Re: Display Pg buffer cache (WIP)

Tom Lane wrote:
> It'd be possible to dispense with the per-buffer spinlocks so long as
> you look only at the tag (and perhaps the TAG_VALID flag bit).  The
> tags can't be changing while you hold the BufMappingLock.

That's what I had thought at first, but this comment in buf_internals.h
dissuaded me: "buf_hdr_lock must be held to examine or change the tag,
flags, usage_count, refcount, or wait_backend_id fields." The comment
already notes this isn't true if you've got the buffer pinned; it would
be worth adding another exception for holding the BufMappingLock, IMHO.

> I'm dubious that there's any point in recording information as
> transient as the refcounts and dirtybits

I think it's worth recording dirty bits -- it provides an indication of
the effectiveness of the bgwriter, for example. Reference counts could
be done away with, although I doubt it would have a significant effect
on the time spent holding the lock.

-Neil