Re: Dynamic configure max_cstate

[Corrado Zoccolo <czoccolo@gmail.com>]

Hi,
On Tue, Jul 28, 2009 at 4:42 AM, Zhang,
Yanmin<yanmin_zhang@linux.intel.com> wrote:
> 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.
>

Is it possible that the different bandwidths figures are due to
incorrect timing, instead of C-state latencies?
Entering a deep C state can cause strange things to timers: some of
them, especially tsc, become unreliable.
Maybe the patch you found that re-enables tsc is actually wrong for
your machine, for which tsc is unreliable in deep C states.

> 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.
>
You can see the latencies (expressed in us) on your machine with:
[root@localhost corrado]# cat
/sys/devices/system/cpu/cpu0/cpuidle/state*/latency
0
0
1
133

Can you post your numbers, to see if they are unusually high?

> 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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-- 
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dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
PhD - Department of Computer Science - University of Pisa, Italy
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