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RE: [kvm-devel] [Qemu-devel] Re: [PATCH 0/4] Rework alarmtimer infrastru

From: Dor Laor
Subject: RE: [kvm-devel] [Qemu-devel] Re: [PATCH 0/4] Rework alarmtimer infrastrucure - take2
Date: Sun, 19 Aug 2007 12:29:10 -0700

>Paul Brook wrote:
>> > Yes, good thinking, but this should only be done if it actually
>> > 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
>> place?
>A benchmark result was posted which is rather interesting:
>>address@hidden ~]$ 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.

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