cvs commit: src/sys/amd64/amd64 cpu_switch.S machdep.c

[Nate Lawson]

Nate Lawson wrote:
> Andrew Gallatin wrote:
> 
>> Poul-Henning Kamp writes:
>>  > In message <20051018094402.A29138 at grasshopper.cs.duke.edu>, Andrew 
>> Gallatin wri
>>  > tes:
>>  >  > >It is a shame we can't find a way to use the TSC as a 
>> timecounter on
>>  > >SMP systems.  It seems that about 40% of the context switch time is
>>  > >spent just waiting for the PIO read of the ACPI-fast or i8254 to
>>  > >return.
>>  >  > No, the shame is that the scheduler tries to partition time rather
>>  > than cpu cycles because that approximation got goldplated in some
>>  > random standard years back.
>>
>> Sorry if I mi-spoke.  I guess the shame twofold. 
>> First we insist on not trying keep the TSC in sync and so we don't use
>> it for SMP timekeeping like other OSes do, which means that getting a
>> micro-second granularity timestamp is orders of magnitude more
>> expensive for us.  To compound the problem, we insist on using the
>> expensive non-TSC binuptime() to get a runtime measurement on each
>> context switch, rather than being able to use something cheap like
>> ticks, or a per-cpu cycle counter.
> 
> 
> I have good information that in the near future, most designs will have 
> guaranteed synchronized TSC across all CPUs.

Oops, I not only meant "synchronized" but also "the same value".

-- 
Nate