Hi, hackers
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)
The problem is not reproducible on PostgreSQL 16.9
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
