Call for performance evaluation: net.isr.direct (fwd)

[Poul-Henning Kamp]

In message <20051014192509.F80520 at delplex.bde.org>, Bruce Evans writes:
>On Fri, 14 Oct 2005, Poul-Henning Kamp wrote:
>
>> In message <17230.62415.991707.840932 at grasshopper.cs.duke.edu>, Andrew Gallatin
>> writes:
>>
>>> Linux already takes care of syncing the TSC between SMP cpus, so we
>>> know it is possible.  This seems like a much more doable optimization.
>>> And it is likely to have other benefits..
>
>The timestamps in mi_switch() are taken on the same CPU and only their
>differences are used, so they don't even need to be synced.  It they
>use the TSC, then the TSCs just need to have the same almost-constant
>frequency (or different almost-constant frequencies if timecounters
>werre per-CPU).

Actually, I think we need to go back a step further.

The task of the scheduler is to hand out a finite resource according
to a set policy.

The finite resource is "instructions executed by a CPU".

It used to be that CPUs ran at constant clock rates, and therefore
implementors made the simplifying assumption that

	instructions = a * time

for some random but constant a and made their scheduling decisions
based on time.


Today CPUs do not run on constant rates but they have counters which
count the number of instruction cycles.  Therefore talking about
computer effort in terms of "CPU second" is like selling rubber
band by the inch.

The scheduler has a side job of accounting for CPU usage and the
API for accesing this info has unfortunately been specified in
terms of time rather than instructions.


The best compromise solution therefore is to change the scheduler
to make decisions based on the TSC ticks (or equivalent on other
archs) and at regular intervals figure out how fast the CPU ran in
the last period and convert the TSC ticks accumulated to a time
unit suitable for resource accounting.


The bad solution is to try to do timekeeping based on hardware
counters which are unsuitable for the purpose, the TSC being
the primary suspect here, and we will not do that.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.