Re: Closing some 8.4 open items

On Wed, 8 Apr 2009, Tom Lane wrote:

> If the kernel can't read-ahead a seqscan by itself, it's unlikely to
> be smart enough to be helped by posix_fadvise ... or at least so I
> would think.

There's some interesting comments on this subject (and about what fadvise 
DONTNEED does) in the RRD research paper about managing their buffer 
cache:

http://www.usenix.org/event/lisa07/tech/full_papers/plonka/plonka_html/index.html

They suggest the Linux read-ahead is pretty aggressive by default, which 
might explain why I wasn't able to replicate any speed-up with the 
seqeuential scan patch on my system.  (The original submission showed a 
significant speedup on Linux, but was from what sounded like a somewhat 
broken kernel--known buggy controller driver I seem to recall)

I suspect we may need to find a platform where the default OS readahead is 
a slacker, *and* that pays attention to POSIX_FADV_SEQUENTIAL, in order to 
show any improvement from that patch.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD