Re: [RFC][PATCH 4/6] Flat hierarchical reclaim by ID

From: Balbir Singh
Date: Tue Dec 09 2008 - 21:52:21 EST


* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2008-12-10 01:34:14]:

> Balbir Singh said:
>
> >> I think your soft-limit idea can be easily merged onto this patch
> >> set.
> >>
> >
> > Yes, potentially. With soft limit, the general expectation is this
> >
> > Let us say you have group A and B
> >
> > groupA, soft limit = 1G
> > groupB, soft limit = 2G
> >
> > Now assume the system has 4G. When groupB is not using its memory,
> > group A can grab all 4G, but when groupB kicks in and tries to use 2G
> > or more, then the expectation is that
> >
> > group A will get 1/3 * 4 = 4/3G
> > group B will get 2/3 * 4 = 8/3G
> >
> > Similar to CPU shares currently.
> >
> I like that idea because it's easy to understand.
>

Excellent, I'll start looking at how to implement it

> >> > Does this order reflect their position in the hierarchy?
> >> No. just scan IDs from last scannned one in RR.
> >> BTW, can you show what an algorithm works well in following case ?
> >> ex)
> >> groupA/ limit=1G usage=300M
> >> 01/ limit=600M usage=600M
> >> 02/ limit=700M usage=70M
> >> 03/ limit=100M usage=30M
> >> Which one should be shrinked at first and why ?
> >> 1) when group_A hit limits.
> >
> > With tree reclaim, reclaim will first reclaim from A and stop if
> > successful, otherwise it will go to 01, 02 and 03 and then go back to
> > A.
> >
> Sorry for my poor example
>
> >> 2) when group_A/01 hit limits.
> >
> > This will reclaim only from 01, since A is under its limit
> >
> I should ask
> 2') when a task in group_A/01 hit limit in group_A
>
> ex)
> group_A/ limtit=1G, usage~0
> /01 limit= unlimited usage=800M
> /02 limit= unlimited usage=200M
> (what limit is allowed to children is another problem to be fixed...)
> when a task in 01 hits limit of group_A
> when a task in 02 hits limit of group_A
> where we should start from ? (is unknown)
> Currenty , this patch uses RR (in A->01->02->A->...).
> and soft-limit or some good algorithm will give us better view.
>
> >> 3) when group_A/02 hit limits.
> >
> > This will reclaim only from 02 since A is under its limit
> >
> > Does RR do the same right now?
> >
> I think so.
>
> Assume
> group_A/
> /01
> /02
> RR does
> 1) when a task under A/01/02 hit limits at A, shrink A, 01, 02,
> 2) when a task under 01 hit limits at 01, shrink only 01.
> 3) when a task under 02 hit limits at 02, shrink only 02.
>
> When 1), start point of shrinking is saved as last_scanned_child.
>
>
> >> I can't now.
> >>
> >> This patch itself uses round-robin and have no special order.
> >> I think implenting good algorithm under this needs some amount of
> >> time.
> >>
> >
> > I agree that fine tuning it will require time, but what we need is
> > something usable that will not have hard to debug or understand corner
> > cases.
>
> yes, we have now. My point is "cgroup_lock()" caused many problems and
> will cause new ones in future, I convince.
>
> And please see 5/6 and 6/6 we need hierarchy consideration in other
> places. I think there are more codes which should take care of hierarchy.
>

Yes, I do have the patches to remove cgroup_lock(), let me post them
indepedent of Daisuke's patches

>
> > > Shouldn't id's belong to cgroups instead of just memory controller?
> >> If Paul rejects, I'll move this to memcg. But bio-cgroup people also use
> >> ID and, in this summer, I posted swap-cgroup-ID patch and asked to
> >> implement IDs under cgroup rather than subsys. (asked by Paul or you.)
> >>
> >
> > We should talk to Paul and convince him.
> >
> yes.
>

Paul, would it be very hard to add id's to control groups?

> >> >From implementation, hierarchy code management at el. should go into
> >> cgroup.c and it gives us clear view rather than implemented under memcg.
> >>
> >
> > cgroup has hierarchy management already, in the form of children and
> > sibling. Walking those structures is up to us, that is all we do
> > currently :)
> >
> yes, but need cgroup_lock(). and you have to keep refcnt to pointer
> just for rememebring it.
>
> This patch doesn't change anything other than removing cgroup_lock() and
> removing refcnt to remember start point.
>

OK, I'll play with it

--
Balbir
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