Anjo Kolk wrote:
Really,
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" <[EMAIL PROTECTED]>
Sent: Thursday, February 28, 2002 7:53 AM
_db_block_hash_bucketsdb_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 giv e 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 <[EMAIL PROTECTED]>
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