IPC/MultixactCreation on the Standby server

<div><div style="background-color:#ffffff;color:#080808"><pre style="font-family:'jetbrains mono' , monospace;font-size:12pt">Hi,

The problem is as follows.
A replication cluster includes a primary server and one hot-standby replica.
The workload on the primary server is represented by multiple requests generating multixact IDs, while the hot-standby replica performs reading requests.

After some time, all requests on the hot-standby are stuck and never get finished.

The `pg_stat_activity` view on the replica reports that processes are stuck waiting for IPC/MultixactCreation,
pg_cancel_backend and pg_terminate_backend cannot cancel the request, SIGQUIT is the only way to stop it.

We tried:
* changing the `autovacuum_multixact_freeze_max_age` parameters,
* increasing `multixact_member_buffers` and `multixact_offset_buffers`,
* disabling `hot_standby_feedback`,
* switching the replica to synchronous and asynchronous mode,
* and much more.
But nothing helped.

We ran the replica in recovery mode from WAL archive, i.e. as warm-standby, the result is the same.

We tried to build from the sources based on REL_17_5 branch with the default configure settings
   ./configure
   make
   make install
But got no luck.

Here is an example with a synthetic workload reproducing the problem.

Test system
===========

-   Architecture: x86_64
-   OS: Ubuntu 24.04.2 LTS (Noble Numbat)
-   Tested postgres version(s):
   -   latest 17 (17.5)
   -   latest 18 (18-beta1)

Steps to reproduce
==================

   postgres=# create table tbl (
       id int primary key,
       val int
   );
   postgres=# insert into tbl select i, 0 from generate_series(1,5) i;

The first and second scripts execute queries on the master server
-----------------------------------------------------------------

   pgbench --no-vacuum --report-per-command -M prepared -c 200 -j 200 -T 300 -P 1 --file=/dev/stdin &lt;&lt;'EOF'
   \set id random(1, 5)
   begin;
   select * from tbl where id = :id for key share;
   commit;
   EOF

   pgbench --no-vacuum --report-per-command -M prepared -c 100 -j 100 -T 300 -P 1 --file=/dev/stdin &lt;&lt;'EOF'
   \set id random(1, 5)
   begin;
   update tbl set val = val+1 where id = :id;
   \sleep 10 ms
   commit;
   EOF

The following script is executed on the replica
-----------------------------------------------

   pgbench --no-vacuum --report-per-command -M prepared -c 100 -j 100 -T 300 -P 1 --file=/dev/stdin &lt;&lt;'EOF'
   begin;
   select sum(val) from tbl;
   \sleep 10 ms
   select sum(val) from tbl;
   \sleep 10 ms
   commit;
   EOF

   pgbench (17.5 (Ubuntu 17.5-1.pgdg24.04+1))
   progress: 1.0 s, 2606.8 tps, lat 33.588 ms stddev 13.316, 0 failed
   progress: 2.0 s, 3315.0 tps, lat 30.174 ms stddev 5.933, 0 failed
   progress: 3.0 s, 3357.0 tps, lat 29.699 ms stddev 5.541, 0 failed
   progress: 4.0 s, 3350.0 tps, lat 29.911 ms stddev 5.311, 0 failed
   progress: 5.0 s, 3206.0 tps, lat 30.999 ms stddev 6.343, 0 failed
   progress: 6.0 s, 3264.0 tps, lat 30.828 ms stddev 6.389, 0 failed
   progress: 7.0 s, 3224.0 tps, lat 31.099 ms stddev 6.197, 0 failed
   progress: 8.0 s, 3168.0 tps, lat 31.486 ms stddev 6.940, 0 failed
   progress: 9.0 s, 3118.0 tps, lat 32.004 ms stddev 6.546, 0 failed
   progress: 10.0 s, 3017.0 tps, lat 33.183 ms stddev 7.971, 0 failed
   progress: 11.0 s, 3157.0 tps, lat 31.697 ms stddev 6.624, 0 failed
   progress: 12.0 s, 3180.0 tps, lat 31.415 ms stddev 6.310, 0 failed
   progress: 13.0 s, 3150.9 tps, lat 31.591 ms stddev 6.280, 0 failed
   progress: 14.0 s, 3329.0 tps, lat 30.189 ms stddev 5.792, 0 failed
   progress: 15.0 s, 3233.6 tps, lat 30.852 ms stddev 5.723, 0 failed
   progress: 16.0 s, 3185.4 tps, lat 31.378 ms stddev 6.383, 0 failed
   progress: 17.0 s, 3035.0 tps, lat 32.920 ms stddev 7.390, 0 failed
   progress: 18.0 s, 3173.0 tps, lat 31.547 ms stddev 6.390, 0 failed
   progress: 19.0 s, 3077.0 tps, lat 32.427 ms stddev 6.634, 0 failed
   progress: 20.0 s, 3266.1 tps, lat 30.740 ms stddev 5.842, 0 failed
   progress: 21.0 s, 2990.9 tps, lat 33.353 ms stddev 7.019, 0 failed
   progress: 22.0 s, 3048.1 tps, lat 32.933 ms stddev 6.951, 0 failed
   progress: 23.0 s, 3148.0 tps, lat 31.769 ms stddev 6.077, 0 failed
   progress: 24.0 s, 1523.2 tps, lat 30.029 ms stddev 5.093, 0 failed
   progress: 25.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 26.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 27.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 28.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 29.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 30.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 31.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 32.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 33.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 34.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed
   progress: 35.0 s, 0.0 tps, lat 0.000 ms stddev 0.000, 0 failed

After some time, all requests on the replica hang waiting for IPC/MultixactCreation.

Output from `pg_stat_activity`
------------------------------

           backend_type        | state  | wait_event_type |    wait_event     |                  query
   ----------------------------+--------+-----------------+-------------------+------------------------------------------
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
   ...
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    client backend             | active | IPC             | MultixactCreation | select sum(val) from tbl;
    startup                    |        | LWLock          | BufferContent     |
    checkpointer               |        | Activity        | CheckpointerMain  |
    background writer          |        | Activity        | BgwriterHibernate |
    walreceiver                |        | Activity        | WalReceiverMain   |

gdb session for `client backend` process
----------------------------------------

   (gdb) bt
   #0  0x00007f0e9872a007 in epoll_wait (epfd=5, events=0x57c4747fc458, maxevents=1, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
   #1  0x000057c440685033 in WaitEventSetWaitBlock (nevents=&lt;optimized out&gt;, occurred_events=0x7ffdaedc8360, cur_timeout=-1, set=0x57c4747fc3f0)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/ipc/latch.c:1577
   #2  WaitEventSetWait (set=0x57c4747fc3f0, timeout=timeout(at)entry=-1, occurred_events=occurred_events(at)entry=0x7ffdaedc8360, nevents=nevents(at)entry=1, wait_event_info=wait_event_info(at)entry=134217765)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/ipc/latch.c:1525
   #3  0x000057c44068541c in WaitLatch (latch=&lt;optimized out&gt;, wakeEvents=&lt;optimized out&gt;, timeout=&lt;optimized out&gt;, wait_event_info=134217765)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/ipc/latch.c:538
   #4  0x000057c44068d8c0 in ConditionVariableTimedSleep (cv=0x7f0cefc50ab0, timeout=-1, wait_event_info=134217765)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/lmgr/condition_variable.c:163
   #5  0x000057c440365a0c in ConditionVariableSleep (wait_event_info=134217765, cv=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/lmgr/condition_variable.c:98
   #6  GetMultiXactIdMembers (multi=45559845, members=0x7ffdaedc84b0, from_pgupgrade=&lt;optimized out&gt;, isLockOnly=&lt;optimized out&gt;)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/transam/multixact.c:1483
   #7  0x000057c4408adc6b in MultiXactIdGetUpdateXid.isra.0 (xmax=xmax(at)entry=45559845, t_infomask=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:7478
   #8  0x000057c44031ecfa in HeapTupleGetUpdateXid (tuple=&lt;error reading variable: Cannot access memory at address 0x0&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:7519
   #9  HeapTupleSatisfiesMVCC (htup=&lt;optimized out&gt;, buffer=404, snapshot=0x57c474892ff0) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam_visibility.c:1090
   #10 HeapTupleSatisfiesVisibility (htup=&lt;optimized out&gt;, snapshot=0x57c474892ff0, buffer=404) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam_visibility.c:1772
   #11 0x000057c44030c1cb in page_collect_tuples (check_serializable=&lt;optimized out&gt;, all_visible=&lt;optimized out&gt;, lines=&lt;optimized out&gt;, block=&lt;optimized out&gt;, buffer=&lt;optimized out&gt;, page=&lt;optimized out&gt;,
       snapshot=&lt;optimized out&gt;, scan=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:480
   #12 heap_prepare_pagescan (sscan=0x57c47495b970) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:579
   #13 0x000057c44030cb59 in heapgettup_pagemode (scan=scan(at)entry=0x57c47495b970, dir=&lt;optimized out&gt;, nkeys=&lt;optimized out&gt;, key=&lt;optimized out&gt;)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:999
   #14 0x000057c44030d1bd in heap_getnextslot (sscan=0x57c47495b970, direction=&lt;optimized out&gt;, slot=0x57c47494b278) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:1319
   #15 0x000057c4404f090a in table_scan_getnextslot (slot=0x57c47494b278, direction=ForwardScanDirection, sscan=&lt;optimized out&gt;)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/include/access/tableam.h:1072
   #16 SeqNext (node=0x57c47494b0e8) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/nodeSeqscan.c:80
   #17 0x000057c4404d5cfc in ExecProcNode (node=0x57c47494b0e8) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/include/executor/executor.h:274
   #18 fetch_input_tuple (aggstate=aggstate(at)entry=0x57c47494aaf0) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/nodeAgg.c:561
   #19 0x000057c4404d848a in agg_retrieve_direct (aggstate=0x57c47494aaf0) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/nodeAgg.c:2459
   #20 ExecAgg (pstate=0x57c47494aaf0) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/nodeAgg.c:2179
   #21 0x000057c4404c2003 in ExecProcNode (node=0x57c47494aaf0) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/include/executor/executor.h:274
   #22 ExecutePlan (dest=0x57c47483d548, direction=&lt;optimized out&gt;, numberTuples=0, sendTuples=true, operation=CMD_SELECT, queryDesc=0x57c474895010)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/execMain.c:1649
   #23 standard_ExecutorRun (queryDesc=0x57c474895010, direction=&lt;optimized out&gt;, count=0, execute_once=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/executor/execMain.c:361
   ...

gdb session for `startup` process
---------------------------------

   (gdb) bt
   #0  0x00007f0e98698ce3 in __futex_abstimed_wait_common64 (private=&lt;optimized out&gt;, cancel=true, abstime=0x0, op=265, expected=0, futex_word=0x7f0ceb34e6b8) at ./nptl/futex-internal.c:57
   #1  __futex_abstimed_wait_common (cancel=true, private=&lt;optimized out&gt;, abstime=0x0, clockid=0, expected=0, futex_word=0x7f0ceb34e6b8) at ./nptl/futex-internal.c:87
   #2  __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word(at)entry=0x7f0ceb34e6b8, expected=expected(at)entry=0, clockid=clockid(at)entry=0, abstime=abstime(at)entry=0x0, private=&lt;optimized out&gt;)
       at ./nptl/futex-internal.c:139
   #3  0x00007f0e986a4f1f in do_futex_wait (sem=sem(at)entry=0x7f0ceb34e6b8, abstime=0x0, clockid=0) at ./nptl/sem_waitcommon.c:111
   #4  0x00007f0e986a4fb8 in __new_sem_wait_slow64 (sem=sem(at)entry=0x7f0ceb34e6b8, abstime=0x0, clockid=0) at ./nptl/sem_waitcommon.c:183
   #5  0x00007f0e986a503d in __new_sem_wait (sem=sem(at)entry=0x7f0ceb34e6b8) at ./nptl/sem_wait.c:42
   #6  0x000057c440696166 in PGSemaphoreLock (sema=0x7f0ceb34e6b8) at port/pg_sema.c:327
   #7  LWLockAcquire (lock=0x7f0cefc58064, mode=LW_EXCLUSIVE) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/lmgr/lwlock.c:1289
   #8  0x000057c44038f96a in LockBuffer (mode=2, buffer=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/buffer/bufmgr.c:5147
   #9  XLogReadBufferForRedoExtended (record=&lt;optimized out&gt;, block_id=&lt;optimized out&gt;, mode=RBM_NORMAL, get_cleanup_lock=false, buf=0x7ffdaedc8b4c)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/transam/xlogutils.c:429
   #10 0x000057c440319969 in XLogReadBufferForRedo (buf=0x7ffdaedc8b4c, block_id=0 '\000', record=0x57c4748994d8) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/transam/xlogutils.c:317
   #11 heap_xlog_lock_updated (record=0x57c4748994d8) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:10230
   #12 heap2_redo (record=0x57c4748994d8) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:10362
   #13 0x000057c44038e1d2 in ApplyWalRecord (replayTLI=&lt;synthetic pointer&gt;, record=0x7f0e983908e0, xlogreader=&lt;optimized out&gt;)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/include/access/xlog_internal.h:380
   #14 PerformWalRecovery () at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/transam/xlogrecovery.c:1822
   #15 0x000057c44037bbf6 in StartupXLOG () at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/transam/xlog.c:5821
   #16 0x000057c4406155ed in StartupProcessMain (startup_data=&lt;optimized out&gt;, startup_data_len=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/startup.c:258
   #17 0x000057c44060b376 in postmaster_child_launch (child_type=B_STARTUP, startup_data=0x0, startup_data_len=0, client_sock=0x0)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/launch_backend.c:277
   #18 0x000057c440614509 in postmaster_child_launch (client_sock=0x0, startup_data_len=0, startup_data=0x0, child_type=B_STARTUP)
       at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:3934
   #19 StartChildProcess (type=type(at)entry=B_STARTUP) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:3930
   #20 0x000057c44061480d in PostmasterStateMachine () at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:3392
   #21 0x000057c4408a3455 in process_pm_child_exit () at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:2683
   #22 ServerLoop.isra.0 () at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:1667
   #23 0x000057c440616965 in PostmasterMain (argc=&lt;optimized out&gt;, argv=&lt;optimized out&gt;) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/postmaster/postmaster.c:1374
   #24 0x000057c4402bcd2d in main (argc=17, argv=0x57c4747fb140) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/main/main.c:199

Could you please help me to fix the problem of stuck 'client backend' processes?

I kindly ask you for any ideas and recommendations!

Best regards,
Dmitry
</pre><div> </div></div></div>