Re: [patch] SLQB slab allocator

Christoph Lameter
2009-01-23T15:44:19Z

On Fri, 23 Jan 2009, Nick Piggin wrote:

> > No it cannot because in SLUB objects must come from the same page.
> > Multiple objects in a queue will only ever require a single page and not
> > multiple like in SLAB.
>
> I don't know how that solves the problem. Task with memory policy A
> allocates an object, which allocates the "fast" page with policy A
> and allocates an object. Then context switch to task with memory
> policy B which allocates another object, which is taken from the page
> allocated with policy A. Right?

Correct. But this is only an issue if you think about policies applying to
individual object allocations (like realized in SLAB). If policies only
apply to pages (which is sufficient for balancing IMHO) then this is okay.

> > (OK this doesn't give the wrong policy 100% of the time; I thought
> there could have been a context switch race during page allocation
> that would result in 100% incorrect, but anyway it could still be
> significantly incorrect couldn't it?)

Memory policies are applied in a fuzzy way anyways. A context switch can
result in page allocation action that changes the expected interleave
pattern. Page populations in an address space depend on the task policy.
So the exact policy applied to a page depends on the task. This isnt an
exact thing.

> "the first cpu will consume more and more memory from the page allocator
> whereas the second will build up huge per cpu lists"
>
> And this is wrong. There is another possible issue where every single
> object on the freelist might come from a different (and otherwise free)
> page, and thus eg 100 8 byte objects might consume 400K.
>
> That's not an invalid concern, but I think it will be quite rare, and
> the periodic queue trimming should naturally help this because it will
> cycle out those objects and if new allocations are needed, they will
> come from new pages which can be packed more densely.

Well but you said that you would defer the trimming (due to latency
concerns). The longer you defer the larger the lists will get.