Re: [Qemu-devel] Re: [PATCH 0/4] Rework alarm timer infrastrucure - take2

Jamie Lokier
2007-08-19T17:15:45Z

Paul Brook wrote:
> > Yes, good thinking, but this should only be done if it actually impacts
> > something. Reducing overhead from 0.1% to 0.05% is not worthwhile if it
> > introduces extra complexity.
>
> If the overhead is that small, why are we touching this code in the first
> place?

Insightful.

A benchmark result was posted which is rather interesting:

>[mkent@localhost ~]$ time ./hackbench 50
>x86_64 host : real 0m10.845s
>x86_64 host, bound to 1 cpu : real 0m21.884s
>i386 guest+unix clock : real 0m49.206s
>i386 guest+hpet clock : real 0m48.292s
>i386 guest+dynticks clock : real 0m28.835s
>
>Results are repeatable and verfied with a stopwatch because I didn't
>believe them at first :)

I am surprised if 1000 redundant SIGALRMs per second is really causing
70% overhead in normal qemu execution, except on a rather old or slow
machine where signal delivery is very slow.

It would be good to understand the cause of that benchmark result.

-- Jamie