The only thing to do is fix the SQL. Each logical I/O or buffer get results
in a cache buffer chain latch get. So by doing less LIO, you will get fewer
latch gets and as a result fewer sleeps on latches. This is how you fix the
*problem*. You can also fix the *symptom*: bump up _spin_count (assuming
that you run on a SMP) or set _db_ block_hash_latches to a higher value.
Fixing the SQL is the right way to go. Are you shooting for a 99.99999
percent buffer cache hit ratio ? If you are than that could also be a reason
for the problem. Oh and there is a bug in Oracle 8.1.6/8.1.7 I believe that
causes an additional buffer get for the index root block (assuming that the
hash latches with the high sleeps cover index root blocks).
Anjo Kolk
http://www.oraperf.com
- Original Message -----
To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
Sent: Thursday, February 28, 2002 7:53 AM
_db_block_hash_buckets
> 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
> --
> James Manning <jmm_at_sublogic.com>
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