Basic premise - if "log file sync" timing isn't a reasonably close match for
the "log file parallel write" timing then a prime suspect is CPU starvation. A
secondary issue may be a large number of processes doing small amounts of work
an committing, leading to a storm of processes restarting at the log writer
clears them for action and is forced off the CPU as they all come on to the CPU
(the failure to get PX processes on remote nodes to query gv$ views is an
indicator that you're hitting your limit on processes which may be relevant to
the log file sync problem).
Nenad's note on controlfile writes and log file switches is also relevant, of
course - so checking log file sizing and completion time would be sensible, and
cross-checking with v$sys_metric_history (if you get within the hour that it
keeps) might give you some confirmation that the problem is highly localised in
time.
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Chris Stephens <cstephens16@xxxxxxxxx>
Sent: 28 January 2020 18:59
To: Noveljic Nenad
Cc: oracle-l
Subject: Re: intermittent long "log file sync" waits
probably just 19c RAC
SQL> @ash_wait_chains event2 session_id=1713 "TIMESTAMP'2020-01-27 15:50:48'"
"TIMESTAMP'2020-01-27 15:52:40'"
-- Display ASH Wait Chain Signatures script v0.2 BETA by Tanel Poder (
http://blog.tanelpoder.com ;)
, TO_CHAR(CASE WHEN session_state = 'WAITING' THEN p1 ELSE null END,
'0XXXXXXXXXXXXXXX') p1hex
*
ERROR at line 14:
ORA-12850: Could not allocate slaves on all specified instances: 3 needed, 1
allocated
SQL>
Session: c2
On Tue, Jan 28, 2020 at 11:30 AM Noveljic Nenad
<nenad.noveljic@xxxxxxxxxxxx<mailto:nenad.noveljic@xxxxxxxxxxxx>> wrote:
The script should work with 19. It must be something else. Could you try with
ash_wait_chains2 - 2 is the rac version? For start, I would group only on
event2 (the first parameter).
If it doesn’t work, please post the error message.
Von: Chris Stephens <cstephens16@xxxxxxxxx<mailto:cstephens16@xxxxxxxxx>>
Datum Dienstag, 28. Jan. 2020, 5:43 PM
An: Noveljic Nenad
<nenad.noveljic@xxxxxxxxxxxx<mailto:nenad.noveljic@xxxxxxxxxxxx>>
Cc: oracle-l <Oracle-L@xxxxxxxxxxxxx<mailto:Oracle-L@xxxxxxxxxxxxx>>
Betreff: Re: intermittent long "log file sync" waits
unfortunately, ash_wait_chains.sql doesn't work on 19.3 but here is ashtop
showing foreground process blocked on lgwr (1710):
SQL> @ashtop inst_id,username,blocking_session,blocking_inst_id,event2 1=1
"TIMESTAMP'2020-01-27 15:50:48'" "TIMESTAMP'2020-01-27 15:52:40'"
TOTALSECONDS AAS %This INST_ID USERNAME
BLOCKING_SESSION BLOCKING_INST_ID EVENT2
FIRST_SEEN LAST_SEEN DIST_SQLEXEC_SEEN
_______________ ______ __________ __________ ________________
___________________ ___________________ _____________________
______________________ ______________________ ____________________
81 0.7 82% | 2 GEN3_MGOWER_3
1710 2 log file sync 2020-01-27 15:50:54
2020-01-27 15:52:14 1
9 0.1 9% | 1 SYS
ON CPU 2020-01-27 15:50:49 2020-01-27
15:52:14 1
3 0 3% | 2 GEN3_MGOWER_3
ON CPU 2020-01-27 15:50:51 2020-01-27
15:52:27 2
2 0 2% | 2 SYS
ON CPU 2020-01-27 15:51:35 2020-01-27
15:51:40 1
1 0 1% | 2 GEN3_MGOWER_3
library cache lock 2020-01-27 15:50:48 2020-01-27
15:50:48 1
1 0 1% | 2 SYS
2321 2 latch free 2020-01-27 15:52:28
2020-01-27 15:52:28 1
1 0 1% | 2 SYS
ges remote message 2020-01-27 15:50:48 2020-01-27
15:50:48 1
1 0 1% | 2 SYS
latch free 2020-01-27 15:52:28 2020-01-27
15:52:28 1
8 rows selected.
SQL> @bg lgwr
NAME DESCRIPTION SID OPID SPID PADDR
SADDR
_______ ______________ _______ _______ ________ ___________________
___________________
LGWR Redo etc. 1710 34 26552 00000001E8718860
00000001D8BFF4A0
On Tue, Jan 28, 2020 at 10:25 AM Noveljic Nenad
<nenad.noveljic@xxxxxxxxxxxx<mailto:nenad.noveljic@xxxxxxxxxxxx>> wrote:
Hi Chris,
log file sync measures much more than IO.
First of all, I’d run Tanel’s ash_wait_chains (
https://github.com/tanelpoder/tpt-oracle/blob/master/ash/ash_wait_chains.sql ;
), because if often points straightly to the root cause .
Here’s is a usage example, when intermittent log file sync waits where caused
by slow control file writes:
https://nenadnoveljic.com/blog/bad-commit-performance-control-file-writes/ ;.
Best regards,
Nenad
From: oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>> On Behalf
Of Chris Stephens
Sent: Dienstag, 28. Januar 2020 17:09
To: oracle-l <Oracle-L@xxxxxxxxxxxxx<mailto:Oracle-L@xxxxxxxxxxxxx>>
Subject: intermittent long "log file sync" waits
3-node Oracle 19.3 RAC
Centos 7
We have a SQLAlchemy/Python based application workload that is running the
exact same steps with widely varying response times which appear to be related
to varying "log file sync" wait times.
Here is a profile of a "fast" run:
CALL-NAME DURATION % CALLS MEAN MIN
MAX
------------------------------ --------- ------ ------ -------- --------
---------
SQL*Net message from client 53.197782 91.8% 10,092 0.005271 0.000177
28.568493
EXEC 3.759177 6.5% 9,816 0.000383 0.000000
0.239592
row cache lock 0.233153 0.4% 541 0.000431 0.000113
0.000941
PARSE 0.140399 0.2% 4,867 0.000029 0.000000
0.006620
DLM cross inst call completion 0.137330 0.2% 956 0.000144 0.000004
0.000505
library cache lock 0.100171 0.2% 215 0.000466 0.000151
0.002133
library cache pin 0.079729 0.1% 216 0.000369 0.000056
0.000710
FETCH 0.058253 0.1% 1,062 0.000055 0.000000
0.004148
log file sync 0.048217 0.1% 149 0.000324 0.000259
0.000505
CLOSE 0.045416 0.1% 4,929 0.000009 0.000000
0.000073
20 others 0.135624 0.2% 11,854 0.000011 0.000000
0.001700
------------------------------ --------- ------ ------ -------- --------
---------
TOTAL (30) 57.935251 100.0% 44,697 0.001296 0.000000
28.568493
Here is a profile of a "slow" run:
CALL-NAME DURATION % CALLS MEAN MIN
MAX
------------------------------ ---------- ------ ------ -------- --------
----------
SQL*Net message from client 131.186118 61.0% 10,092 0.012999 0.000212
106.789360
log file sync 79.291166 36.8% 150 0.528608 0.000264
2.986575
EXEC 3.728402 1.7% 9,816 0.000380 0.000000
0.221403
row cache lock 0.248868 0.1% 542 0.000459 0.000111
0.001036
PARSE 0.164267 0.1% 4,867 0.000034 0.000000
0.004652
DLM cross inst call completion 0.146981 0.1% 957 0.000154 0.000005
0.001188
library cache lock 0.104354 0.0% 218 0.000479 0.000160
0.000728
library cache pin 0.082504 0.0% 202 0.000408 0.000157
0.000672
FETCH 0.056687 0.0% 1,062 0.000053 0.000000
0.003969
CLOSE 0.043590 0.0% 4,929 0.000009 0.000000
0.000180
20 others 0.142044 0.1% 11,866 0.000012 0.000000
0.001792
------------------------------ ---------- ------ ------ -------- --------
----------
TOTAL (30) 215.194981 100.0% 44,701 0.004814 0.000000
106.789360
looking at even histogram for that event:
SQL> @evh "log file sync"
EVH_EVENT EVH_WAIT_TIME_MILLI WAIT_COUNT EVH_EST_TIME
LAST_UPDATE_TIME
________________ ______________________ _____________ _______________
______________________________________
log file sync < 1 200051 100.026 27-JAN-20
11.39.57.344734 PM -06:00
log file sync < 2 165 0.248 28-JAN-20
12.18.10.429089 AM -06:00
log file sync < 4 150 0.45 27-JAN-20
11.18.31.158102 PM -06:00
log file sync < 8 199 1.194 27-JAN-20
11.19.14.209947 PM -06:00
log file sync < 16 253 3.036 28-JAN-20
08.03.17.851328 AM -06:00
log file sync < 32 472 11.328 27-JAN-20
11.20.22.746033 PM -06:00
log file sync < 64 728 34.944 28-JAN-20
01.13.37.364541 AM -06:00
log file sync < 128 691 66.336 27-JAN-20
11.31.37.400504 PM -06:00
log file sync < 256 414 79.488 28-JAN-20
12.18.10.423987 AM -06:00
log file sync < 512 405 155.52 28-JAN-20
03.27.50.540383 AM -06:00
log file sync < 1024 459 352.512 27-JAN-20
11.35.14.378363 PM -06:00
log file sync < 2048 482 740.352 28-JAN-20
01.18.20.556248 AM -06:00
log file sync < 4096 576 1769.472 27-JAN-20
11.21.05.084998 PM -06:00
log file sync < 8192 89 546.816 27-JAN-20
11.57.36.436460 AM -06:00
log file sync < 16384 60 737.28 25-JAN-20
07.48.31.460408 AM -06:00
log file sync < 32768 39 958.464 27-JAN-20
11.59.09.869286 AM -06:00
log file sync < 65536 27 1327.104 25-JAN-20
09.49.13.856563 AM -06:00
The weird thing is that I don't see corresponding log I/O waits (awaits) in
iostat output.
I have a ticket open w/ oracle but does anyone have any suggestions to discover
root cause and/or solution?
____________________________________________________
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