Re: aio: worker: Free SMGR objects when idle

Hi,

Right, the per IO check puts the spinlock in the hot path.

On dropping SHAREDINVALSMGR_ID, one thing to check first. The barrier
is only emitted by DROP and ALTER DATABASE and by DROP TABLESPACE.
Plain DROP TABLE and TRUNCATE go through CacheInvalidateSmgr, from
smgrdounlinkall and smgrtruncate. With only the barrier left, a
backend would hold descriptors for dropped relations until someone
drops a database or a tablespace. That is a behavior change, not a
cleanup.

The barrier also only releases, it does not destroy, so it does not
fix what Nazir reported. The hash table in the IO worker keeps
growing either way. Nothing pins those entries in an IO worker, and
smgrdestroyall only zaps unpinned ones, so destroying them there
should be safe.

That suggests a version with no lock at all. Keep the cleanup where
Nazir put it, at a safe point in the worker loop, but trigger it on a
local condition, the number of unpinned entries being over a cap,
rather than on a checkpoint generation. No shared state, and it also
covers a worker that never goes idle.

What this does not answer is how an IO worker finds out that a
relation was unlinked, so it can close the descriptor without waiting
for its own eviction. That is the sinval question you raised, and it
looks separate from Nazir's patch to me.

Thanks,
Shihao