Messages in this thread | | | Subject | Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support | From | Yang Zhang <> | Date | Fri, 1 Sep 2017 14:21:53 +0800 |
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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
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