2017-03-16 10:08 GMT+08:00 Longpeng (Mike) <longpeng2@xxxxxxxxxx>: > Hi, Radim, > > On 2017/3/16 1:32, Radim Krčmář wrote: > >> 2017-03-13 15:12+0800, Longpeng (Mike): >>> Hi guys, >>> >>> I'm confusing about the behavior of HLT instruction in VMX guest mode. >>> >>> I set "hlt exiting" bit to 0 in VMCS, and the vcpu didn't vmexit when execute >>> HLT as expected. However, I used powertop/cpupower on host to watch the pcpu's >>> c-states, it seems that the pcpu didn't enter C1/C1E state during this period. >>> >>> I searched the Intel spec vol-3, and only found that guest MWAIT won't entering >>> a low-power sleep state under certain conditions(ch 25.3), but not mentioned HLT. >>> >>> My questions are >>> 1) Does executing HLT instruction in guest-mode won't enter C1/C1E state ? >> >> Do you get a different result when running HLT outside VMX? >> > > > Yep, I'm sure that executing HLT in host will enter C1/C1E state, but it won't > when executing in guest. Execute hlt will enter C1 state, just mwait can enter C1E state. Regards, Wanpeng Li > >>> 2) If it won't, then whether it would release the hardware resources shared with >>> another hyper-thread ? >> > >> No idea. Aren't hyperthreaded resources scheduled dynamically, so even >> a nop-spinning VCPU won't hinder the other hyper-thread? >> > > > I had wrote a testcase in kvm-unit-tests, and it seems that guest-mode HLT-ed > vcpu won't compete the hardware resources( maybe including the pipeline ) any more. > > My testcase is: binding vcpu1 and vcpu2 to a core's 2 hyper-threads, and > > (vcpu1) > t1 = rdtsc(); > for (int i = 0; i < 10000000; ++i) ; > t2 = rdtsc(); > costs = t2 - t1; > > (vcpu2) > "halt" or "while (1) ;" > > The result is: > ----------------------------------------------------------------------- > (vcpu2)idle=poll (vcpu2)idle=halt > (HLT exiting=1) > vcpu1 costs 3800931 1900209 > > (HLT exiting=0) > vcpu1 costs 3800193 1913514 > ----------------------------------------------------------------------- > > I found that https://www.spinics.net/lists/kvm-commits/msg00137.html had maked > "HLT exiting" configurable, while > http://lkml.iu.edu/hypermail/linux/kernel/1202.0/03309.html removed it due to > redundant with CFS hardlimit. > > I focus on the VM's performance. According the result, I think running HLT in > guest-mode is better than idle=poll with HLT-exiting in *certain* scenarios. > >>> Any suggestion would be greatly appreciated, thanks! >> >> Mostly just more questions, sorry ... >> >> I'd look at temperature sensors while halting inside guests on all cores >> to see if they really enter a power saving mode -- I expect a noticeable >> difference from idle=poll. :) >> >> > > > -- > Regards, > Longpeng(Mike) >
