Re: 2nd Level Buffer Cache

On Thu, Mar 24, 2011 at 5:34 PM, Greg Stark <gsstark(at)mit(dot)edu> wrote:
> On Thu, Mar 24, 2011 at 8:59 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> It seems at least plausible that buffer allocation could be
>> significantly faster if it need only pop the head of a list, rather
>> than scanning until it finds a suitable candidate.  Moving as much
>> buffer allocation work as possible into the background seems like it
>> ought to be useful.
>
> Linked lists are notoriously non-concurrent, that's the whole reason
> for the clock sweep algorithm to exist at all instead of just using an
> LRU directly. That said, an LRU needs to be able to remove elements
> from the middle and not just enqueue elements on the tail, so the
> situation isn't exactly equivalent.
>
> Just popping off the head is simple enough but the bgwriter would need
> to be able to add elements to the tail of the list and the people
> popping elements off the head would need to compete with it for the
> lock on the list. And I think you need a single lock for the whole
> list because of the cases where the list becomes a single element or
> empty.
>
> The main impact this list would have is that it would presumably need
> some real number of buffers to satisfy the pressure for victim buffers
> for a real amount of time. That would represent a decrease in cache
> size, effectively evicting buffers from cache as if the cache were
> smaller by that amount.
>
> Theoretical results are that a small change in cache size affects
> cache hit rates substantially. I'm not sure that's born out by
> practical experience with Postgres though. People tend to either be
> doing mostly i/o or very little i/o. Cache hit rate only really
> matters and is likely to be affected by small changes in cache size in
> the space in between

You wouldn't really have to reduce the effective cache size - there's
logic in there to just skip to the next buffer if the first one you
pull off the freelist has been reused. But the cache hit rates on
those buffers would (you'd hope) be fairly low, since they are the
ones we're about to reuse. Maybe it doesn't work out to a win,
though.

--
Robert Haas
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