Re: [BENCHMARKS] Namesys VM patches improve kbuild

Nick Piggin
2004-01-24T01:07:03Z

Nikita Danilov wrote:

>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.
>

Yes, but why doesn't my small patch have the same effect?
It doesn't do it by nicely seperating mapped and non mapped
pages like yours, but it should do something similar: ignore
all mapped pages until the reclaim_mapped threshold.

>
>
> >
> > 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 by clearing the referenced bit when below the reclaim_mapped
threshold, you're throwing this information away.

Say you have 16 mapped pages on the active list, 8 referenced, 8 not.
You do a !reclaim_mapped scan. Your 16 pages are now in the same
order and none are referenced. You now do a reclaim_mapped scan and
reclaim 8 pages. 4 of them were the referenced ones, 4 were not.

With my change, you would reclaim all 8 non referenced pages.