[Devel] Re: [RFC][PATCH 4/7] RSS accounting hooks over the code

[Balbir Singh]

Nick Piggin wrote:
> Kirill Korotaev wrote:
> 
>>> The approaches I have seen that don't have a struct page pointer, do
>>> intrusive things like try to put hooks everywhere throughout the kernel
>>> where a userspace task can cause an allocation (and of course end up
>>> missing many, so they aren't secure anyway)... and basically just
>>> nasty stuff that will never get merged.
>>
>>
>> User beancounters patch has got through all these...
>> The approach where each charged object has a pointer to the owner 
>> container,
>> who has charged it - is the most easy/clean way to handle
>> all the problems with dynamic context change, races, etc.
>> and 1 pointer in page struct is just 0.1% overehad.
> 
> The pointer in struct page approach is a decent one, which I have
> liked since this whole container effort came up. IIRC Linus and Alan
> also thought that was a reasonable way to go.
> 
> I haven't reviewed the rest of the beancounters patch since looking
> at it quite a few months ago... I probably don't have time for a
> good review at the moment, but I should eventually.
> 

This patch is not really beancounters.

1. It uses the containers framework
2. It is similar to my RSS controller (http://lkml.org/lkml/2007/2/26/8)

I would say that beancounters are changing and evolving.

>>> Struct page overhead really isn't bad. Sure, nobody who doesn't use
>>> containers will want to turn it on, but unless you're using a big PAE
>>> system you're actually unlikely to notice.
>>
>>
>> big PAE doesn't make any difference IMHO
>> (until struct pages are not created for non-present physical memory 
>> areas)
> 
> The issue is just that struct pages use low memory, which is a really
> scarce commodity on PAE. One more pointer in the struct page means
> 64MB less lowmem.
> 
> But PAE is crap anyway. We've already made enough concessions in the
> kernel to support it. I agree: struct page overhead is not really
> significant. The benefits of simplicity seems to outweigh the downside.
> 
>>> But again, I'll say the node-container approach of course does avoid
>>> this nicely (because we already can get the node from the page). So
>>> definitely that approach needs to be discredited before going with this
>>> one.
>>
>>
>> But it lacks some other features:
>> 1. page can't be shared easily with another container
> 
> I think they could be shared. You allocate _new_ pages from your own
> node, but you can definitely use existing pages allocated to other
> nodes.
> 
>> 2. shared page can't be accounted honestly to containers
>>    as fraction=PAGE_SIZE/containers-using-it
> 
> Yes there would be some accounting differences. I think it is hard
> to say exactly what containers are "using" what page anyway, though.
> What do you say about unmapped pages? Kernel allocations? etc.
> 
>> 3. It doesn't help accounting of kernel memory structures.
>>    e.g. in OpenVZ we use exactly the same pointer on the page
>>    to track which container owns it, e.g. pages used for page
>>    tables are accounted this way.
> 
> ?
> page_to_nid(page) ~= container that owns it.
> 
>> 4. I guess container destroy requires destroy of memory zone,
>>    which means write out of dirty data. Which doesn't sound
>>    good for me as well.
> 
> I haven't looked at any implementation, but I think it is fine for
> the zone to stay around.
> 
>> 5. memory reclamation in case of global memory shortage
>>    becomes a tricky/unfair task.
> 
> I don't understand why? You can much more easily target a specific
> container for reclaim with this approach than with others (because
> you have an lru per container).
> 

Yes, but we break the global LRU. With these RSS patches, reclaim not
triggered by containers still uses the global LRU, by using nodes,
we would have lost the global LRU.

>> 6. You cannot overcommit. AFAIU, the memory should be granted
>>    to node exclusive usage and cannot be used by by another containers,
>>    even if it is unused. This is not an option for us.
> 
> I'm not sure about that. If you have a larger number of nodes, then
> you could assign more free nodes to a container on demand. But I
> think there would definitely be less flexibility with nodes...
> 
> I don't know... and seeing as I don't really know where the google
> guys are going with it, I won't misrepresent their work any further ;)
> 
> 
>>> Everyone seems to have a plan ;) I don't read the containers list...
>>> does everyone still have *different* plans, or is any sort of consensus
>>> being reached?
>>
>>
>> hope we'll have it soon :)
> 
> Good luck ;)
> 

I think we have made some forward progress on the consensus.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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