linux-kernel - Re: [PATCH 1/2] blk-throtl: make latency= absolute

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Message-ID: <20171109234258.GD983427@devbig577.frc2.facebook.com>
Date:   Thu, 9 Nov 2017 15:42:58 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Shaohua Li <shli@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 1/2] blk-throtl: make latency= absolute

Hello, Shaohua.

On Thu, Nov 09, 2017 at 03:12:12PM -0800, Shaohua Li wrote:
> The percentage latency makes sense, but the absolute latency doesn't to me. A
> 4k IO latency could be much smaller than 1M IO latency. If we don't add
> baseline latency, we can't specify a latency target which works for both 4k and
> 1M IO.

It isn't adaptive for sure.  I think it's still useful for the
following reasons.

1. The absolute latency target is by nature both workload and device
   dependent.  For a lot of use cases, coming up with a decent number
   should be possible.

2. There are many use cases which aren't sensitive to the level where
   they care much about the different between small and large
   requests.  e.g. protecting a managerial job so that it doesn't
   completely stall doesn't require tuning things to that level.  A
   value which is comfortably higher than usually expected latencies
   would often be enough (say 100ms).

3. It's also useful for verification / testing.

Thanks.

-- 
tejun