Re: WIP patch for hint bit i/o mitigation

> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-
> owner@postgresql.org] On Behalf Of Merlin Moncure
> Sent: Wednesday, November 07, 2012 5:26 AM
> To: PostgreSQL-development
> Cc: Atri Sharma
> Subject: [HACKERS] WIP patch for hint bit i/o mitigation
> 
> Following the sig is a first cut at a patch (written by Atri) that
> attempts to mitigate hint bit i/o penalty when many pages worth of
> tuples are sequentially written out with the same transaction id.
> There have been other attempts to deal with this problem that fit
> niche cases (especially those that create the table in the same
> transaction as the one inserting) that work but don't really solve the
> problem generally.
> 
> I previously attacked this problem ([1], [2]) and came up with a patch
> that cached hint bits inside tqual.c.  The patch was pulled for a few
> reasons:
> 
> 1) a lot of complexity without proper justification
> 2) sketchy cache replacement algorithm
> 3) I manged to misspell 'committed' just about everywhere
> 4) invalidation?
> 
> Issues 1-3 could have been worked out but #4 was making me think the
> problem was a nonstarter, or at least, 'too much too soon'.  The tuple
> visibility routines are in a very tight code path and having to deal
> with various things in the backend that could cause the xid to become
> stale were making me nervous.   A smaller, simpler patch might be the
> ticket.

About invalidation, I think the cached xid can become invalid due to xid
wraparound.
So for that one way could be to invalidate it through Vacuum.

Though I am not sure what all other things can make cached id as invalid,
but I think once we
can think what other ways can make cached id invalid, then we can see if
there is a solution to address
them.


With Regards,
Amit Kapila.