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IDS Forum

Re: Performance issue

Posted By: Marcus Haarmann
Date: Monday, 8 June 2015, at 12:17 p.m.

In Response To: Re: Performance issue (Spokey Wheeler)

lets see, I have stopped the btree scanner temporarily ...
Waiting for tomorrow, cause in the evening, we never had this situation.

Marcus Haarmann

----- Ursprüngliche Mail -----

Von: "Spokey Wheeler" <spokey.wheeler@gmail.com>
An: ids@iiug.org
Gesendet: Montag, 8. Juni 2015 17:50:10
Betreff: Re: Performance issue [35234]

BTree Cleaner?

> On 8 Jun 2015, at 16:42, Marcus Haarmann <marcus.haarmann@midoco.de> wrote:
>
> 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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>

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