On Fri, 02 May 2008 14:48:27 -0700 (PDT)
David Miller <davem@davemloft.net> wrote:
it's a sad affair indeed. On some systems it counts cycles, on other
systems it counts time. On most systems it stops while idle, on others
it keeps running. On most systems its not very synchronized between
packages, and on some systems it's not even synchronized between cores.
I'm not convinced TSC is the right thing for the scheduler in the first
place; on current x86 systems TSC counts "time" not "work done". Now of
course "time" is an approximation for "work done", but not a very good
one given the presence of what effectively is variable cpu speeds
(software CPU frequency control is only part of that; there's also the
finegrained hardware level frequency control as done by what marketing
people call "Intel Dynamic Acceleration technology"). [*]
I and others have talked to Peter about this already, and I'm sure we'll
talk more about it in the future as well.. at some point this part of
CFS needs to fundamentally be cleaned up. Since this gets into a debate
about what fairness means ;(
[*] The converse is also true; cycles aren't a good representation of
time either; this makes cycle based profilers a bit iffy if you're
interested in where the system spends time rather than where it spends
cycles.
--
