On 2017/7/14 17:37, Alexander Graf wrote:



On 13.07.17 13:49, Yang Zhang wrote:
On 2017/7/4 22:13, Radim Krčmář wrote:
2017-07-03 17:28+0800, Yang Zhang:
The background is that we(Alibaba Cloud) do get more and more
complaints
from our customers in both KVM and Xen compare to bare-mental.After
investigations, the root cause is known to us: big cost in message
passing
workload(David show it in KVM forum 2015)

A typical message workload like below:
vcpu 0                             vcpu 1
1. send ipi                     2.  doing hlt
3. go into idle                 4.  receive ipi and wake up from hlt
5. write APIC time twice        6.  write APIC time twice to
   to stop sched timer              reprogram sched timer

One write is enough to disable/re-enable the APIC timer -- why does
Linux use two?

One is to remove the timer and another one is to reprogram the timer.
Normally, only one write to remove the timer.But in some cases, it
will reprogram it.


7. doing hlt                    8.  handle task and send ipi to
                                    vcpu 0
9. same to 4.                   10. same to 3

One transaction will introduce about 12 vmexits(2 hlt and 10 msr
write). The
cost of such vmexits will degrades performance severely.

Yeah, sounds like too much ... I understood that there are

  IPI from 1 to 2
  4 * APIC timer
  IPI from 2 to 1

which adds to 6 MSR writes -- what are the other 4?

In the worst case, each timer will touch APIC timer twice.So it will
add additional 4 msr writse. But this is  not always true.


                                                         Linux kernel
already provide idle=poll to mitigate the trend. But it only
eliminates the
IPI and hlt vmexit. It has nothing to do with start/stop sched timer. A
compromise would be to turn off NOHZ kernel, but it is not the default
config for new distributions. Same for halt-poll in KVM, it only
solve the
cost from schedule in/out in host and can not help such workload much.

The purpose of this patch we want to improve current idle=poll
mechanism to

Please aim to allow MWAIT instead of idle=poll -- MWAIT doesn't slow
down the sibling hyperthread.  MWAIT solves the IPI problem, but doesn't
get rid of the timer one.

Yes, i can try it. But MWAIT will not yield CPU, it only helps the
sibling hyperthread as you mentioned.

If you implement proper MWAIT emulation that conditionally gets en- or
disabled depending on the same halt poll dynamics that we already have
for in-host HLT handling, it will also yield the CPU.

It is hard to do . If we not intercept MWAIT instruction, there is no chance to wake up the CPU unless an interrupt arrived or a store to the address armed by MONITOR which is the same with idle=polling.


As for the timer - are you sure the problem is really the overhead of
the timer configuration, not the latency that it takes to actually fire
the guest timer?

No, the main cost is introduced by vmexit, includes IPIs, Timer program, HLT. David detailed it in KVM forum, you can search "Message Passing Workloads in KVM" in google and the first link give the whole analysis of the problem.


One major problem I see is that we configure the host hrtimer to fire at
the point in time when the guest wants to see a timer event. But in a
virtual environment, the point in time when we have to start switching
to the VM really should be a bit *before* the guest wants to be woken
up, as it takes quite some time to switch back into the VM context.


Alex


--
Yang
Alibaba Cloud Computing

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