Re: [PATCH 4/10] alternate 4-level page tables patches

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:
> Nick Piggin wrote:
> 
>> Andi Kleen wrote:
>>
> 
>>> Another way I thought about was to have a reference count of the used
>>> ptes/pmds per page table page in struct page and free the page when 
>>> it goes to zero. That would give perfect garbage collection. Drawback 
>>> is that
>>> it may be a bit intrusive again.
>>>
>>
>> Yes I thought about that a bit too.
>>
>> Note that this (4/10) patch should give perfect garbage collection too
>> (modulo bugs). The difference is in where the overheads lie. I suspect
>> refcounting may be too much overhead (at least, SMP overhead); especially
>> in light of Christoph's results.
>>
> 
> Hmm... you could refcount just the pud and pmd directories, and

.. or to hide my bias, the pgd and pmd!

My thinking is that populating of these directories should happen
infrequently enough that an atomic counter shouldn't show up on the
radar, neither for single threaded nor massively multithreaded
perfomance.

In the current fully ptl locked scheme, it wouldn't even have to be
an atomic counter...
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