Re: Why are these ARC variables per-backend?

Tom Lane wrote:

> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
>> Tom Lane wrote:
>>> I've got a problem with these variables in freelist.c:
>>>
>>> static int strategy_cdb_found;
>>> static int strategy_cdb_replace;
>
>> These two most definitely are per backend because they hold status
>> information about the blocks this backend specifically is mucking with.
>> If it all would be in one function, they would be local variables.
>
> Would you object if I made 'em actual local variables? This would
> clutter the strategy API slightly since the vars would have to be passed
> out of some routines and into others, but I think it would be logically
> cleaner. (In the back of my mind is the idea to support two instances
> of the ARC datastructure, one for global and one for local buffers, so
> minimizing the number of static variables is a prerequisite.)

Good idea.

>
>>> static int strategy_get_from;
>
>> This one is questionable. The point of it is to steer the T1/T2 list
>> sizes towards the goal of the strategy (T1_target). It doesn't matter
>> much if this is off by one every now and then, it will be corrected at
>> the next buffer eviction ... or the one after that.
>
> Why do we have it at all? Seems like it would be at least as good to
> make the T1-or-T2 decision in StrategyGetBuffer, rather than earlier.

If it'd be possible. The problem is that it can get called multiple
times during one logical buffer lookup/fault cycle. Because of the
possible IO, which is done in the caller of the strategy, this would
screw up the decision process about the queue sizes.

The original ARC logic was built on a single process/thread model, where
you do the IO just in the middle of the decision process. With our
buffer manager who's now calling the strategy logic this needs a few
status switches.

Jan

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