Re: Dynamic configure max_cstate

[Zhang, Yanmin]

On Tue, 2009-07-28 at 17:00 +0800, Zhang, Yanmin wrote:
> On Tue, 2009-07-28 at 09:20 +0200, Corrado Zoccolo wrote:
> > Hi,
> > On Tue, Jul 28, 2009 at 4:42 AM, Zhang,
> > Yanmin<yanmin_zhang@xxxxxxxxxxxxxxx> 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 ïprocessor.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?
> I'm not sure.
> 
> > 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'm using a SDV machine, not an official product. But it's rare that cpuid
> reports non-stop tsc feature while it doesn't support it.
> 
> 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.
> 
> > 
> > > 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?
> [ymzhang@lkp-ne02 ~]$ cat /proc/acpi/processor/CPU0/power 
> active state:            C0
> max_cstate:              C8
> maximum allowed latency: 2000000000 usec
> states:
>     C1:                  type[C1] promotion[--] demotion[--] latency[003] usage[00001661] duration[00000000000000000000]
>     C2:                  type[C3] promotion[--] demotion[--] latency[205] usage[00000687] duration[00000000000000732028]
>     C3:                  type[C3] promotion[--] demotion[--] latency[245] usage[00011509] duration[00000000000115186065]
> 
> [ymzhang@lkp-ne02 ~]$ cat /sys/devices/system/cpu/cpu0/cpuidle/state*/latency
> 0
> 3
> 205
> 245
> 
> > 
> > > 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.
Another interesting testing with netperf has the similiar behavior. I start 1 netperf client
and bind client and server to different physical cpus to run a UDP-RR-1 loopback testing.
The result is about 54000 without idle=poll while the one is 88000 with idle=poll.

If I start CPU_NUM netperf clients, there is no such issue, because all cpu are busy.


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