Re: StrategyGetBuffer optimization, take 2

On Wed, Aug 7, 2013 at 7:40 AM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:

> I agree; at least then it's not unambiguously better. if you (in
> effect) swap all contention on allocation from a lwlock to a spinlock
> it's not clear if you're improving things; it would have to be proven
> and I'm trying to keep things simple.
>
> Attached is a scaled down version of the patch that keeps the freelist
> lock but still removes the spinlock during the clock sweep. This
> still hits the major objectives of reducing the chance of scheduling
> out while holding the BufFreelistLock and mitigating the worst case
> impact of doing so if it does happen. An even more scaled down
> version would keep the current logic exactly as is except for
> replacing buffer lock in the clock sweep with a trylock (which is
> IMNSHO a no-brainer).

Since usage_count is unsigned, are you sure that changing the tests
from "buf->usage_count == 0" to "buf->usage_count <= 0" accomplishes
what you need it to? If usage_count gets decremented when it already
zero, it will wrap around to 65,535, at least on some compilers some
of the time, won't it?

It seem safer just not to decrement if we can't get the lock.

Cheers,

Jeff