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SubjectRe: [RFC 0/3] Implementation of cgroup isolation
On Tue 29-03-11 21:15:59, Zhu Yanhai wrote:
> Michal,

Hi,

> Maybe what we need here is some kind of trade-off?
> Let's say a new configuable parameter reserve_limit, for the cgroups
> which want to
> have some guarantee in the memory resource, we have:
>
> limit_in_bytes > soft_limit > reserve_limit
>
> MEM[limit_in_bytes..soft_limit] are the bytes that I'm willing to contribute
> to the others if they are short of memory.
>
> MEM[soft_limit..reserve_limit] are the bytes that I can afford if the others
> are still eager for memory after I gave them MEM[limit_in_bytes..soft_limit].
>
> MEM[reserve_limit..0] are the bytes which is a must for me to guarantee QoS.
> Nobody is allowed to steal them.
>
> And reserve_limit is 0 by default for the cgroups who don't care about Qos.
>
> Then the reclaim path also needs some changes, i.e, balance_pgdat():
> 1) call mem_cgroup_soft_limit_reclaim(), if nr_reclaimed is meet, goto finish.
> 2) shrink the global LRU list, and skip the pages which belong to the cgroup
> who have set a reserve_limit. if nr_reclaimed is meet, goto finish.

Isn't this an overhead that would slow the whole thing down. Consider
that you would need to lookup page_cgroup for every page and touch
mem_cgroup to get the limit.
The point of the isolation is to not touch the global reclaim path at
all.

> 3) shrink the cgroups who have set a reserve_limit, and leave them with only
> the reserve_limit bytes they need. if nr_reclaimed is meet, goto finish.
> 4) OOM
>
> Does it make sense?

It sounds like a good thing - in that regard it is more generic than
a simple flag - but I am afraid that the implementation wouldn't be
that easy to preserve the performance and keep the balance between
groups. But maybe it can be done without too much cost.

Thanks
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic


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