On 11/30/2015 12:33 PM, 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: > > BM v4.4-rc2 v4.4-rc2-wsinc overhead > -- -------- -------------- -------- > Apache 5297.11 5243.77 101.02% > fio rand read 4354.33 4294.50 101.39% > fio rand write 2465.33 2231.33 110.49% > hackbench 17.48 19.78 113.16% > memcached 96442.69 101274.04 95.23% > TCP_MAERTS 5966.89 6029.72 98.96% > TCP_STREAM 6284.60 6351.74 98.94% > TCP_RR 15044.71 14324.03 105.03% > pbzip2 c 18.13 17.89 98.68% > pbzip2 d 11.42 11.45 100.26% > kernbench 50.13 50.28 100.30% > mysql 1 152.84 154.01 100.77% > mysql 2 98.12 98.94 100.84% > mysql 4 51.32 51.17 99.71% > mysql 8 27.31 27.70 101.42% > mysql 20 16.80 17.21 102.47% > mysql 100 13.71 14.11 102.92% > mysql 200 15.20 15.20 100.00% > mysql 400 17.16 17.16 100.00% > > (you want to see this with a viewer that renders clear-text and tabs > properly) > > 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. > > -Christoffer > _______________________________________________ > kvmarm mailing list > kvmarm@xxxxxxxxxxxxxxxxxxxxx > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm > I ran some of the lmbench 'micro benchmarks' - currently the usleep one consistently stands out by about .4% or extra 300ns per sleep. Few other ones have some outliers, I will look at these closer. Tests were ran on Juno. - Mario -- 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
