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

From: Jack Steiner (
Date: Mon Dec 10 2001 - 11:32:59 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:

I can give you 1 data point. This is for the SGI SN1 platform. This is a NUMA
platform & is running with the DISCONTIGMEM patch that is on sourceforge.

The virt_to_page() function currently generates the following code:

        23 instructions
                18 "real" instructions
                 5 noop (I would like to believe the compiler can eventually
                   use these instructions slots for something else)

The code has
        2 load instructions that are always reference node-local memory & have
          a high probability of hitting in the caches
        1 load to the node that contains the target page

I think I see a couple opportunities for reducing the amount of code. However,
I consider the code to be "fast enough" for most purposes.

> * 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.

BTW, I think Tony Luck (at Intel) is currently changing the slab allocator
to be numa-aware. Are coordinating your work with his???


Jack Steiner (651-683-5302) (vnet 233-5302)

- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to More majordomo info at Please read the FAQ at

This archive was generated by hypermail 2b29 : Sat Dec 15 2001 - 21:00:17 EST