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1. TruncateMultiXact() performs the SLRU truncations in a critical
section. Deleting the SLRU segments calls ForwardSyncRequest(), which
will try to compact the request queue if it's full
(CompactCheckpointerRequestQueue()). That in turn allocates memory,
which is not allowed in a critical section. Backtrace:
TRAP: failed Assert("CritSectionCount == 0 || (context)->allowInCritSection"), File: "../src/backend/utils/mmgr/mcxt.c", Line: 1353, PID: 920981
postgres: autovacuum worker template0(ExceptionalCondition+0x6e)[0x560a501e866e]
postgres: autovacuum worker template0(+0x5dce3d)[0x560a50217e3d]
postgres: autovacuum worker template0(ForwardSyncRequest+0x8e)[0x560a4ffec95e]
postgres: autovacuum worker template0(RegisterSyncRequest+0x2b)[0x560a50091eeb]
postgres: autovacuum worker template0(+0x187b0a)[0x560a4fdc2b0a]
postgres: autovacuum worker template0(SlruDeleteSegment+0x101)[0x560a4fdc2ab1]
postgres: autovacuum worker template0(TruncateMultiXact+0x2fb)[0x560a4fdbde1b]
postgres: autovacuum worker template0(vac_update_datfrozenxid+0x4b3)[0x560a4febd2f3]
postgres: autovacuum worker template0(+0x3adf66)[0x560a4ffe8f66]
postgres: autovacuum worker template0(AutoVacWorkerMain+0x3ed)[0x560a4ffe7c2d]
postgres: autovacuum worker template0(+0x3b1ead)[0x560a4ffecead]
postgres: autovacuum worker template0(+0x3b620e)[0x560a4fff120e]
postgres: autovacuum worker template0(+0x3b3fbb)[0x560a4ffeefbb]
postgres: autovacuum worker template0(+0x2f724e)[0x560a4ff3224e]
/lib/x86_64-linux-gnu/libc.so.6(+0x27c8a)[0x7f62cc642c8a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f62cc642d45]
postgres: autovacuum worker template0(_start+0x21)[0x560a4fd16f31]
To fix, bail out in CompactCheckpointerRequestQueue() without doing
anything, if it's called in a critical section. That covers the above
call path, as well as any other similar cases where
RegisterSyncRequest might be called in a critical section.
2. After fixing that, another problem became apparent: Autovacuum
process doing that truncation can deadlock with the checkpointer
process. TruncateMultiXact() sets "MyProc->delayChkptFlags |=
DELAY_CHKPT_START". If the sync request queue is full and cannot be
compacted, the process will repeatedly sleep and retry, until there is
room in the queue. However, if the checkpointer is trying to start a
checkpoint at the same time, and is waiting for the DELAY_CHKPT_START
processes to finish, the queue will never shrink.
More concretely, the autovacuum process is stuck here:
#0 0x00007fc934926dc3 in epoll_wait () from /lib/x86_64-linux-gnu/libc.so.6
#1 0x000056220b24348b in WaitEventSetWaitBlock (set=0x56220c2e4b50, occurred_events=0x7ffe7856d040, nevents=1, cur_timeout=<optimized out>) at ../src/backend/storage/ipc/latch.c:1570
#2 WaitEventSetWait (set=0x56220c2e4b50, timeout=timeout@entry=10, occurred_events=<optimized out>, occurred_events@entry=0x7ffe7856d040, nevents=nevents@entry=1,
wait_event_info=wait_event_info@entry=150994949) at ../src/backend/storage/ipc/latch.c:1516
#3 0x000056220b243224 in WaitLatch (latch=<optimized out>, latch@entry=0x0, wakeEvents=wakeEvents@entry=40, timeout=timeout@entry=10, wait_event_info=wait_event_info@entry=150994949)
at ../src/backend/storage/ipc/latch.c:538
#4 0x000056220b26cf46 in RegisterSyncRequest (ftag=ftag@entry=0x7ffe7856d0a0, type=type@entry=SYNC_FORGET_REQUEST, retryOnError=true) at ../src/backend/storage/sync/sync.c:614
#5 0x000056220af9db0a in SlruInternalDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1495
#6 0x000056220af9dab1 in SlruDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1566
#7 0x000056220af98e1b in PerformMembersTruncation (oldestOffset=<optimized out>, newOldestOffset=<optimized out>) at ../src/backend/access/transam/multixact.c:3006
#8 TruncateMultiXact (newOldestMulti=newOldestMulti@entry=3221225472, newOldestMultiDB=newOldestMultiDB@entry=4) at ../src/backend/access/transam/multixact.c:3201
#9 0x000056220b098303 in vac_truncate_clog (frozenXID=749, minMulti=<optimized out>, lastSaneFrozenXid=749, lastSaneMinMulti=3221225472) at ../src/backend/commands/vacuum.c:1917
#10 vac_update_datfrozenxid () at ../src/backend/commands/vacuum.c:1760
#11 0x000056220b1c3f76 in do_autovacuum () at ../src/backend/postmaster/autovacuum.c:2550
#12 0x000056220b1c2c3d in AutoVacWorkerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/autovacuum.c:1569
and the checkpointer is stuck here:
#0 0x00007fc9348ebf93 in clock_nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007fc9348fe353 in nanosleep () from /lib/x86_64-linux-gnu/libc.so.6
#2 0x000056220b40ecb4 in pg_usleep (microsec=microsec@entry=10000) at ../src/port/pgsleep.c:50
#3 0x000056220afb43c3 in CreateCheckPoint (flags=flags@entry=108) at ../src/backend/access/transam/xlog.c:7098
#4 0x000056220b1c6e86 in CheckpointerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/checkpointer.c:464
To fix, add AbsorbSyncRequests() to the loops where the checkpointer
waits for DELAY_CHKPT_START or DELAY_CHKPT_COMPLETE operations to
finish.
Backpatch to v14. Before that, SLRU deletion didn't call
RegisterSyncRequest, which avoided this failure. I'm not sure if there
are other similar scenarios on older versions, but we haven't had
any such reports.
Discussion: https://www.postgresql.org/message-id/ccc66933-31c1-4f6a-bf4b-45fef0d4f22e@iki.fi
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.