Jamie Lokier wrote: > 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: > > >> [EMAIL PROTECTED] ~]$ 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. > No, something else is happening here. On kvm, a signal is maybe 10us, to the SIGALRMs represent 1% of overhead. Respectable, but not more.
I suspect that the increased accuracy causes something to behave better (or just differently). This won't be representative. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
