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

From: Manfred Spraul (
Date: Sun Dec 09 2001 - 05:57:55 EST

>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.


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