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SubjectRe: [RFC][PATCH 8/8] RSS controller support reclamation
Pavel Emelianov wrote:
> Balbir Singh wrote:
> [snip]
>>> And what about a hard limit - how would you fail in page fault in
>>> case of limit hit? SIGKILL/SEGV is not an option - in this case we
>>> should run synchronous reclamation. This is done in beancounter
>>> patches v6 we've sent recently.
>> I thought about running synchronous reclamation, but then did not follow
>> that approach, I was not sure if calling the reclaim routines from the
>> page fault context is a good thing to do. It's worth trying out, since
> Each page fault potentially calls reclamation by allocating
> required page with __GFP_IO | __GFP_FS bits set. Synchronous
> reclamation in page fault is really normal.

True. I don't know what I was thinking, thanks for making me think

> [snip]
>>> Please correct me if I'm wrong, but does this reclamation work like
>>> "run over all the zones' lists searching for page whose controller
>>> is sc->container" ?
>> Yeah, that's correct. The code can also reclaim memory from all over-the-limit
> OK. What if I have a container with 100 pages limit in a 4Gb
> (~ million of pages) machine and this group starts reclaiming
> its pages. In case this group uses its pages heavily they will
> be at the beginning of an LRU list and reclamation code would
> have to scan through all (million) pages before it finds proper
> ones. This is not optimal!

Yes, thats possible. The trade off is between

The cost associated with traversing that list while reclaiming
and the complexity associated with task migration. If we keep
a per-container list of pages, during task migration, you'll have
to migrate pages (of the task) from the list to the new container.

>> containers (by passing SC_OVERLIMIT_ALL). The idea behind using such a scheme
>> is to ensure that the global LRU list is not broken.
> isolate_lru_pages() helps in this. As far as I remember this
> was introduced to reduce lru lock contention and keep lru
> lists integrity.
> In beancounters patches this is used to shrink BC's pages.

I'll look at isolate_lru_pages() to see if the reclaim can be optimized.

Thanks for your feedback,


Balbir Singh,
Linux Technology Center,
IBM Software Labs
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