Oracle-L: Perf Advice Needed: cache buffers chains, high waits, _db_block_hash

From: James Manning <oracle_at_sublogic.com>
Date: Wed, 27 Feb 2002 22:53:29 -0800
Message-ID: <F001.0041AA9D.20020227225329@fatcity.com>

db_block_buffers = 360448
db_block_lru_latches = 4
db_block_size = 8192

_db_block_hash_buckets = 720896

Ok, what I have so far is:

using itrprof, I saw that 35% of my elapsed time was based on waits of "cache buffers chains" latches.
checking v$latch_children (latch#=66), there are a good number (8-10 I'd guess) of the 4096 children that have a very high (10k+) number of sleeps - the rest of the children (of this type latch) have sleep counts are 10-12, so we have a ton of contention on a low # of "cache buffers chains" latches.
joining with x$bh (v$latch_children.addr=x$bh.hladdr), I see that the most contentioned-for of these latches (51,240 sleeps!) has 66 blocks on the chain. Checking with all_objects, I'm noticing that these blocks are scattered in some of the more important (and most-accessed) tables and indexes
The other latch children that have high sleep counts also have 30-50 buffers in their chains

Questions:

to me, 66 seems awfully high - is it?
the sleep count is obviously high from what I can tell - is it definitely tied to the buffer chain this latch is protecting being so long and just happening to be 66 buffers that are mostly important tables and indexes?
I haven't set it by hand, but _db_block_hash_buckets = 720896 and this is 11 * 2^16. Everything I've read says it should be a prime number (and that jives with my comp sci background) - why is it not prime, why is it exactly twice db_block_buffers?
the number of children for "cache buffers chains" is 4096. Now, increasing that could have a positive effect on distributing the contention, but since the sleps are so heavily skewed to only a few of the children as it stands, I don't get the feeling that's the right fix.

Anyone have any advice to offer? Pages/URL's that can help give some advice?

It's worth noting that these latches are basically non-existant as wait events at low load - "log file sync" is about the only wait event I see at low loads, and I'm working on reducing my commit counts much further to help tackle that.

Thanks!!

James

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James Manning <jmm_at_sublogic.com>
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Received on Thu Feb 28 2002 - 00:53:29 CST

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