Re: buffer cache structure in SGA

"It's probably worth including the point that the linked list is still there".

Am I correct in assuming the linked list points to buffers in the hash table?

On Tue, Apr 3, 2018 at 2:29 AM, Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk> wrote:

>
> It's probably worth including the point that the linked list is still
> there - the effect of the touch count is that a buffer is not moved to the
> head of the list every time it is touched (which is why the latch activity
> became a threat), but it only moved when it reaches the tail of the list,
> and then only if it has been touched on the way down.
>
> (Obviously lots more detail I've omitted - but it's in the book)
>
> Regards
> Jonathan Lewis
>
> ________________________________________
> From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> on
> behalf of Mladen Gogala <gogala.mladen_at_gmail.com>
> Sent: 03 April 2018 08:07
> To: oracle-l_at_freelists.org
> Subject: Re: buffer cache structure in SGA
>
> Well, not quite the same. In Oracle 8i, buffers were managed by a linked
> list. The oldest buffers were at the tail of list, the most recently
> touched buffers were at the head of the list. Since Oracle 9.2 buffers are
> managed by the "touch count". Basically, when Oracle would need to free
> buffer in Oracle 8i, it would take certain number of buffers from the end
> of the linked list, save them and allocate them to processes that needed
> them. The problem was that to manipulate the linked list, Oracle would need
> to acquire the latch. And latches can be expensive. So, with touch counts,
> the organization was different, the need for latches was much smaller.
>
> In Oracle 11.2, the whole system of pins and latches was completely
> reorganized, which resulted in Oracle 11.2.0.1 being practically unusable.
> Situation has improved greatly in 11.2.0.4. Now, there are some additional
> changes in 12c, to prevent connections to different PDB's from stealing
> buffers from each other. If you check db_cache_size parameter in Oracle
> 12.2, you will see that it's modifiable within PDB:
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>
>

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