On Mon, Apr 27, 2026 at 3:18 PM shveta malik <[email protected]> wrote: > > 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. > >
I agree with the problem statement. I can see it in my debugging. > > 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. Yes, this should work. Let's see what others have to say on this. > > 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) > > Just curious, how did you catch this problem? Did it show up in any of your profiling reports? thanks Shveta
