Re: [Lse-tech] [RFC] [PATCH] Scalable Statistics Counters

[Manfred Spraul]

>
>
>Assuming the slab allocator manages by node, kmem_cache_alloc_node() & 
>kmem_cache_alloc_cpu() would be identical (exzcept for spelling :-). 
>Each would pick up the nodeid from the cpu_data struct, then allocate 
>from the slab cache for that node.
>

kmem_cache_alloc is simple - the complex operation is kmem_cache_free.

The current implementation
- assumes that virt_to_page() and reading one cacheline from the page 
structure is fast. Is that true for your setups?
- uses an array to batch several free calls together: If the array 
overflows, then up to 120 objects are freed in one call, to reduce 
cacheline trashing.

If virt_to_page is fast, then a NUMA allocator would be a simple 
extention of the current implementation:

* one slab chain for each node, one spinlock for each node.
* 2 per-cpu arrays for each cpu: one for "correct node" kmem_cache_free 
calls , one for "foreign node" kmem_cache_free calls.
* kmem_cache_alloc allocates from the "correct node" per-cpu array, 
fallback to the per-node slab chain, then fallback to __get_free_pages.
* kmem_cache_free checks to which node the freed object belongs and adds 
it to the appropriate per-cpu array. The array overflow function then 
sorts the objects into the correct slab chains.

If virt_to_page is slow we need a different design. Currently it's 
called in every kmem_cache_free/kfree call.

--
    Manfred