linux-kernel - Re: [PATCH V4 10/15] blk-throttle: add a simple idle detection

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Date:   Wed, 23 Nov 2016 16:46:19 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Shaohua Li <shli@...com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kernel-team@...com, axboe@...com, vgoyal@...hat.com
Subject: Re: [PATCH V4 10/15] blk-throttle: add a simple idle detection

Hello, Shaohua.

On Mon, Nov 14, 2016 at 02:22:17PM -0800, Shaohua Li wrote:
> Unfortunately it's very hard to determine if a cgroup is real idle. This
> patch uses the 'think time check' idea from CFQ for the purpose. Please
> note, the idea doesn't work for all workloads. For example, a workload
> with io depth 8 has disk utilization 100%, hence think time is 0, eg,
> not idle. But the workload can run higher bandwidth with io depth 16.
> Compared to io depth 16, the io depth 8 workload is idle. We use the
> idea to roughly determine if a cgroup is idle.

Hmm... I'm not sure thinktime is the best measure here.  Think time is
used by cfq mainly to tell the likely future behavior of a workload so
that cfq can take speculative actions on the prediction.  However,
given that the implemented high limit behavior tries to provide a
certain level of latency target, using the predictive thinktime to
regulate behavior might lead to too unpredictable behaviors.

Moreover, I don't see why we need to bother with predictions anyway.
cfq needed it but I don't think that's the case for blk-throtl.  It
can just provide idle threshold where a cgroup which hasn't issued an
IO over that threshold is considered idle.  That'd be a lot easier to
understand and configure from userland while providing a good enough
mechanism to prevent idle cgroups from clamping down utilization for
too long.

Thanks.

-- 
tejun