Re: Performance issue - high Free buff wait

Hello Pap,

Is not just those wait events, there is clearly a significant difference in load between the 2 periods you mentioned.

-> the moment you open the excel you can see DB time 237.29 (mins) vs 874.29 (mins)
Based on the fact that you have 64 cores most probably you reached almost peak CPU usage during that period maybe you was in the position where processes competed to get time on cpu.

going down
-> db block changes 9720600 vs 53870404 looks like transactions do more work ( more than 5 times more)
-> redo size 2448278000 vs 11624470400 correlated with above metric you generated 5 times more redo info
-> user rollbacks / user commits -> both look high , you can check with dev guys why you commit so often and why you have the rollbacks (do you have any errors in alertlog, to they have any timeout at app level)

As you might be cpu bound during this peak period you might be storage bound as well. It is possible for the whole system to be slowed down by LGWR.
You can validate this using Tanel`s chain waits: https://github.com/tanelpoder/tpt-oracle/blob/master/ash/dash_wait_chains.sql

you can use it like this:
_at_dash_wait_chains event2 1=1 "TIMESTAMP'2021-01-12 20:00:00'" "TIMESTAMP'2021-01-12 20:15:00'"
and post output

Good luck.

În mie., 13 ian. 2021 la 16:05, Pap <oracle.developer35_at_gmail.com> a scris:

> Not sure why , but the first email I dropped on this question was bounced
> back from oracle-l, so trying again.
>
> Hello experts,
>
> We have a database with Oracle version 11.2.0.4. We are getting
> complaints of slowness in one of our jobs mainly for ~15minutes duration on
> a daily basis. And there is no plan change or any Undo reads for the
> underlying sqls and also there is not one sql which we can singled out for
> that job duration and that job consists of many small queries(selects,
> inserts running many thousand times in that duration). But while
> seeing/comparing the AWR for that ~15minutes period VS another ~15minutes
> just before that, we are seeing a few odd waits , they are a combination of
> Configuration(free buffer waits) followed by concurrency(buffer busy, index
> contention etc) foreground waits. We do see comparatively higher sessions
> during this ~15minutes window as compared to normal time.
>
> In the AWR under section "IOStat by Function summary" the DB writer avg
> response time is logged as ~103 ms vs in normal period it stays ~35ms. And
> then checking v$iostat_file , we see it has ASYNCH_IO set as OFF for both
> "data file" and "temp file" which I have seen in many databases normally
> set to ON. Also in dba_hist_active_sess_history the top waits events are
> showing as below. I have attached excel with specific sections of AWR
> during the issue period and normal period in two different tabs.
>
> 1)So , is it correct that by turning this ASYNCH_IO "ON" for data/temp
> file , will cater all these issues because these all seem to be triggering
> from the slow DB writer performance?
>
> Or
>
> 2)Should we ask the application team to reduce the total number of
> sessions(maybe by decreasing the max connection limit) which are submitting
> to the Database at that point to reduce contention?
>
> *Below is count of waits from Dba_hist_active_sess_history for that
> ~15minutes window:-*
>
> Top two(log file sync and db file async I/O submit) are not associated
> with application user/session.
>
> EVENT COUNT(*)
> log file sync 260
> db file async I/O submit 188
> free buffer waits 72
> 66
> 65
> 63
> 62
> db file sequential read 56
> write complete waits 54
> 54
> db file sequential read 51
>
> Regards
>
> Pap
>
>
>
>

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