Re: [RFC][PATCH][0/4] Memory controller (RSS Control)

[Balbir Singh <balbir@in.ibm.com>]

Magnus Damm wrote:
> On 2/19/07, Balbir Singh <balbir@in.ibm.com> wrote:
>> Magnus Damm wrote:
>> > On 2/19/07, Andrew Morton <akpm@linux-foundation.org> wrote:
>> >> On Mon, 19 Feb 2007 12:20:19 +0530 Balbir Singh <balbir@in.ibm.com>
>> >> wrote:
>> >>
>> >> > This patch applies on top of Paul Menage's container patches (V7)
>> >> posted at
>> >> >
>> >> >       http://lkml.org/lkml/2007/2/12/88
>> >> >
>> >> > It implements a controller within the containers framework for 
>> limiting
>> >> > memory usage (RSS usage).
>> >
>> >> The key part of this patchset is the reclaim algorithm:
>> >>
>> >> Alas, I fear this might have quite bad worst-case behaviour.  One 
>> small
>> >> container which is under constant memory pressure will churn the
>> >> system-wide LRUs like mad, and will consume rather a lot of system 
>> time.
>> >> So it's a point at which container A can deleteriously affect things
>> >> which
>> >> are running in other containers, which is exactly what we're 
>> supposed to
>> >> not do.
>> >
>> > Nice with a simple memory controller. The downside seems to be that it
>> > doesn't scale very well when it comes to reclaim, but maybe that just
>> > comes with being simple. Step by step, and maybe this is a good first
>> > step?
>> >
>>
>> Thanks, I totally agree.
>>
>> > Ideally I'd like to see unmapped pages handled on a per-container LRU
>> > with a fallback to the system-wide LRUs. Shared/mapped pages could be
>> > handled using PTE ageing/unmapping instead of page ageing, but that
>> > may consume too much resources to be practical.
>> >
>> > / magnus
>>
>> Keeping unmapped pages per container sounds interesting. I am not quite
>> sure what PTE ageing, will it look it up.
> 
> You will most likely have no luck looking it up, so here is what I
> mean by PTE ageing:
> 
> The most common unit for memory resource control seems to be physical
> pages. Keeping track of pages is simple in the case of a single user
> per page, but for shared pages tracking the owner becomes more
> complex.
> 
> I consider unmapped pages to only have a single user at a time, so the
> unit for unmapped memory resource control is physical pages. Apart
> from implementation details such as fun with struct page and
> scalability, handling this case is not so complicated.
> 
> Mapped or shared pages should be handled in a different way IMO. PTEs
> should be used instead of using physical pages as unit for resource
> control and reclaim. For the user this looks pretty much the same as
> physical pages, apart for memory overcommit.
> 
> So instead of using a global page reclaim policy and reserving
> physical pages per container I propose that resource controlled shared
> pages should be handled using a PTE replacement policy. This policy is
> used to keep the most active PTEs in the container backed by physical
> pages. Inactive PTEs gets unmapped in favour over newer PTEs.
> 
> One way to implement this could be by populating the address space of
> resource controlled processes with multiple smaller LRU2Qs. The
> compact data structure that I have in mind is basically an array of
> 256 bytes, one byte per PTE. Associated with this data strucuture are
> start indexes and lengths for two lists. The indexes are used in a
> FAT-type of chain to form single linked lists. So we create active and
> inactive list here - and we move PTEs between the lists when we check
> the young bits from the page reclaim and when we apply memory
> pressure. Unmapping is done through the normal page reclaimer but
> using information from the PTE LRUs.
> 
> In my mind this should lead to more fair resource control of mapped
> pages, but if it is possible to implement with low overhead, that's
> another question. =)
> 
> Thanks for listening.
> 
> / magnus
> 

Thanks for explaining PTE aging.

-- 
	Warm Regards,
	Balbir Singh