Re: Dynamic configure max_cstate

[Zhang, Yanmin]

On Thu, 2009-07-30 at 21:43 -0600, Robert Hancock wrote:
> On 07/28/2009 04:11 AM, Andreas Mohr wrote:
> > Hi,
> >
> > On Tue, Jul 28, 2009 at 05:00:35PM +0800, Zhang, Yanmin wrote:
> >> I tried different clocksources. For exmaple, I could get a better (30%) result with
> >> hpet. With hpet, cpu utilization is about 5~8%. Function hpet_read uses too much cpu
> >> time. With tsc, cpu utilization is about 2~3%. I think more cpu utilization causes fewer
> >> C state transitions.
> >>
> >> With idle=poll, the result is about 10% better than the one of hpet. If using idle=poll,
> >> I didn't find result difference among different clocksources.
> >
> > IOW, this seems to clearly point to ACPI Cx causing it.
> >
> > Both Corrado and me have been thinking that one should try skipping all
> > bigger-latency ACPI Cx states whenever there's an ongoing I/O request where an
> > immediate reply interrupt is expected.
> >
> > I've been investigating this a bit, and interesting parts would perhaps include
> >    . kernel/pm_qos_params.c
> >    . drivers/cpuidle/governors/menu.c (which acts on the ACPI _cx state
> > structs as configured by drivers/acpi/processor_idle.c)
> >    . and e.g. the wait_for_completion_timeout() part in drivers/ata/libata-core.c
> >      (or other sources in case of other disk I/O mechanisms)
> >
> > One way to do some quick (and dirty!!) testing would be to set a flag
> > before calling wait_for_completion_timeout() and testing for this flag in
> > drivers/cpuidle/governors/menu.c and then skip deeper Cx states
> > conditionally.
> >
> > As a very quick test, I tried a
> > while :; do :; done
> > loop in shell and renicing shell to 19 (to keep my CPU out of ACPI idle),
> > but bonnie -s 100 results initially looked promising yet turned out to
> > be inconsistent. The real way to test this would be idle=poll.
> > My test system was Athlon XP with /proc/acpi/processor/CPU0/power
> > latencies of 000 and 100 (the maximum allowed value, BTW) for C1/C2.
> >
> > If the wait_for_completion_timeout() flag testing turns out to help,
> > then one might intend to use the pm_qos infrastructure to indicate
> > these conditions, however it might be too bloated for such a
> > purpose, a relatively simple (read: fast) boolean flag mechanism
> > could be better.
> >
> > Plus one could then create a helper function which figures out a
> > "pretty fast" Cx state (independent of specific latency times!).
> > But when introducing this mechanism, take care to not ignore the
> > requirements defined by pm_qos settings!
> >
> > Oh, and about the places which submit I/O requests where one would have to
> > flag this: are they in any way correlated with the scheduler I/O wait
> > value? Would the I/O wait mechanism be a place to more easily and centrally
> > indicate that we're waiting for a request to come back in "very soon"?
> > OTOH I/O requests may have vastly differing delay expectations,
> > thus specifically only short-term expected I/O replies should be flagged,
> > otherwise we're wasting lots of ACPI deep idle opportunities.
> 
> Did the results show a big difference in performance between maximum C2 
> and maximum C3? 
No big difference. I tried different max cstate by processor.max_cstate.
Mostly, processor.max_cstate=1 could get the similiar result like idle=poll.

> Thing with C3 is that it likely will have some 
> interference with bus-master DMA activity as the CPU has to wake up at 
> least partially before the SATA controller can complete DMA operations, 
> which will likely stall the controller for some period of time. There 
> would be an argument for avoiding going into deep C-states which can't 
> handle snooping while IO is in progress and DMA will shortly be occurring..

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