Re: [PATCH] Identify LWLocks in tracepoints

On Sat, 19 Dec 2020 at 13:00, Craig Ringer <craig(dot)ringer(at)enterprisedb(dot)com>
wrote:

> Hi all
>
> The attached patch set follows on from the discussion in [1] "Add LWLock
> blocker(s) information" by adding the actual LWLock* and the numeric
> tranche ID to each LWLock related TRACE_POSTGRESQL_foo tracepoint.
>
>
I've attached a systemtap script that makes use of the information exported
by the enhanced LWLock tracepoints. It offers something akin to dynamic
-DLWLOCK_STATS with automatic statistical aggregation and some selective
LWLOCK_DEBUG output.

The script isn't super pretty. I didn't try to separate event-data
collection from results output, and there's some duplication in places. But
it gives you an idea what's possible when we report lock pointers and
tranche IDs to tracepoints and add entry/exit tracepoints.

Key features:

* Collect statistical aggregates on lwlock hold and wait durations across
all processes. Stats are grouped by lockmode (shared or exclusive) and by
tranche name, as well as rollup stats across all tranches.
* Report lock wait and hold durations for each process when that process
exits. Again, reported by mode and tranche name.
* For long lock waits, print the waiter pid and waiting pid, along with
each process's backend type and application_name if known, the acquire
mode, and the acquire function

The output is intended to be human-readable, but it'd be quite simple to
convert it into raw tsv-style output suitable for ingestion into
statistical postprocessing or graphing tools.

It should be fairly easy to break down the stats by acquire function if
desired, so LWLockAcquire(), LWLockWaitForVar(), and LWLockAcquireOrWait()
are reported separately. They're combined in the current output.

Capturing the current query string is pretty simple if needed, but I didn't
think it was likely to be especially useful.

Sample output for a pg_regress run attached. Abridged version follows. Here
the !!W!! lines are "waited a long time", the !!H!! lines are "held a long
time". Then [pid]:MyBackendType tranche_name wait_time_us (wait_time) in
wait_func (appliation_name) => [blocker_pid] (blocker_application_name) .
If blocker pid wasn't identified it won't be reported - I know how to fix
that and will do so soon.

!!W!! [ 93030]:3 BufferContent 12993 (0m0.012993s) in
lwlock__acquire__start (pg_regress/text)
!!W!! [ 93036]:3 LockManager 14540 (0m0.014540s) in
lwlock__acquire__start (pg_regress/float8) => [ 93045] (pg_regress/regproc)
!!W!! [ 93035]:3 BufferContent 12608 (0m0.012608s) in
lwlock__acquire__start (pg_regress/float4) => [ 93034] (pg_regress/oid)
!!W!! [ 93036]:3 LockManager 10301 (0m0.010301s) in
lwlock__acquire__start (pg_regress/float8)
!!W!! [ 93043]:3 LockManager 10356 (0m0.010356s) in
lwlock__acquire__start (pg_regress/pg_lsn)
!!H!! [ 93033]:3 BufferContent 20579 (0m0.020579s)
(pg_regress/int8)
!!W!! [ 93027]:3 BufferContent 10766 (0m0.010766s) in
lwlock__acquire__start (pg_regress/char) => [ 93037] (pg_regress/bit)
!!W!! [ 93036]:3 OidGen 12876 (0m0.012876s) in
lwlock__acquire__start (pg_regress/float8)
...

Then the summary rollup at the end of the run. This can also be output
periodically during the run. Abbreviated for highlights:

wait locks: all procs tranche mode count
total avg variance min max
W LW_EXCLUSIVE (all) E 54185
14062734 259 1850265 1 44177
W LW_SHARED (all) S 3668
1116022 304 1527261 2 18642

held locks: all procs tranche mode count
total avg variance min max
H LW_EXCLUSIVE (all) E 10438060
153077259 14 37035 1 195043
H LW_SHARED (all) S 14199902
65466934 4 5318 1 44030

all procs by tranche tranche mode count
total avg variance min max
W tranche (all) S 3668
1116022 304 1527261 2 18642
W tranche (all) E 54185
14062734 259 1850265 1 44177
W tranche WALInsert E 9839
2393229 243 1180294 2 14209
W tranche BufferContent E 3012
1726543 573 3869409 2 28186
W tranche BufferContent S 1664
657855 395 2185694 2 18642
W tranche LockFastPath E 28314
6327801 223 1278053 1 26133
W tranche LockFastPath S 87
59401 682 3703217 19 9454
W tranche LockManager E 7223
2764392 382 2514863 2 44177

Hope this is interesting to someone.