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

On 1/29/23 19:08, Tomas Vondra wrote:
>
>
> 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.
>

FWIW the machine is now on run ~2700 without any further lockups :-/

Seems it was quite lucky we hit it twice in a handful of attempts.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company