Re: [Patch]: Fix excessive ProcArrayLock acquisitions with subscription max_retention_duration=0

On Tue, Apr 28, 2026 at 9:38 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Mon, Apr 27, 2026 at 10:32 PM SATYANARAYANA NARLAPURAM
> <satyanarlapuram(at)gmail(dot)com> wrote:
> >
> > Hi,
> >
> > On Mon, Apr 27, 2026 at 2:48 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >>
> >> On Mon, Apr 27, 2026 at 2:11 PM SATYANARAYANA NARLAPURAM
> >> <satyanarlapuram(at)gmail(dot)com> wrote:
> >> >
> >> > Hi Hackers,
> >> >
> >> > When a subscription has retain_dead_tuples enabled with maxretention set
> >> > to zero (unlimited retention), adjust_xid_advance_interval() caps
> >> > xid_advance_interval to Min(interval, maxretention). Since maxretention
> >> > is zero, this always collapses the interval to zero milliseconds.
> >> >
> >> > A zero makes TimestampDifferenceExceeds(last_time, now, 0) always
> >> > true in get_candidate_xid(). This causes the apply worker to call
> >> > GetOldestActiveTransactionId() on every single WAL message. This results in
> >> > a huge number of ProcArrayLock acquisitions under moderate write load.
> >> >
> >> > Fix by adding a maxretention > 0 guard to the cap. When maxretention is zero ,
> >> > the exponential back-off in adjust_xid_advance_interval()
> >> > now works correctly, growing the interval from 100 ms toward the 180 s
> >> > ceiling.
> >> >
> >> > Measured with perf uprobe counting GetOldestActiveTransactionId calls
> >> > at ~39K TPS (pgbench, 5 clients):
> >> >
> >> > Before fix: 25,104 calls / 5 s (~5,021/s)
> >> > After fix: 31 calls / 5 s (~6/s)
> >> >
> >>
> >> Thanks for reporting it. I am reveiwing the problem sattement.
> >> Meanwhile can you please look at it, I am getting the following error
> >> while applying the patch on my Ubuntu setup (git am):
> >>
> >> error: corrupt patch at line 22
> >
> >
> > Thanks! Please find the updated v2 patch.
>
> Thanks. The patch looks good.
>

LGTM as well, so pushed.

--
With Regards,
Amit Kapila.