Re: [Lsf] [LSF][MM] page allocation & direct reclaim latency

[Ying Han <yinghan@google.com>]

On Tue, Mar 29, 2011 at 1:45 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> On Tue, Mar 29, 2011 at 01:35:24PM -0700, Ying Han wrote:
>> In page reclaim, I would like to discuss on the magic "8" *
>> high_wmark() in balance_pgdat(). I recently found the discussion on
>> thread "too big min_free_kbytes", where I didn't find where we proved
>> it is still a problem or not. This might not need reserve time slot,
>> but something I want to learn more on.
>
> That is merged in 2.6.39-rc1. It's hopefully working good enough. We
> still use high+balance_gap but the balance_gap isn't high*8 anymore. I
> still think the balance_gap may as well be zero but the gap now is
> small enough (not 600M on 4G machine anymore) that it's ok and this
> was a safer change.
>
> This is an LRU ordering issue to try to keep the lru balance across
> the zones and not just rotate a lot a single one. I think it can be
> covered in the LRU ordering topic too. But we could also expand it to
> a different slot if we expect too many issues to showup in that
> slot... Hugh what's your opinion?

Yes, that is what I got from the thread discussion and thank you for
confirming that. Guess my question is :

Do we need to do balance across zones by giving the fact that each
zone does its own balancing?
What is the problem we saw without doing the cross-zone balancing?

I don't have data to back-up either way, and that is something I am
interested too :)

--Ying


>
> The subtopics that comes to mind for that topic so far would be:
>
> - reclaim latency
> - compaction issues (Mel)
> - lru ordering altered by compaction/migrate/khugepaged or other
>  features requiring lru page isolation (Minchan)
> - lru rotation balance across zones in kswapd (balance_gap) (Ying)
>

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