Re: cbc latch assignment on db_cache resize

Andrew,
After your first email, I did a couple of quick tests on an NT 9.2.

I happen to have an 8K database with 4K blocks Starting with

    sga_max_size = 256Mb
    db_cache_size = 8M
    db_4K_cache-size = 8M
I see 1024 cbc latches.

If I restart the database, but change the sga_max_size to 512MB, I get 1536 cbc
latches.

So I was going to test the theory that the number of cbc latches is determined by
largest number of buffers you could demand given the smallest block size with a defined cache.

BTW - the number of latches in 9.2 seems to revolve around significant powers to 2. I think you are thinking of the number of hash buckets when you say 'prime number
close to twice the buffers'.

--
Jonathan Lewis
http://www.jlcomp.demon.co.uk

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Andrew Mobbs wrote in message ...


>

>Having now had a chance to play with this, it appears that, at least,

>the values for _db_block_hash_latches and _db_block_hash_buckets in

>x$ksppcv stay constant when db_cache_size changes.

>

>This, unfortunately for me, means that shrinking the SGA shouldn't make

>cache buffers chains contention worse. However, should we then be wary

>of starting with a small SGA and dynamically growing it to something

>quite large?

>

>Interestingly, 9iR2 (at least, maybe 9.0.1 too), _db_block_hash_latches

>has gone back to being prime but now is a prime "near" (not sure exactly

>how it's chosen) twice the number of block buffers.

>

>--

>Andrew Mobbs - http://www.chiark.greenend.org.uk/~andrewm/