On 30/11/15 20:33, Christoffer Dall wrote: > On Fri, Nov 27, 2015 at 06:49:54PM +0000, Marc Zyngier wrote: >> Once upon a time, the KVM/arm64 world switch was a nice, clean, lean >> and mean piece of hand-crafted assembly code. Over time, features have >> crept in, the code has become harder to maintain, and the smallest >> change is a pain to introduce. The VHE patches are a prime example of >> why this doesn't work anymore. >> >> This series rewrites most of the existing assembly code in C, but keeps >> the existing code structure in place (most function names will look >> familiar to the reader). The biggest change is that we don't have to >> deal with a static register allocation (the compiler does it for us), >> we can easily follow structure and pointers, and only the lowest level >> is still in assembly code. Oh, and a negative diffstat. >> >> There is still a healthy dose of inline assembly (system register >> accessors, runtime code patching), but I've tried not to make it too >> invasive. The generated code, while not exactly brilliant, doesn't >> look too shaby. I do expect a small performance degradation, but I >> believe this is something we can improve over time (my initial >> measurements don't show any obvious regression though). > > I ran this through my experimental setup on m400 and got this:
[...] > What this tells me is that we do take a noticable hit on the > world-switch path, which shows up in the TCP_RR and hackbench workloads, > which have a high precision in their output. > > Note that the memcached number is well within its variability between > individual benchmark runs, where it varies to 12% of its average in over > 80% of the executions. > > I don't think this is a showstopper thought, but we could consider > looking more closely at a breakdown of the world-switch path and verify > if/where we are really taking a hit. Thanks for doing so, very interesting. As a data point, what compiler are you using? I'd expect some variability based on the compiler version... Thanks, M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html