Re: PrivateRefCount (for 8.3)

Simon Riggs wrote:
> > Most likely a waste of development effort --- have you got any evidence
> > of a real effect here?  With 200 max_connections the size of the arrays
> > is still less than 10% of the space occupied by the buffers themselves,
> > ergo there isn't going to be all that much cache-thrashing compared to
> > what happens in the buffers themselves.  You're going to be hard pressed
> > to buy back the overhead of the hashing.
> 
> And at 2000 connections we waste RAM the size of shared_buffers... that
> isn't something to easily ignore.
> 
> > It might be interesting to see whether we could shrink the refcount
> > entries to int16 or int8.  We'd need some scheme to deal with overflow,
> > but given that the counts are now backed by ResourceOwner entries, maybe
> > extra state could be kept in those entries to handle it.
> 
> int8 still seems like overkjll. When will the ref counts go above 2 on a
> regular basis? Surely refcount=2 is just chance at the best of times.
> 
> Refcount -> 2 bits per value, plus a simple overflow list? That would
> allow 0,1,2 ref counts plus 3 means look in hashtable to find real
> refcount.

At two bits, would we run into contention for the byte by multiple
backends?

--  Bruce Momjian   bruce@momjian.us EnterpriseDB    http://www.enterprisedb.com
 + If your life is a hard drive, Christ can be your backup. +