RE: Keep buffer cache question

Oy, yes. Sorry about that. My brain was fuzzy from spending all day looking into logs and AWR reports while having people stop by my desk and IM and set up conference calls.

Jay Miller
Sr. Oracle DBA
201.369.8355

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of John Thomas Sent: Thursday, March 14, 2019 5:50 PM
To: dmarc-noreply_at_freelists.org
Cc: ORACLE-L
Subject: Re: Keep buffer cache question

Jay,

Can I confirm you meant milliseconds (10^-3) not microseconds (10^-6)?

Regards,

John Thomas
Database Designer and Administrator
https://oracleexpert.net
<https://urldefense.proofpoint.com/v2/url?u=https-3A__oracleexpert.net_&d=DwMFaQ&c=nulvIAQnC0yOOjC0e0NVa8TOcyq9jNhjZ156R-JJU10&r=aiKV3Uv2Wo7GqYQcis9TSvB1MZslPOnintrOY1rjG58&m=2D0jZBl_Tlic3qxfNxpz8NfsE_t8u1Ntx0JC54Q7DNE&s=VQY68E1SAaCbd1_T9d-7ArDSHlv1aH7q_ZbGxY1mqrk&e=>

On Thu, 14 Mar 2019 at 21:23, Redacted sender Jay.Miller for DMARC <dmarc-noreply_at_freelists.org<mailto:dmarc-noreply_at_freelists.org>> wrote: I should add that there has been no memory pool resizing since over 2 months ago.

Jay Miller
Sr. Oracle DBA
From: Miller, Jay
Sent: Thursday, March 14, 2019 5:22 PM
To: oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org> Subject: Keep buffer cache question

Odd issue here. One of our apps reported slightly increased latency on a heavily used database which started Monday evening and has been consistent since. This is not large from a database perspective but the increase of average response time from 1 to 3 microseconds has had a noticeable impact on their performance.

No execution plan changes, a slightly heavier load at peak times (up about 10% from last week) but nothing that I would expect to have such an impact. We still see the increased latency when the server is 90% idle and the load average is 5 (32 cpus, 16 cores).

In doing an AWR report comparison for comparable times one major difference I saw was that 2 frequently run queries were suddenly doing a lot of physical i/o. For a comparable 2 hour period they went from 1.5 million to 1.8 million executions but physical reads increased from 0 to 1.2 million. I sampled a few other random times and this was consistent. The queries are both doing index access. One is an index range scan and the other a unique scan against the primary key.

I checked with the app group and they have no explanation for why the app might suddenly be querying blocks that aren't in cache whereas they weren't last week.

I am reluctantly considering adding the tables to the keep cache even though they are huge (4-5G_at_). They are frequently accessed so I don't see any real downside to this other than the huge amount of memory I would be reserving for them.

We are going to test this in a non-production environment tomorrow but my two questions are:

1. Is anyone aware of any problems this might cause? We have not used the keep cache in the past.
2. Is there a way to preload the data once the db_keep_cache_size has been set, the sga increased to accommodate it, and the table assigned to the keep cache? A full table scan would usually bypass the buffer cache so I'm not sure of the best approach here.
3. Any other ideas on what might cause this?

TIA! Jay Miller
Sr. Oracle DBA

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