On Tue, Apr 06, 2010 at 04:22:37PM +0300, Avi Kivity wrote: > On 04/06/2010 04:10 PM, Nick Piggin wrote: > >Actual workloads are infinitely more useful. And in > >most cases, quite possibly hardware improvements like asids will > >be more useful. > > This already has ASIDs for the guest; and for the host they wouldn't > help much since there's only one process running. I didn't realize these improvements were directed completely at the virtualized case. > I don't see how > hardware improvements can drastically change the numbers above, it's > clear that for the 4k case the host takes a cache miss for the pte, > and twice for the 4k/4k guest case. It's because you're missing the point. You're taking the most unrealistic and pessimal cases and then showing that it has fundamental problems. Speedups like Linus is talking about would refer to ways to speed up actual workloads, not ways to avoid fundamental limitations. Prefetching, memory parallelism, caches. It's worked for 25 years :) > >I don't really agree with how virtualization problem is characterised. > >Xen's way of doing memory virtualization maps directly to normal > >hardware page tables so there doesn't seem like a fundamental > >requirement for more memory accesses. > > The Xen pv case only works for modified guests (so no Windows), and > doesn't support host memory management like swapping or ksm. Xen > hvm (which runs unmodified guests) has the same problems as kvm. > > Note kvm can use a single layer of translation (and does on older > hardware), so it would behave like the host, but that increases the > cost of pte updates dramatically. So it is fundamentally possible. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>