Re: spin_delay() for ARM

On Sat, 18 Apr 2020 at 03:30, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Sat, Apr 18, 2020 at 2:00 AM Ants Aasma <ants(at)cybertec(dot)at> wrote:
> > On Thu, 16 Apr 2020 at 10:33, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> > > what I know, pgbench cannot be used for testing spinlocks problems.
> > >
> > > Maybe you can see this issue when a) use higher number clients - hundreds, thousands. Decrease share memory, so there will be press on related spin lock.
> >
> > There really aren't many spinlocks left that could be tickled by a
> > normal workload. I looked for a way to trigger spinlock contention
> > when I prototyped a patch to replace spinlocks with futexes. The only
> > one that I could figure out a way to make contended was the lock
> > protecting parallel btree scan. A highly parallel index only scan on a
> > fully cached index should create at least some spinlock contention.
>
> I suspect the snapshot-too-old "mutex_threshold" spinlock can become
> contended under workloads that generate a high rate of
> heap_page_prune_opt() calls with old_snapshot_threshold enabled. One
> way to do that is with a bunch of concurrent index scans that hit the
> heap in random order. Some notes about that:
>
> https://www.postgresql.org/message-id/flat/CA%2BhUKGKT8oTkp5jw_U4p0S-7UG9zsvtw_M47Y285bER6a2gD%2Bg%40mail.gmail.com

Thanks all for the inputs. Will keep these two particular scenarios in
mind, and try to get some bandwidth on this soon.

--
Thanks,
-Amit Khandekar
Huawei Technologies