Re: Perf Advice Needed: cache buffers chains, high waits,

James Manning wrote:

> [Mogens Nørgaard]
> > Amen. Contention for cache buffers chains means too much logical IO,
> > ie. find and exterminate heavy SQL.
>
> I don't see why the heavy SQL would result in the chain having 66 buffer
> heads in it, though, or why the sleep count would be so skewed.

The basic thought about the new parameters is that 1) you don't need so many latches, actually one per cpu should be good enough 2) having more hash buckets than latches helps to reduce the number of

>
>
> And my core question is still whether the number of buckets being
> non-prime is "normal" or not - it seems awfully wrong to me.
>
> That there's a lot of contention *is* a factor of the SQL, but the
> fact that it's so skewed to only a few chains is what worries me more.
>
> Once I have the contention down to a particular latch, but that latch
> protects a buffer chain with 66 buffer heads in it, how can I find out
> which ones of the 66 are generating the most attempts at that latch?
>
> Tell ya what - can I get a few ppl to run this query? It tells the
> min/max/avg for the number of buffers associated with each chain and if
> my numbers are high I can at least have a chance of spreading out the
> buffers over more chains (by upping the number of latches from 4k to 16k,
> 32, whatever) - it won't drop the actual IO any, of course, but since
> I don't have a hard fix on which buffers of the 66 are really the source
> of my contention, I'm not sure where to go from here.
>
> SELECT min(buffers_per), max(buffers_per),
> avg(buffers_per), sum(buffers_per)
> FROM (
> SELECT count(*) buffers_per, hladdr
> FROM x$bh b, all_objects o, v$latch_children v
> WHERE
> b.HLADDR=v.addr
> AND b.obj=o.object_id
> AND v.name LIKE '%cache buffers %'
> GROUP BY hladdr
> )

Actually this shows you the number of buffers per latch not per hash bucket. That information can't be easily retrieved from Oracle. Each latch will cover a number of buckets. Each bucket will hold a number of buffer headers. The buffers are hashed over the hash buckets depending on (tsn#,DBA).

>
>
> My results:
> min = 39
> max = 119
> avg = 55.06
> sum = 225555
>
> If this shows to be about the same in other (well-tuned) Oracle DB's, then
> I won't worry as much about the number of buffers in each chain and would
> then focus on trying to isolate the specific buffers, then the source SQL
> causing the problem, etc.
>
> Given my previous sql trace analyses, I have a good idea what the problem
> SQL statement is, but it's a bit of a necessary evil right now (a join
> of a table (260k rows) and a materialized view (2k rows), 6 conditions
> in there where, and it gets executed a ton, probably on the order of 10x
> a second at peak) - all indexes that helped performance are created and
> around already. :( But, ideally I'd like to be able to prove this is
> the cause of the "hot buffers" before fixing anything.
>

Well I think it is easier to fix the SQL than the shutdown and restart oracle to try the "correct settings" of the parameters. By fixing the SQL not only will you remove the contention but it will so improve peformance for the end user.

>
> Thanks, guys!!
>
> James
> --
> James Manning <jmm_at_sublogic.com>
> GPG Key fingerprint = B913 2FBD 14A9 CE18 B2B7 9C8E A0BF B026 EEBB F6E4
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.com
> --
> Author: James Manning
> INET: oracle_at_sublogic.com
>
> Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
> San Diego, California -- Public Internet access / Mailing Lists
> --------------------------------------------------------------------
> To REMOVE yourself from this mailing list, send an E-Mail message
> to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
> the message BODY, include a line containing: UNSUB ORACLE-L
> (or the name of mailing list you want to be removed from). You may
> also send the HELP command for other information (like subscribing).

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Anjo Kolk
  INET: anjo_at_oraperf.com

Fat City Network Services    -- (858) 538-5051  FAX: (858) 538-5051
San Diego, California        -- Public Internet access / Mailing Lists
--------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB ORACLE-L
(or the name of mailing list you want to be removed from).  You may
also send the HELP command for other information (like subscribing).