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 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? > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
