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Re: posix_fadvise v22

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Robert Haas" <robertmhaas(at)gmail(dot)com>
Cc: "Gregory Stark" <stark(at)enterprisedb(dot)com>, "ITAGAKI Takahiro" <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Postgres <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: posix_fadvise v22
Date: 2009-01-11 19:14:50
Message-ID: 1098.1231701290@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
"Robert Haas" <robertmhaas(at)gmail(dot)com> writes:
> OK, here's an update of Greg's patch with the runtime configure test
> ripped out, some minor documentation tweaks, and a few unnecessary
> whitespace diff hunks quashed.  I think this is about ready for
> committer review.

I've started to look through this, and the only part I seriously don't
like is the nbtsearch.c changes.  I've got three complaints about that:

* Doing it inside the index AMs is wrong, or at the very least forces
us to do it over for each index AM (which the patch fails to do).

* As coded, it generates prefetch bursts that are much too large and too
widely spaced to be effective, not to mention that they entirely
ignore the effective_io_concurrency control knob as well as the order
in which the pages will actually be needed.  I wonder now whether
Robert's inability to see any benefit came because he was testing
indexscans and not bitmap scans.

* It's only accidental that it's not kicking in during a bitmap
indexscan and bollixing up the much-more-carefully-written
nodeBitmapHeapscan prefetch logic.

What I intend to do over the next day or so is commit the prefetch
infrastructure and the bitmap scan prefetch logic, but I'm bouncing the
indexscan part back for rework.  I think that it should be implemented
in or near index_getnext() and pay attention to
effective_io_concurrency.

			regards, tom lane

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