On 12/15/2016 04:48 PM, Li, Liang Z wrote: >>> It seems we leave too many bit for the pfn, and the bits leave for >>> length is not enough, How about keep 45 bits for the pfn and 19 bits >>> for length, 45 bits for pfn can cover 57 bits physical address, that should >>> be >> enough in the near feature. >>> What's your opinion? >> I still think 'order' makes a lot of sense. But, as you say, 57 bits is >> enough for >> x86 for a while. Other architectures.... who knows?
Thinking about this some more... There are really only two cases that matter: 4k pages and "much bigger" ones. Squeezing each 4k page into 8 bytes of metadata helps guarantee that this scheme won't regress over the old scheme in any cases. For bigger ranges, 8 vs 16 bytes means *nothing*. And 16 bytes will be as good or better than the old scheme for everything which is >4k. How about this: * 52 bits of 'pfn', 5 bits of 'order', 7 bits of 'length' * One special 'length' value to mean "actual length in next 8 bytes" That should be pretty simple to produce and decode. We have two record sizes, but I think it is manageable.
