Re: [BENCHMARKS] Namesys VM patches improve kbuild

Nikita Danilov
2004-01-23T15:40:10Z

Nick Piggin writes:
>
>
> Nikita Danilov wrote:
>
> >Nick Piggin writes:
> > >
> >
> >[...]
> >
> > >
> > > But those cold mapped pages are basically ignored until the
> > > reclaim_mapped threshold, however they do continue to have their
> > > referenced bits cleared - hence page_referenced check should
> > > become a better estimation when reclaim_mapped is reached, right?
> >
> >Right.
> >
>
>
> I still am a bit skeptical that the LRU lists are actually LRU,
> however I'm running out of other explainations for your patch's
> improvements :)

They are not LRU, it is impossible (and useless) to have LRU in the VM.

The problem that we have, and that dont-rotate-active-list tries to
address, is that two different LRU _approximations_ are maintained
within the same page queues. This patch tries (lazily) to separate pages
handled differently so that they don't interfere with each other.

>
> One ideas I had turns out to have little effect for kbuild, but
> it might still be worth including?
>
> When reclaim_mapped == 0 mapped referenced pages are treated
> the same way as mapped unreferenced pages, and the referenced
> info is thrown out. Fixed by not clearing referenced bits.

I think that purpose of having active/inactive lists in the first place
is to tell hot pages from cold one. Hotness of page is estimated on the
basis of how frequently it has been accessed _recently_: if page was
accessed while migrating through the active list---it is hot. When the
memory pressure increases, the active list is scanned more aggressively
and the time that the page spends on it (which is the time it has to get
a reference) decreases, thus adjusting VM's notion of the hotness.

By not clearing the referenced bit, one loses the ability to tell recent
accesses from the old ones. As a result, all mapped pages that were ever
accessed from the bootup would appear as hot when reclaim_mapped is
reached.

But,

"The practice is the criterion of the truth" :)

>
> linux-2.6-npiggin/mm/vmscan.c | 10 ++++++----
> 1 files changed, 6 insertions(+), 4 deletions(-)
>
> diff -puN mm/vmscan.c~vm-info mm/vmscan.c
> --- linux-2.6/mm/vmscan.c~vm-info 2004-01-24 00:50:15.000000000 +1100

Nikita.