Re: [RFC PATCH 0/3] generic hypercall support

Anthony Liguori
2009-05-08T19:05:50Z

Gregory Haskins wrote:
> Its more of an issue of execution latency (which translates to IO
> latency, since "execution" is usually for the specific goal of doing
> some IO). In fact, per my own design claims, I try to avoid exits like
> the plague and generally succeed at making very few of them. ;)
>
> So its not really the .4% reduction of cpu use that allures me. Its the
> 16% reduction in latency. Time/discussion will tell if its worth the
> trouble to use HC or just try to shave more off of PIO. If we went that
> route, I am concerned about falling back to MMIO, but Anthony seems to
> think this is not a real issue.
>

It's only a 16% reduction in latency if your workload is entirely
dependent on the latency of a hypercall. What is that workload? I
don't think it exists.

For a network driver, I have a hard time believing that anyone cares
that much about 210ns of latency. We're getting close to the cost of a
few dozen instructions here.

Regards,

Anthony Liguori