physical memory granularity
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? Or any strong opinions on what granule sizes we should support?
Bjorn
Quoted 13 lines
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.
Quoted 4 lines
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.
Quoted 10 lines
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) [email protected]