Re: Track the amount of time waiting due to cost_delay

On 6/11/24 13:13, Robert Haas wrote:
> On Tue, Jun 11, 2024 at 5:49 AM Bertrand Drouvot
> <bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>> As we can see the actual wait time is 30ms less than the intended wait time with
>> this simple test. So I still think we should go with 1) actual wait time and 2)
>> report the number of waits (as mentioned in [1]). Does that make sense to you?
>
> I like the idea of reporting the actual wait time better, provided
> that we verify that doing so isn't too expensive. I think it probably
> isn't, because in a long-running VACUUM there is likely to be disk
> I/O, so the CPU overhead of a few extra gettimeofday() calls should be
> fairly low by comparison. I wonder if there's a noticeable hit when
> everything is in-memory. I guess probably not, because with any sort
> of normal configuration, we shouldn't be delaying after every block we
> process, so the cost of those gettimeofday() calls should still be
> getting spread across quite a bit of real work.

Does it even require a call to gettimeofday()? The code in vacuum
calculates an msec value and calls pg_usleep(msec * 1000). I don't think
it is necessary to measure how long that nap was.

Regards, Jan