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IDS Forum

Performance issue

Posted By: Marcus Haarmann
Date: Monday, 8 June 2015, at 11:42 a.m.

Hi experts,

we are encountering a performance problem on one of our database systems, which happens periodically,
not limited to a specific high load.
The effect is that all our application servers are blocked with some transactions (which are committing
at this time), until 20-30 seconds later all the threads are unblocked all of a sudden.
(in the same millisecond, spread over all client machines).
The common thing is that all these threads have a connection to a specific database instance and
have all done mostly read-only work (within a XA transaction, which is necessary to support multiple
datasources).
Since this effect is not really happening really regularly, but between 4-8 times spread over the day,
it is not easy to find out what is the main blocker here.

We have now done some measurements in the application to get the correct time when the situation occurs
and executed a script each 10 seconds to get some status information from the common DB instance.
The script put a timestamp in a log and executed onstat -g sql, followed by onstat -g stk

What seems to be interesting:
1) online.log does not show anything when the effect occurs (no checkpoint, no full log, no entry at all)
2) What we found as common output ist the following thread during the blocking period:

Stack for thread: 1101943 sqlexec
base: 0x00000003aff07000
len: 135168
pc: 0x00000000012ce881
tos: 0x00000003aff25190
state: sleeping
vp: 1

0x00000000012ce881 (/opt/informix/bin/oninit) yield_processor_mvp
0x00000000012d76df (/opt/informix/bin/oninit) mt_yield
0x0000000000e975a7 (/opt/informix/bin/oninit) logm_flush
0x0000000000d526ef (/opt/informix/bin/oninit) logflush
0x0000000000d21e55 (/opt/informix/bin/oninit) bfphysflush
0x0000000000d22a37 (/opt/informix/bin/oninit) bfphyslogx
0x0000000000cfe082 (/opt/informix/bin/oninit) btadditem
0x0000000000d00382 (/opt/informix/bin/oninit) rsbtadditem
0x00000000013be6b0 (/opt/informix/bin/oninit) fm_idxinsert
0x000000000139bdea (/opt/informix/bin/oninit) fmwrite
0x0000000000688d82 (/opt/informix/bin/oninit) aud_sqiswrite
0x000000000082a233 (/opt/informix/bin/oninit) chkrowcons
0x00000000007da790 (/opt/informix/bin/oninit) addone
0x00000000007de57a (/opt/informix/bin/oninit) insone_next
0x00000000008399b7 (/opt/informix/bin/oninit) doinsert
0x00000000005af784 (/opt/informix/bin/oninit) excommand
0x00000000009abfd0 (/opt/informix/bin/oninit) sq_execute
0x0000000000a6a3fb (/opt/informix/bin/oninit) sqmain
0x00000000013dbe3b (/opt/informix/bin/oninit) spawn_thread
0x00000000013151c0 (/opt/informix/bin/oninit) th_init_initgls
0x00000000012e1a38 (/opt/informix/bin/oninit) startup

This seems to be an insert, which triggers an index (btree) write, which triggers a physical log flush.
(there have been many inserts between the blocking times, which did not trigger such a flush.
This only occurs sometimes.)

3) also a very high number of threads does the following:

Stack for thread: 905431 sqlexec
base: 0x00000003a7b48000
len: 135168
pc: 0x00000000012ce881
tos: 0x00000003a7b67250
state: cond wait
vp: 8

0x00000000012ce881 (/opt/informix/bin/oninit) yield_processor_mvp
0x00000000012e1216 (/opt/informix/bin/oninit) mt_fast_wait
0x0000000000d52e82 (/opt/informix/bin/oninit) logwrite
0x0000000000e99b9e (/opt/informix/bin/oninit) logm_write
0x0000000000d504bb (/opt/informix/bin/oninit) logputx
0x0000000000ccd40f (/opt/informix/bin/oninit) rscommit
0x0000000000de2a6b (/opt/informix/bin/oninit) xa_commit
0x00000000009dd04b (/opt/informix/bin/oninit) ifx_common_xcommit
0x00000000009dd39d (/opt/informix/bin/oninit) sq_xcommit
0x0000000000a6a3fb (/opt/informix/bin/oninit) sqmain
0x00000000013dbe3b (/opt/informix/bin/oninit) spawn_thread
0x00000000013151c0 (/opt/informix/bin/oninit) th_init_initgls
0x00000000012e1a38 (/opt/informix/bin/oninit) startup

This seems to be one of the threads blocking in commit. The reason is obviously a logwrite call,
which it is waiting for. We have 95 threads a this time which have a logwrite stack member !

Now my question: How can we prevent this flushing ? Or make it happen faster ?
Or are we on the wrong way when looking at the source of the problem ?
What is really happening during logwrite ? Since the hardware is relatively new,
and the disk array is very fast, I/O should not be the problem.

We have here: IDS 11.70FC8XB8GE running on Linux x64, using huge memory pages.

Machine has 24GB Ram, Database uses ~15GB.
Checkpoints are done on 3-4 seconds, flush time 1.5-2.4 seconds (this cannot trigger 20 seconds of
blocking from my point of view).
onstat -F shows only chunk writes. onstat -p shows 99,8% read cached and 86,47% write cached.
Physical log is 5GB, rarely used > 5%
Checkpoints occur each 300sec, no other triggers than interval.

The machine is paired with a secondary instance via HDR. HDR pair was up and running without any trace of
"real" activity (checkpoints 1 second).

Any idea will be appreciated, we cannot file a bug, since we are not able to reproduce the behaviour
or even present an onstat -a output.
Yes, the instance is loaded very high, but we have other instances with equal or higher load, which
are not behaving like this.

Marcus Haarmann

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