Re: memory resource accounting (was Re: [RFC, PATCH 0/5] Going forward with Resource Management - A cpu controller)

[Martin Bligh <mbligh@mbligh.org>]

>> It also saves you from maintaining huge lists against each page.
>>
>> Worse case, you want to bill everyone who opens that address_space
>> equally. But the semantics on exit still suck.
>>
>> What was Alan's quote again? "unfair, unreliable, inefficient ...
>> pick at least one out of the three". or something like that.
> 
> What's the sucking semantics on exit? I haven't looked much at the
> existing memory controllers going around, but the implementation I
> imagine looks something like this (I think it is conceptually similar
> to the basic beancounters idea):

You have to increase the other processes allocations, putting them
over their limits. If you then force them into reclaim, they're going
to stall, and give bad latency.

> - anyone who allocates a page for anything gets charged for that page.
>   Except interrupt/softirq context. Could we ignore these for the moment?
> 
>   This does give you kernel (slab, pagetable, etc) allocations as well as
>   userspace. I don't like the idea of doing controllers for inode cache
>   and controllers for dentry cache, etc, etc, ad infinitum.
> 
> - each struct page has a backpointer to its billed container. At the mm
>   summit Linus said he didn't want back pointers, but I clarified with him
>   and he isn't against them if they are easily configured out when not 
>   using memory controllers.
> 
> - memory accounting containers are in a hierarchy. If you want to destroy a
>   container but it still has billed memory outstanding, that gets charged
>   back to the parent. The data structure itself obviously still needs to
>   stay around, to keep the backpointers from going stale... but that could
>   be as little as a word or two in size.
> 
> The reason I like this way of accounting is that it can be done with a 
> couple
> of hooks into page_alloc.c and an ifdef in mm.h, and that is the extent of
> the impact on core mm/ so I'd be against anything more intrusive unless 
> this
> really doesn't work.
> 

See "inefficent" above (sorry ;-)) What you've chosen is more correct,
but much higher overhead. The point was that there's tradeoffs either
way - the conclusion we came to last time was that to make it 100%
correct, you'd be better off going with a model like Xen.

1. You're adding a backpointer to struct page.

2. Each page is not accounted to one container, but shared across them,
so the billing changes every time someone forks or exits. And not just
for that container, but all of them. Think pte chain based rmap ...
except worse.

3. When a container needs to "shrink" when somebody else exits, how do
we do reclaim pages from a specific container?

M.