linux-kernel - Re: [PATCH V4 07/15] blk-throttle: make throtl

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Message-ID: <20161122231824.GC14628@shli-mbp.local>
Date:   Tue, 22 Nov 2016 15:18:24 -0800
From:   Shaohua Li <shli@...com>
To:     Tejun Heo <tj@...nel.org>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <Kernel-team@...com>, <axboe@...com>, <vgoyal@...hat.com>
Subject: Re: [PATCH V4 07/15] blk-throttle: make throtl_slice tunable

On Tue, Nov 22, 2016 at 04:27:15PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Mon, Nov 14, 2016 at 02:22:14PM -0800, Shaohua Li wrote:
> > throtl_slice is important for blk-throttling. A lot of stuffes depend on
> > it, for example, throughput measurement. It has 100ms default value,
> > which is not appropriate for all disks. For example, for SSD we might
> > use a smaller value to make the throughput smoother. This patch makes it
> > tunable.
> 
> It bothers me a bit because time slice doesn't mean anything inherent
> to throttling.  It really is an implementation detail - throttling can
> be implemented at per-operation level without time slice involved at
> all.  It's okay to expose the knob if necessary but the meaning of the
> knob is almost completely arbitrary to users (as it has no inherent
> meaning).

Hmm, it's not a real 'time slice'. The name is a bit confusion. Maybe rename it
to 'throtl_interval' or 'throtl_sampling_time'? not sure. bandwidth and iops
are always in terms of a time interval we measure them. We can't say the
iops/bw for a single io. So this is really a tuable knob.

Thanks,
Shaohua