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SubjectRe: KSWAPD Algorithm
On Wed, 03 Dec 2008 15:51:11 +0100
Andi Kleen <andi@firstfloor.org> wrote:

> "wassim dagash" <wassim.dagash@gmail.com> writes:
>
> > Hi All,
> > Description:
> > I countered a weird problem with kswapd:
> > it runs in some infinite loop trying to swap until order 10 of zone
> > highmem is OK, While zone higmem (as I understand) has nothing to do
> > with contiguous physical memory (cause there is no 1-1 mapping) which
> > means kswapd will continue to try to balance order 10 of zone highmem
> > forever (or until someone release a very large chunk of highmem).
> > Can anyone please explain me the algorithm of kswapd and why it tries
> > to balance order 10 of zone higmem ?
>
> hugetlbfs requires large continuous mappings even in highmem
> to map 2MB pages or similar to user space.
> However order 10 seems pretty high, normally that's only order 8
> on a i386 system.
>

Reading Kosaki's explanation, it seems

- order-10 allocation occurs at ZONE_NORMAL.

- kswapd is kicked with order=10.
wakeup_kswaps(zone, 10) -> pgdat->kswapd_max_order = 10.

- kswapd(balance_pgdat()) does
if (!zone_watermark_ok(zone, order, zone->pages_high, end_zone, 0))
all_zones_ok = 0;
And this "zone" is all zone, i.e. DMA, NORMAL, HIGHMEM.

Then, loop again,

if (!all_zones_ok) {
cond_resched();

try_to_freeze();

goto loop_again;
}

Maybe...

-Kame



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