Re: mincore() & fincore()

["Cédric Villemain" <cedric@2ndquadrant.com>]

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> >> Johannes, I add you in CC because you're the last one who proposed
> >> something. Can I update your patch with previous suggestions from
> >> reviewers ?
> >
> >Absolutely!

OK.

> >> I'm also asking for feedback in this area, others ideas are very
> >> welcome.
> >
> >Andrew didn't like the idea of the one byte per covered page
> >representation but all proposals to express continuous ranges in a
> 
> mincore utilize byte array and the least significant bit is used to
> check if the corresponding page is currently resident in memory, I
> don't know the history, what's the reason for not using bitmap?
> 
> >more compact fashion had worse worst cases and a much more involved
> >interface.
> >
> >I do wonder if we should model fincore() after mincore() and add a
> >separate syscall to query page cache coverage with statistical output
> >(x present [y dirty, z active, whatever] in specified area) rather
> >than describing individual pages or continuous chunks of pages in
> >address order.  That might leave us with better interfaces than trying
> >to integrate all of this into one arcane syscall.

It should works too. My tool pgfincore (for postgresql) also outputs the number 
of group of contiguous in-memory page, it is to get a quick idea of 
the access pattern: from large number of groups (random) to few groups 
(sequential). So for this usage, I don't really need the full vector and page 
level information, but some stats are needed to make those sums useful.

However another usage is to snapshot/restore in-memory pages, it is useful in 
at least 2 scenarios. One for simple server restart, PostgreSQL is back to 
full speed faster when you're able to restore the previous cache content. The 
other one is similar, switchover to a previously 'cold' server or prepare a 
server to get traffic.
For those use-cases, it is interesting to have the details.

-- 
Cédric Villemain +33 (0)6 20 30 22 52
http://2ndQuadrant.fr/
PostgreSQL: Support 24x7 - Développement, Expertise et Formation

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