, or just the guest?

In any case, can I talk you into trying with two patches on top of the 
current code? It's the two newest patches here:

http://git.kernel.dk/cgit/linux-block/log/?h=wb-buf-throttle

The first treats REQ_META|REQ_PRIO like they should be treated, like 
high priority IO. The second disables throttling for virtual devices, so 
we only throttle on the backend. The latter should probably be the other 
way around, but we need some way of conveying that information to the 
backend.

-- 
Jens Axboe