From: Rusty Russell <rusty@rustcorp.com.au>
To: Eric Dumazet <dada1@cosmosbay.com>
Subject: Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access
Date: Sun, 8 Jun 2008 16:00:12 +1000
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On Friday 06 June 2008 15:33:22 Eric Dumazet wrote:
> 1) NUMA case
>
> For a 64 bit NUMA arch, chunk size of 2Mbytes
>
> Allocates 2Mb for each possible processor (on its preferred memory
> node), and compute values to setup offset_of_cpu[NR_CPUS] array.
>
> Chunk 0
> CPU 0 : virtual address XXXXXX
> CPU 1 : virtual address XXXXXX + offset_of_cpu[1]
> ...
> CPU n : virtual address XXXXXX + offset_of_cpu[n]
> + a shared bitmap
>
>
> For next chunks, we could use vmalloc() zone to find
> nr_possible_cpus virtual addresses ranges where you can map
> a 2Mb page per possible cpu, as long as we respect the relative
> delta between each cpu block, that was computed when
> chunk 0 was setup.
>
> Chunk 1..n
> CPU 0 : virtual address YYYYYYYYYYYYYY
> CPU 1 : virtual address YYYYYYYYYYYYYY + offset_of_cpu[1]
> ...
> CPU n : virtual address YYYYYYYYYYYYYY + offset_of_cpu[n]
> + a shared bitmap (32Kbytes if 8 bytes granularity in allocator)
>
> For a variable located in chunk 0, its 'address' relative to current
> cpu %gs will be some number between [0 and 2^20-1]
>
> For a variable located in chunk 1, its 'address' relative to current
> cpu %gs will be some number between
> [YYYYYYYYYYYYYY - XXXXXX  and YYYYYYYYYYYYYY - XXXXXX + 2^20 - 1],
> not necessarly [2^20 to 2^21 - 1]
>
>
> Chunk 0 would use normal memory (no vmap TLB cost), only next ones need
> vmalloc().
>
> So the extra TLB cost would only be taken for very special NUMA setups
> (only if using a lot of percpu allocations)
>
> Also, using a 2Mb page granularity probably wastes about 2Mb per cpu, but
> this is nothing for NUMA machines :)

If you're prepared to have mappings for chunk 0, you can simply make it 
virtually linear and creating a new chunk is simple.  If not, you need to 
reserve the virtual address space(s) for future mappings.  Otherwise you're 
unlikely to get the same layout for allocations.

This is not a show-stopper: we've lived with limited vmalloc room since 
forever.  It just has to be sufficient.

Otherwise, your analysis is correct, if a little verbose :)

Cheers,
Rusty.
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