Re: [Lhms-devel] [RFC] free_area[] bitmap elimination [0/3]

[Dave Hansen <haveblue@us.ibm.com>]

On Fri, 2004-08-20 at 19:31, Hiroyuki KAMEZAWA wrote: 
> This patch removes bitmap from buddy allocator used in
> alloc_pages()/free_pages() in the kernel 2.6.8.1.

Looks very interesting.  The most mysterious thing about it that I can
think of right now would be its cache behavior.  Since struct pages are
at least 1/2 a cacheline on most architectures, you're going to dirty
quite a few more cachelines than if you were accessing a quick bitmap. 
However, if the page was recently accessed you might get *better*
cacheline performance because the struct page itself may have been
hotter than its bitmap.  

The use of page_count()==0 is a little worrisome.  There's almost
certainly some race conditions where a page can be mistaken for free
while it's page_count()==0, but before it's reached free_pages_bulk().

BTW, even if page_count()==0 isn't a valid check like you fear, you
could always steal a bit in the page->flags.  Check out 
free_pages_check() in mm/page_alloc.c for a nice summary of what state
pages have to be in before they're freed.  

I'll try and give these patches a run on a NUMA-Q today.  Those machines
are very cache-sensitive and should magnify any positive or negative
effects.  

-- Dave

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