Re: [PATCH 0/3] readahead drop behind and size adjustment

[Rik van Riel <riel@redhat.com>]

Nick Piggin wrote:
> Eric St-Laurent wrote:
> > On Wed, 2007-25-07 at 15:19 +1000, Nick Piggin wrote:
> >
> >
> > > What *I* think is supposed to happen is that newly read in pages get
> > > put on the inactive list, and unless they get accessed againbefore
> > > being reclaimed, they are allowed to fall off the end of the list
> > > without disturbing active data too much.
> > >
> > > I think there is a missing piece here, that we used to ease the reclaim
> > > pressure off the active list when the inactive list grows relatively
> > > much larger than it (which could indicate a lot of use-once pages in
> > > the system).
> >
> >
> > Maybe a new list should be added to put newly read pages in it. If they
> > are not used or used once after a certain period, they can be moved to
> > the inactive list (or whatever).
> >
> > Newly read pages...
> >
> > - ... not used after this period are excessive readahead, we discard
> > immediately.
> > - ... used only once after this period, we discard soon.
> > - ... used many/frequently are moved to active list.
> >
> > Surely the scan rate (do I make sense?) should be different for this
> > newly-read list and the inactive list. 
>
> A new list could be a possibility. One problem with adding lists is just
> trying to work out how to balance scanning rates between them, another
> problem is CPU overhead of moving pages from one to another... but don't
> let me stop you if you want to jump in and try something :)
>
>
> > I also remember your split mapped/unmapped active list patches from a
> > while ago.
> >
> > Can someone point me to a up-to-date documentation about the Linux VM?
> > The books and documents I've seen are outdated.
>
> If you just want to play with page reclaim algorithms, try reading over
> mm/vmscan.c. If you don't know much about the Linux VM internals before,
> don't worry too much about the fine details and start by getting an idea
> of how pages move between the active and inactive lists.

I've got a nice list of problems with the VM at
http://linux-mm.org/PageoutFailureModes :)

I should post my latest version of the split LRU
lists code.  I'm still working on SEQ replacement
for the anonymous pages...

> OK here is one which just changes the rate that the active and inactive
> lists get scanned. Data corruption bugs should be minimal ;)

I had something like that in mind for the file
pages.  I am a bit nervous about introducing
such a change while file and anonymous pages
share the same LRU, though...

--
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.
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