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 -- 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
