Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support

[Yang Zhang]

On 2017/8/29 19:58, Alexander Graf wrote:
> On 08/29/2017 01:46 PM, Yang Zhang wrote:
> > Some latency-intensive workload will see obviously performance
> > drop when running inside VM. The main reason is that the overhead
> > is amplified when running inside VM. The most cost i have seen is
> > inside idle path.
> >
> > This patch introduces a new mechanism to poll for a while before
> > entering idle state. If schedule is needed during poll, then we
> > don't need to goes through the heavy overhead path.
> >
> > Here is the data we get when running benchmark contextswitch to measure
> > the latency(lower is better):
> >
> >     1. w/o patch:
> >        2493.14 ns/ctxsw -- 200.3 %CPU
> >     2. w/ patch:
> >        halt_poll_threshold=10000 -- 1485.96ns/ctxsw -- 201.0 %CPU
> >        halt_poll_threshold=20000 -- 1391.26 ns/ctxsw -- 200.7 %CPU
> >        halt_poll_threshold=30000 -- 1488.55 ns/ctxsw -- 200.1 %CPU
> >        halt_poll_threshold=500000 -- 1159.14 ns/ctxsw -- 201.5 %CPU
> >     3. kvm dynamic poll
> >        halt_poll_ns=10000 -- 2296.11 ns/ctxsw -- 201.2 %CPU
> >        halt_poll_ns=20000 -- 2599.7 ns/ctxsw -- 201.7 %CPU
> >        halt_poll_ns=30000 -- 2588.68 ns/ctxsw -- 211.6 %CPU
> >        halt_poll_ns=500000 -- 2423.20 ns/ctxsw -- 229.2 %CPU
> >     4. idle=poll
> >        2050.1 ns/ctxsw -- 1003 %CPU
> >     5. idle=mwait
> >        2188.06 ns/ctxsw -- 206.3 %CPU
>
> Could you please try to create another metric for guest initiated, host 
> aborted mwait?
>
> For a quick benchmark, reserve 4 registers for a magic value, set them 
> to the magic value before you enter MWAIT in the guest. Then allow 
> native MWAIT execution on the host. If you see the guest wants to enter 

I guess you want to allow native MWAIT execution on the guest not host?

> with the 4 registers containing the magic contents and no events are 
> pending, directly go into the vcpu block function on the host.

Mmm..It is not very clear to me. If guest executes MWAIT without vmexit, 
how to check the register?

> That way any time a guest gets naturally aborted while in mwait, it will 
> only reenter mwait when an event actually occured. While the guest is 
> normally running (and nobody else wants to run on the host), we just 
> stay in guest context, but with a sleeping CPU.
>
> Overall, that might give us even better performance, as it allows for 
> turbo boost and HT to work properly.

In our testing,  we have enough cores(32cores) but only 10VCPUs, so in 
the best case, we may see the same performance as poll.

--
Yang
Alibaba Cloud Computing