Re: sleeps and waits during io_submit

From: Avi Kivity <avi@scylladb.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Brian Foster <bfoster@redhat.com>,
	Glauber Costa <glauber@scylladb.com>,
	xfs@oss.sgi.com
Subject: Re: sleeps and waits during io_submit
Date: Tue, 8 Dec 2015 15:56:52 +0200	[thread overview]
Message-ID: <5666E1A4.4070805@scylladb.com> (raw)
In-Reply-To: <20151208060343.GC19802@dastard>

On 12/08/2015 08:03 AM, Dave Chinner wrote:
> On Wed, Dec 02, 2015 at 10:34:14AM +0200, Avi Kivity wrote:
>> On 12/02/2015 02:13 AM, Brian Foster wrote:
>>> Metadata is modified in-core and handed off to the logging
>>> infrastructure via a transaction. The log is flushed to disk some time
>>> later and metadata writeback occurs asynchronously via the xfsaild
>>> thread.
>> Unless, I expect, if the log is full.  Since we're hammering on the
>> disk quite heavily, the log would be fighting with user I/O and
>> possibly losing.
>>
>> Does XFS throttle user I/O in order to get the log buffers recycled faster?
> No. XFS tags the metadata IO with REQ_META that the IO schedulers
> can tell the difference between metadata and data IO, and schedule
> them appropriately. Further. log buffers are also tagged with
> REQ_SYNC to indicate they are latency sensitive IOs, whcih the IO
> schedulers again treat differently to minimise latency in the face
> of bulk async IO which is not latency sensitive.
>
> IOWs, IO prioritisation and dispatch scheduling is the job of the IO
> scheduler, not the filesystem. The filesystem just tells the
> scheduler how to treat the different types of IO...
>
>> Is there any way for us to keep track of it, and reduce disk
>> pressure when it gets full?
> Only if you want to make more problems for yourself - second
> guessing what the filesystem is going to do will only lead you to
> dancing the Charlie Foxtrot on a regular basis. :/

So far the best approach I found that doesn't conflict with this is to 
limit io_submit iodepth to the natural disk iodepth (or a small multiple 
thereof).  This seems to keep XFS in its comfort zone, and is good for 
latency anyway.

The only issue is that the only way to obtain this parameter is to 
measure it.

I wrote a small tool to do this [1], but it's a hassle for users.

[1] https://github.com/avikivity/diskplorer

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