RE: [Patch] Optimize dropping of relation buffers using dlist

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> Why would all client backends wait for AccessExclusive lock on this
> relation? Say, a client needs a buffer for some other relation and
> that might evict this buffer after we release the lock on the
> partition. In StrategyGetBuffer, it is important to either have a pin
> on the buffer or the buffer header itself must be locked to avoid
> getting picked as victim buffer. Am I missing something?

Ouch, right. (The year-end business must be making me crazy...)

So, there are two choices here:

1) The current patch.
2) Acquire the buffer header spinlock before releasing the buffer mapping lwlock, and eliminate the buffer tag comparison as follows:

BufTableLookup();
LockBufHdr();
LWLockRelease();
InvalidateBuffer();

I think both are okay. If I must choose either, I kind of prefer 1), because LWLockRelease() could take longer time to wake up other processes waiting on the lwlock, which is not very good to do while holding a spinlock.

Regards
Takayuki Tsunakawa