Re: [PATCH 2/5] ioengines: don't record issue_time if ioengines already do it

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On Tue, Jun 14, 2022 at 10:59 AM Vincent Fu <vincent.fu@xxxxxxxxxxx> wrote:
>
> io_uring, io_uring_cmd, and libaio record issue_time inside the ioengine
> code when their commit functions are called. So we don't need to record
> issue_time again for these ioengines in td_io_queue.
>
> If we do fill issue_time twice, then mean(slat) + mean(clat) !=
> mean(lat):

I'm a little bit confused though why not recording issue_time again in
td_io_queue is considered the right fix. By having some ioengines that
record issue_time in their commit functions, and others that do not
(and thus have it recorded in td_io_queue), comparing clat between two
different engines isn't an apples-to-apples comparison. An ioengine
that does not record issue_time in its commit gets a "discount" of
sorts in its measured clat, since its issue_time is not recorded until
later. Does that make sense, or am I missing something? (Has been a
few months since I've been in the code in detail, so that's possible.)



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