Thread: Re: IPC/MultixactCreation on the Standby server
On 23.06.2025 16:33, Dmitry wrote:
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 <<'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 <<'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 <<'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=<optimized out>, 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@entry=-1, occurred_events=occurred_events@entry=0x7ffdaedc8360, nevents=nevents@entry=1, wait_event_info=wait_event_info@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=<optimized out>, wakeEvents=<optimized out>, timeout=<optimized out>, 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=<optimized out>) 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=<optimized out>, isLockOnly=<optimized out>) 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@entry=45559845, t_infomask=<optimized out>) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:7478 #8 0x000057c44031ecfa in HeapTupleGetUpdateXid (tuple=<error reading variable: Cannot access memory at address 0x0>) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/access/heap/heapam.c:7519 #9 HeapTupleSatisfiesMVCC (htup=<optimized out>, 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=<optimized out>, 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=<optimized out>, all_visible=<optimized out>, lines=<optimized out>, block=<optimized out>, buffer=<optimized out>, page=<optimized out>, snapshot=<optimized out>, scan=<optimized out>) 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@entry=0x57c47495b970, dir=<optimized out>, nkeys=<optimized out>, key=<optimized out>) 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=<optimized out>, 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=<optimized out>) 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@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=<optimized out>, 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=<optimized out>, count=0, execute_once=<optimized out>) 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=<optimized out>, 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=<optimized out>, 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@entry=0x7f0ceb34e6b8, expected=expected@entry=0, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=<optimized out>) at ./nptl/futex-internal.c:139 #3 0x00007f0e986a4f1f in do_futex_wait (sem=sem@entry=0x7f0ceb34e6b8, abstime=0x0, clockid=0) at ./nptl/sem_waitcommon.c:111 #4 0x00007f0e986a4fb8 in __new_sem_wait_slow64 (sem=sem@entry=0x7f0ceb34e6b8, abstime=0x0, clockid=0) at ./nptl/sem_waitcommon.c:183 #5 0x00007f0e986a503d in __new_sem_wait (sem=sem@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=<optimized out>) at /usr/src/postgresql-17-17.5-1.pgdg24.04+1/build/../src/backend/storage/buffer/bufmgr.c:5147 #9 XLogReadBufferForRedoExtended (record=<optimized out>, block_id=<optimized out>, 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=<synthetic pointer>, record=0x7f0e983908e0, xlogreader=<optimized out>) 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=<optimized out>, startup_data_len=<optimized out>) 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@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=<optimized out>, argv=<optimized out>) 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
A small addition to the description of the problem:
- the problem is not reproducible on the 16th version of PostgreSQL
Best regards,
Dmitry