Re: #tj-percpu has been rebased

[H. Peter Anvin]

Rusty Russell wrote:
>>
>> All in all I think a dedicated virtual zone per CPU as opposed to
>> interleaving them seems to make more sense.  Even with 4096 CPUs and
>> reserving, say, 256 MB per CPU it's not that much address space in the
>> context of a 47-bit kernel space.  On 32 bits I don't think anything but
>> the most trivial amount of percpu space is going to fly no matter what.
> 
> It's the TLB cost which I really don't want to pay; num_possible_cpus()
> 4096 non-NUMA is a little silly (currently impossible).
> 
> I'm happy to limit per-cpu allocations to pagesize, then you only need to
> find num_possible_cpus() contig pages, and if you can't, you fall back to
> vmalloc.
> 

num_possible_cpus() can be very large though, so in many cases the
likelihood of finding that many pages approach zero.  Furthermore,
num_possible_cpus() may be quite a bit larger than the actual number of
CPUs in the system.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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