Messages in this thread | | | Date | Fri, 10 Nov 2017 07:43:14 -0800 | From | Tejun Heo <> | Subject | Re: [PATCH 1/2] blk-throtl: make latency= absolute |
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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
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