5d7962c679
Previously, we used an lwlock that was held from the time we began seeking a candidate buffer until the time when we found and pinned one, which is disastrous for concurrency. Instead, use a spinlock which is held just long enough to pop the freelist or advance the clock sweep hand, and then released. If we need to advance the clock sweep further, we reacquire the spinlock once per buffer. This represents a significant increase in atomic operations around buffer eviction, but it still wins on many workloads. On others, it may result in no gain, or even cause a regression, unless the number of buffer mapping locks is also increased. However, that seems like material for a separate commit. We may also need to consider other methods of mitigating contention on this spinlock, such as splitting it into multiple locks or jumping the clock sweep hand more than one buffer at a time, but those, too, seem like separate improvements. Patch by me, inspired by a much larger patch from Amit Kapila. Reviewed by Andres Freund.
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