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From: shveta malik <[email protected]>
To: SATYANARAYANA NARLAPURAM <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Amit Kapila <[email protected]>
Cc: shveta malik <[email protected]>
Subject: Re: [Patch]: Fix excessive ProcArrayLock acquisitions with subscription max_retention_duration=0
Date: Mon, 27 Apr 2026 15:18:23 +0530
Message-ID: <CAJpy0uDGM8eVqMORHKw61mm24AjH7jPZAhUaN3YeJrZ1bdpQFQ@mail.gmail.com> (raw)
In-Reply-To: <CAHg+QDdKVnCLHot=AcoPpEiSyDzGz7wGYjAFHVOw57oDtmUDWQ@mail.gmail.com>
References: <CAHg+QDdKVnCLHot=AcoPpEiSyDzGz7wGYjAFHVOw57oDtmUDWQ@mail.gmail.com>

On Mon, Apr 27, 2026 at 2:11 PM SATYANARAYANA NARLAPURAM
<[email protected]> 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
Shveta

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