Re: [Linux-ia64] [RFD] physical memory granularity

[Jack Steiner <steiner@sgi.com>]

> 
> Linux on IA64 currently supports "granules" of 16MB and 64MB.  
> This is the page size used for the kernel identity-mapped
> regions.  Everything mapped by a page must actually exist (to
> avoid MCAs when prefetching into a hole) and must have the same
> attributes (to avoid attribute aliasing), so we assume that
> physical memory comes in granule-sized chunks.
> 
> DIG64 2.1 says nothing about the granularity of physical memory, 
> but it seems that for all existing IA64 machines, memory comes in
> chunks of at least 64MB.  But there's an HP machine in the pipe
> that may support only 4MB granularity.
> 
> I'm not keen on reducing the granule size to 4MB, because I'm 
> afraid the extra TLB misses will be a noticeable performance
> problem.  Does anybody have any quantitative data on the effect
> of granule size on performance?  

Certainly nothing conclusive, but ....

About 6 months ago, I ran the AIM7 benchmark with 16MB & 64MB 
granules. No measurable difference. The runs were indistinguishable.


However, with other benchmarks (or 4MB granules), YMMV.




> Or any strong opinions on what
> granule sizes we should support?
> 
> Bjorn
> 
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-- 
Thanks

Jack Steiner    (651-683-5302)   (vnet 233-5302)      steiner@sgi.com