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SubjectRe: Dynamic configure max_cstate
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On Mon, 2009-07-27 at 09:33 +0200, Andreas Mohr wrote:
> Hi,
>
> > When running a fio workload, I found sometimes cpu C state has
> > big impact on the result. Mostly, fio is a disk I/O workload
> > which doesn't spend much time with cpu, so cpu switch to C2/C3
> > freqently and the latency is big.
>
> Rather than inventing ways to limit ACPI Cx state usefulness, we should
> perhaps be thinking of what's wrong here.
Andreas,

Thanks for your kind comments.

>
> And your complaint might just fit into a thought I had recently:
> are we actually taking ACPI Cx exit latency into account, for timers???
I tried both tickless kernel and non-tickless kernels. The result is similiar.

Originally, I also thought it's related to timer. As you know, I/O block layer
has many timers. Such timers don't expire normally. For example, an I/O request
is submitted to driver and driver delievers it to disk and hardware triggers
an interrupt after finishing I/O. Mostly, the I/O submit and interrupt, not
the timer, drive the I/O.

>
> If we program a timer to fire at some point, then it is quite imaginable
> that any ACPI Cx exit latency due to the CPU being idle at that moment
> could add to actual timer trigger time significantly.
>
> To combat this, one would need to tweak the timer expiration time
> to include the exit latency. But of course once the CPU is running
> again, one would need to re-add the latency amount (read: reprogram the
> timer hardware, ugh...) to prevent the timer from firing too early.
>
> Given that one would need to reprogram timer hardware quite often,
> I don't know whether taking Cx exit latency into account is feasible.
> OTOH analysis of the single next timer value and actual hardware reprogramming
> would have to be done only once (in ACPI sleep and wake paths each),
> thus it might just turn out to be very beneficial after all
> (minus prolonging ACPI Cx path activity and thus aggravating CPU power
> savings, of course).
>
> Arjan mentioned examples of maybe 10us for C2 and 185us for C3/C4 in an
> article.
>
> OTOH even 185us is only 0.185ms, which, when compared to disk seek
> latency (around 7ms still, except for SSD), doesn't seem to be all that much.
> Or what kind of ballpark figure do you have for percentage of I/O
> deterioration?
I have lots of FIO sub test cases which test I/O on single disk and JBOD (a disk
bos which mostly has 12~13 disks) on nahelam machines. Your analysis on disk seek
is reasonable. I found sequential buffered read has the worst regression while rand
read is far better. For example, I start 12 processes per disk and every disk has 24
1-G files. There are 12 disks. The sequential read fio result is about 593MB/second
with idle=poll, and about 375MB/s without idle=poll. Read block size is 4KB.

Another exmaple is single fio direct seqential read (block size is 4K) on a single
SATA disk. The result is about 28MB/s without idle=poll and about 32.5MB with
idle=poll.

How did I find C state has impact on disk I/O result? Frankly, I found a regression
between kernel 2.6.27 and 2.6.28. Bisect located a nonstop tsc patch, but the patch
is quite good. I found the patch changes the default clocksource from hpet to
tsc. Then, I tried all clocksources and got the best result with acpi_pm clocksource.
But oprofile data shows acpi_pm has more cpu utilization. clocksource jiffies has
worst result but least cpu utilization. As you know, fio calls gettimeofday frequently.
Then, I tried boot parameter processor.max_cstate and idle=poll.
I get the similar result with processor.max_cstate=1 like the one with idle=poll.

I also run the testing on 2 stoakley machines and don't find such issues.
/proc/acpi/processor/CPUXXX/power shows stoakley cpu only has C1.

> I'm wondering whether we might have an even bigger problem with disk I/O
> related to this than just the raw ACPI exit latency value itself.
We might have. I'm still doing more testing. With Venki's tool (write/read MSR registers),
I collected some C state switch stat.

Current cpuidle has a good consideration on cpu utilization, but doesn't have
consideration on devices. So with I/O delivery and interrupt drive model
with little cpu utilization, performance might be hurt if C state exit has a long
latency.

Yanmin


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