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From: shveta malik <shveta.malik@gmail.com>
Date: Tue, 28 Apr 2026 09:38:29 +0530
Message-ID: 
 <CAJpy0uD8nmRCbhH1KBnb9Ch9SyxZh4OdeBaShBOkPKJM5A8Ujg@mail.gmail.com>
Subject: Re: [Patch]: Fix excessive ProcArrayLock acquisitions with
 subscription max_retention_duration=0
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
	Amit Kapila <amit.kapila16@gmail.com>, shveta malik <shveta.malik@gmail.com>
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On Mon, Apr 27, 2026 at 10:32=E2=80=AFPM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Hi,
>
> On Mon, Apr 27, 2026 at 2:48=E2=80=AFAM shveta malik <shveta.malik@gmail.=
com> wrote:
>>
>> On Mon, Apr 27, 2026 at 2:11=E2=80=AFPM SATYANARAYANA NARLAPURAM
>> <satyanarlapuram@gmail.com> wrote:
>> >
>> > Hi Hackers,
>> >
>> > When a subscription has retain_dead_tuples enabled with maxretention s=
et
>> > to zero (unlimited retention), adjust_xid_advance_interval() caps
>> > xid_advance_interval to Min(interval, maxretention).  Since maxretenti=
on
>> > 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 resul=
ts in
>> > a huge number of ProcArrayLock acquisitions under moderate write load.
>> >
>> > Fix by adding a maxretention > 0 guard to the cap. When maxretention i=
s 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