Horton

Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Andres Freund <andres@anarazel.de>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, melanieplageman@gmail.com, vignesh21@gmail.com, lukas@fittl.com, alvherre@alvh.no-ip.org, magnus@hagander.net, pgsql-hackers@postgresql.org, thomas.munro@gmail.com, m.sakrejda@gmail.com
Date: 2023-02-22T04:09:37Z
Lists: pgsql-hackers 
On Tue, Feb 21, 2023 at 07:50:35PM -0600, Justin Pryzby wrote:
> On Sat, Feb 11, 2023 at 10:24:37AM -0800, Andres Freund wrote:
> > On 2023-02-08 21:03:19 -0800, Andres Freund wrote:
> > > Pushed the first (and biggest) commit. More tomorrow.
> > 
> > Just pushed the actual pg_stat_io view, the splitting of the tablespace test,
> > and the pg_stat_io tests.
> 
> pg_stat_io says:
> 
>          * Some BackendTypes do not currently perform any IO in certain
>          * IOContexts, and, while it may not be inherently incorrect for them to
>          * do so, excluding those rows from the view makes the view easier to use.
> 
>         if (bktype == B_AUTOVAC_LAUNCHER && io_context == IOCONTEXT_VACUUM)
>                 return false;
> 
>         if ((bktype == B_AUTOVAC_WORKER || bktype == B_AUTOVAC_LAUNCHER) &&
>                 io_context == IOCONTEXT_BULKWRITE)
>                 return false;
> 
> What about these combinations?  Aren't these also "can't happen" ?
> 
>  relation      | bulkread   | autovacuum worker
>  relation      | bulkread   | autovacuum launcher
>  relation      | vacuum     | startup

Nevermind - at least these are possible.

(gdb) p MyBackendType
$1 = B_AUTOVAC_WORKER
(gdb) p io_object
$2 = IOOBJECT_RELATION
(gdb) p io_context
$3 = IOCONTEXT_BULKREAD
(gdb) p io_op
$4 = IOOP_EVICT
(gdb) bt
...
#9  0x0000557b2f6097a3 in ReadBufferExtended (reln=0x7ff5ccee36b8, forkNum=forkNum@entry=MAIN_FORKNUM, blockNum=blockNum@entry=16, mode=mode@entry=RBM_NORMAL, strategy=0x557b305fb568) at ../src/include/utils/rel.h:573
#10 0x0000557b2f3057c0 in heapgetpage (sscan=sscan@entry=0x557b305fb158, block=block@entry=16) at ../src/backend/access/heap/heapam.c:405
#11 0x0000557b2f305d6c in heapgettup_pagemode (scan=scan@entry=0x557b305fb158, dir=dir@entry=ForwardScanDirection, nkeys=0, key=0x0) at ../src/backend/access/heap/heapam.c:885
#12 0x0000557b2f306956 in heap_getnext (sscan=sscan@entry=0x557b305fb158, direction=direction@entry=ForwardScanDirection) at ../src/backend/access/heap/heapam.c:1122
#13 0x0000557b2f59be0c in do_autovacuum () at ../src/backend/postmaster/autovacuum.c:2061
#14 0x0000557b2f59ccf7 in AutoVacWorkerMain (argc=argc@entry=0, argv=argv@entry=0x0) at ../src/backend/postmaster/autovacuum.c:1716
#15 0x0000557b2f59cdd8 in StartAutoVacWorker () at ../src/backend/postmaster/autovacuum.c:1494
#16 0x0000557b2f5a561a in StartAutovacuumWorker () at ../src/backend/postmaster/postmaster.c:5481
#17 0x0000557b2f5a5a39 in process_pm_pmsignal () at ../src/backend/postmaster/postmaster.c:5192
#18 0x0000557b2f5a5d7e in ServerLoop () at ../src/backend/postmaster/postmaster.c:1770
#19 0x0000557b2f5a73da in PostmasterMain (argc=9, argv=<optimized out>) at ../src/backend/postmaster/postmaster.c:1463
#20 0x0000557b2f4dfc39 in main (argc=9, argv=0x557b30568f50) at ../src/backend/main/main.c:200

-- 
Justin

Commits

  1. Stabilize pg_stat_io writes test

  2. Fix flakey pg_stat_io test

  3. Suppress more compiler warnings in new pgstats code.

  4. Suppress compiler warnings in new pgstats code.

  5. Add tests for pg_stat_io

  6. Create regress_tblspc in test_setup

  7. Add pg_stat_io view, providing more detailed IO statistics

  8. pgstat: Track more detailed relation IO statistics

  9. pgstat: Infrastructure for more detailed IO statistics

  10. doc: Fix some issues in logical replication section

  11. Manual cleanup and pgindent of pgstat and bufmgr related code

  12. Have the planner consider Incremental Sort for DISTINCT

  13. Use actual backend IDs in pg_stat_get_backend_idset() and friends.

  14. Remove redundant call to pgstat_report_wal()

  15. Add BackendType for standalone backends

  16. Initialize backend status reporting during bootstrap.