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

Dor Laor
2007-08-19T19:29:10Z

>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.

while I don't know the benchmark [I head it's something like paralled
chat messaging,
the performance gain is probably achieved by improved latency and
response times that
the dyn-tick provides.