Re: [RFC 1/3] Slab infrastructure for array operations

[Joonsoo Kim]

On Wed, Jan 28, 2015 at 09:30:56AM -0600, Christoph Lameter wrote:
> On Wed, 28 Jan 2015, Joonsoo Kim wrote:
> 
> > > GFP_SLAB_ARRAY new is best for large quantities in either allocator since
> > > SLAB also has to construct local metadata structures.
> >
> > In case of SLAB, there is just a little more work to construct local metadata so
> > GFP_SLAB_ARRAY_NEW would not show better performance
> > than GFP_SLAB_ARRAY_LOCAL, because it would cause more overhead due to
> > more page allocations. Because of this characteristic, I said that
> > which option is
> > the best is implementation specific and therefore we should not expose it.
> 
> For large amounts of objects (hundreds or higher) GFP_SLAB_ARRAY_LOCAL
> will never have enough objects. GFP_SLAB_ARRAY_NEW will go to the page
> allocator and bypass free table creation and all the queuing that objects
> go through normally in SLAB. AFAICT its going to be a significant win.
> 
> A similar situation is true for the freeing operation. If the freeing
> operation results in all objects in a page being freed then we can also
> bypass that and put the page directly back into the page allocator (to be
> implemented once we agree on an approach).
> 
> > Even if we narrow down the problem to the SLUB, choosing correct option is
> > difficult enough. User should know how many objects are cached in this
> > kmem_cache
> > in order to choose best option since relative quantity would make
> > performance difference.
> 
> Ok we can add a function call to calculate the number of objects cached
> per cpu and per node? But then that is rather fluid and could change any
> moment.
> 
> > And, how many objects are cached in this kmem_cache could be changed
> > whenever implementation changed.
> 
> The default when no options are specified is to first exhaust the node
> partial objects, then allocate new slabs as long as we have more than
> objects per page left and only then satisfy from cpu local object. I think
> that is satisfactory for the majority of the cases.
> 
> The detailed control options were requested at the meeting in Auckland at
> the LCA. I am fine with dropping those if they do not make sense. Makes
> the API and implementation simpler. Jesper, are you ok with this?

IMHO, it'd be better to choose a proper way of allocation by slab itself
and not to expose options to API user. We could decide the best option
according to current status of kmem_cache and requested object number
and internal implementation.

Is there any obvious example these option are needed for user?

Thanks.
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