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

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Message-ID: <20171110154314.GE983427@devbig577.frc2.facebook.com>
Date:   Fri, 10 Nov 2017 07:43:14 -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 08:27:13PM -0800, Shaohua Li wrote:
> I think the absolute latency would only work for HD. For a SSD, a 4k latency
> probably is 60us and 1M latency is 500us. The disk must be very contended to
> make 4k latency reach 500us. Not sensitive doesn't mean no protection. If the
> use case sets rough latency, say 1ms, there will be no protection for 4k IO at
> all. The baseline latency is pretty reliable for SSD actually. So I'd rather

I don't understand how that would mean no protection.  The latency
naturally includes the queueing time on the host side and, even for a
fast SSD device, it isn't too difficult to saturate the device to the
point where the host-side waiting time becomes pretty long.  All
that's necessary is IOs being issued faster than completed and we can
almost always do that.

> keeping the baseline latency for SSD but using absolute latency for HD, which
> can be done easily by setting DFL_HD_BASELINE_LATENCY to 0.

I don't think that'd be a good interface choice.  It's too misleading.
If we actually need to specify baseline + margin, it'd probably be
better to add another notation - say, "+N" - than overloading the
meaning of "N".

Thanks.

-- 
tejun