What is the size of your SGA?
Are you using large pages?
What is your value for db_writer_processes?
Cheers
jko
From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Pap
Sent: 13 January 2021 15:04
To: Oracle L <oracle-l_at_freelists.org>
Subject: Performance issue - high Free buff wait
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
