Re: StrategyGetBuffer optimization, take 2

On 2013-08-05 10:49:08 -0500, Merlin Moncure wrote:
> optimization 4: remove free list lock (via Jeff Janes). This is the
> other optimization: one backend will no longer be able to shut down
> buffer allocation

I think splitting off the actual freelist checking into a spinlock makes
quite a bit of sense. And I think a separate one should be used for the
actual search for the clock sweep.
I don't think the unlocked increment of nextVictimBuffer is a good idea
though. nextVictimBuffer jumping over NBuffers under concurrency seems
like a recipe for disaster to me. At the very, very least it will need a
good wad of comments explaining what it means and how you're allowed to
use it. The current way will lead to at least bgwriter accessing a
nonexistant/out of bounds buffer via StrategySyncStart().
Possibly it won't even save that much, it might just increase the
contention on the buffer header spinlock's cacheline.

Greetings,

Andres Freund