Messages in this thread | | | Date | Tue, 21 Aug 2007 23:03:17 -0700 | From | Zachary Amsden <> | Subject | Re: [PATCH] Add I/O hypercalls for i386 paravirt |
| |
H. Peter Anvin wrote: > Zachary Amsden wrote: > >> In general, I/O in a virtual guest is subject to performance problems. >> The I/O can not be completed physically, but must be virtualized. This >> means trapping and decoding port I/O instructions from the guest OS. >> Not only is the trap for a #GP heavyweight, both in the processor and >> the hypervisor (which usually has a complex #GP path), but this forces >> the hypervisor to decode the individual instruction which has faulted. >> Worse, even with hardware assist such as VT, the exit reason alone is >> not sufficient to determine the true nature of the faulting instruction, >> requiring a complex and costly instruction decode and simulation. >> >> This patch provides hypercalls for the i386 port I/O instructions, which >> vastly helps guests which use native-style drivers. For certain VMI >> workloads, this provides a performance boost of up to 30%. We expect >> KVM and lguest to be able to achieve similar gains on I/O intensive >> workloads. >> >> > > What about cost on hardware? >
On modern hardware, port I/O is about the most expensive thing you can do. The extra function call cost is totally masked by the stall. We have measured with port I/O converted like this on real hardware, and have seen zero measurable impact on macro-benchmarks. Micro-benchmarks that generate massively repeated port I/O might show some effect on ancient hardware, but I can't even imagine a workload which does such a thing, other than a polling port I/O loop perhaps - which would not be performance critical in any case I can reasonably imagine.
Zach - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |