Re: [Patch]: Fix excessive ProcArrayLock acquisitions with subscription max_retention_duration=0 - Mailing list pgsql-hackers

On Mon, Apr 27, 2026 at 10:32 PM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Hi,
>
> On Mon, Apr 27, 2026 at 2:48 AM shveta malik <shveta.malik@gmail.com> wrote:
>>
>> On Mon, Apr 27, 2026 at 2:11 PM SATYANARAYANA NARLAPURAM
>> <satyanarlapuram@gmail.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.

thanks
Shveta