On Tue, Dec 01, 2015 at 09:58:23AM +0000, Marc Zyngier wrote: > 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... > I used the following (compiling natively on the m400): gcc version 4.8.2 (Ubuntu/Linaro 4.8.2-19ubuntu1) -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
