Re: lockup in parallel hash join on dikkop (freebsd 14.0-current)

On 1/29/23 18:53, Andres Freund wrote:
> Hi,
>
> On 2023-01-29 18:39:05 +0100, Tomas Vondra wrote:
>> Will do, but I'll wait for another lockup to see how frequent it
>> actually is. I'm now at ~90 runs total, and it didn't happen again yet.
>> So hitting it after 15 runs might have been a bit of a luck.
>
> Was there a difference in how much load there was on the machine between
> "reproduced in 15 runs" and "not reproed in 90"? If indeed lack of barriers
> is related to the issue, an increase in context switches could substantially
> change the behaviour (in both directions). More intra-process context
> switches can amount to "probabilistic barriers" because that'll be a
> barrier. At the same time it can make it more likely that the relatively
> narrow window in WaitEventSetWait() is hit, or lead to larger delays
> processing signals.
>

No. The only thing the machine is doing is

while /usr/bin/true; do
make check
done

I can't reduce the workload further, because the "join" test is in a
separate parallel group (I cut down parallel_schedule). I could make the
machine busier, of course.

However, the other lockup I saw was when using serial_schedule, so I
guess lower concurrency makes it more likely.

But who knows ...

regards

--
Tomas Vondra
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