Re: mincore() & fincore()

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Thu, Jul 25, 2013 at 11:32:07AM -0400, Johannes Weiner wrote:
>On Thu, Jul 25, 2013 at 05:07:10PM +0200, Cedric Villemain wrote:
>> [sorry, previous mail was sent earlier than expected]
>> 
>> > First, the proposed changes in this email are to be used at least for 
>> > PostgreSQL extensions, maybe for core.
>> > 
>> > Purpose is to offer better monitoring/tracking of the hot/cold areas (and 
>> > read/write paterns) in the tables and indexes, in PostgreSQL those are by default 
>> > written in segments of 1GB.
>> > 
>> > There are some possible usecase already:
>> > 
>> >  * planning of hardware upgrade
>> >  * easier configuration setup (both PostgreSQL and linux)
>> >  * provide more informations to the planner/executor of PostgreSQL
>> > 
>> > My ideas so far are to 
>> > 
>> >  * improve mincore() in linux and add it information like in freeBSD (at 
>> >    least adding 'mincore_modified' to track clean vs dirty pages).
>> >  * adding fincore() to make the information easier to grab from PostgreSQL (no 
>> >    mmap)
>> >  * maybe some access to those stats in /proc/
>> > 
>> > It makes years that libprefetch, mincore() and fincore() are discussed on linux 
>> > mailling lists. And they got a good feedback... So I hope it is ok to keep on 
>> > those and provide updated patches.
>> 
>> 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!
>
>> 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.
>
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