Re: [patch] SLQB slab allocator

Nick Piggin
2009-01-23T15:53:21Z

On Fri, Jan 23, 2009 at 10:41:15AM -0500, Christoph Lameter wrote:
> 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.

According to memory policies, a task's memory policy is supposed to
apply to its slab allocations too.

> > > (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.

There are other memory policies than just interleave though.

> > "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.

But that is wrong. The lists obviously have high water marks that
get trimmed down. Periodic trimming as I keep saying basically is
alrady so infrequent that it is irrelevant (millions of objects
per cpu can be allocated anyway between existing trimming interval)