From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757560AbdKODPo (ORCPT ); Tue, 14 Nov 2017 22:15:44 -0500 Received: from mail-ot0-f195.google.com ([74.125.82.195]:49639 "EHLO mail-ot0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755117AbdKODPh (ORCPT ); Tue, 14 Nov 2017 22:15:37 -0500 X-Google-Smtp-Source: AGs4zMagIvdfsPGzZbTfvmZVDrojOcFH63BQ8tvFuGVt1qm9FPf31bWm0ynDSRMw1Mz8AmNhIDZYKQ== Subject: Re: [PATCH RFC v3 4/6] Documentation: Add three sysctls for smart idle poll To: Ingo Molnar References: <1510570064-6178-1-git-send-email-quan.xu0@gmail.com> <20171113150842.epdffyxhlu5nq37u@gmail.com> <35da8c93-95fb-8bf0-8538-4e1ef5f126f4@gmail.com> <20171114074410.c5h5zg43xtcmelez@gmail.com> Cc: "linux-kernel@vger.kernel.org" , kvm , Quan Xu From: "quan.xu04@gmail.com" Message-ID: Date: Wed, 15 Nov 2017 11:15:30 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: <20171114074410.c5h5zg43xtcmelez@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2017年11月14日 15:44, Ingo Molnar wrote: > * Quan Xu wrote: > >> >> On 2017/11/13 23:08, Ingo Molnar wrote: >>> * Quan Xu wrote: >>> >>>> From: Quan Xu >>>> >>>> To reduce the cost of poll, we introduce three sysctl to control the >>>> poll time when running as a virtual machine with paravirt. >>>> >>>> Signed-off-by: Yang Zhang >>>> Signed-off-by: Quan Xu >>>> --- >>>> Documentation/sysctl/kernel.txt | 35 +++++++++++++++++++++++++++++++++++ >>>> arch/x86/kernel/paravirt.c | 4 ++++ >>>> include/linux/kernel.h | 6 ++++++ >>>> kernel/sysctl.c | 34 ++++++++++++++++++++++++++++++++++ >>>> 4 files changed, 79 insertions(+), 0 deletions(-) >>>> >>>> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt >>>> index 694968c..30c25fb 100644 >>>> --- a/Documentation/sysctl/kernel.txt >>>> +++ b/Documentation/sysctl/kernel.txt >>>> @@ -714,6 +714,41 @@ kernel tries to allocate a number starting from this one. >>>> ============================================================== >>>> +paravirt_poll_grow: (X86 only) >>>> + >>>> +Multiplied value to increase the poll time. This is expected to take >>>> +effect only when running as a virtual machine with CONFIG_PARAVIRT >>>> +enabled. This can't bring any benifit on bare mental even with >>>> +CONFIG_PARAVIRT enabled. >>>> + >>>> +By default this value is 2. Possible values to set are in range {2..16}. >>>> + >>>> +============================================================== >>>> + >>>> +paravirt_poll_shrink: (X86 only) >>>> + >>>> +Divided value to reduce the poll time. This is expected to take effect >>>> +only when running as a virtual machine with CONFIG_PARAVIRT enabled. >>>> +This can't bring any benifit on bare mental even with CONFIG_PARAVIRT >>>> +enabled. >>>> + >>>> +By default this value is 2. Possible values to set are in range {2..16}. >>>> + >>>> +============================================================== >>>> + >>>> +paravirt_poll_threshold_ns: (X86 only) >>>> + >>>> +Controls the maximum poll time before entering real idle path. This is >>>> +expected to take effect only when running as a virtual machine with >>>> +CONFIG_PARAVIRT enabled. This can't bring any benifit on bare mental >>>> +even with CONFIG_PARAVIRT enabled. >>>> + >>>> +By default, this value is 0 means not to poll. Possible values to set >>>> +are in range {0..500000}. Change the value to non-zero if running >>>> +latency-bound workloads in a virtual machine. >>> I absolutely hate it how this hybrid idle loop polling mechanism is not >>> self-tuning! >> Ingo, actually it is self-tuning.. > Then why the hell does it touch the syscall ABI? just for more data about performance and CPU utilization with different the maximum poll time. there are 3 parameters, paravirt_poll_{grow|shrink|threshold_ns}.. we didn't touch paravirt_poll_{grow|shrink} since we sent out v1. We tested it based on  benchmark contextswitch / netperf with different paravirt_poll_threshold_ns. Here is the data we get when running benchmark contextswitch to measure the latency(lower is better):       halt_poll_threshold=0      -- 3402.9 ns/ctxsw -- 199.8 %CPU       halt_poll_threshold=10000  -- 1151.4 ns/ctxsw -- 200.1 %CPU       halt_poll_threshold=20000  -- 1149.7 ns/ctxsw -- 199.9 %CPU       halt_poll_threshold=30000  -- 1151.0 ns/ctxsw -- 199.9 %CPU       halt_poll_threshold=40000  -- 1155.4 ns/ctxsw -- 199.3 %CPU       halt_poll_threshold=50000  -- 1161.0 ns/ctxsw -- 200.0 %CPU       halt_poll_threshold=100000 -- 1163.8 ns/ctxsw -- 200.4 %CPU       halt_poll_threshold=200000 -- 1163.8 ns/ctxsw -- 201.4 %CPU       halt_poll_threshold=300000 -- 1159.4 ns/ctxsw -- 201.9 %CPU       halt_poll_threshold=500000 -- 1163.5 ns/ctxsw -- 205.5 %CPU Here is the data we get when running benchmark netperf:       halt_poll_threshold=0      -- 29031.6 bit/s -- 76.1  %CPU       halt_poll_threshold=10000  -- 29021.7 bit/s -- 105.1 %CPU       halt_poll_threshold=20000  -- 33463.5 bit/s -- 128.2 %CPU       halt_poll_threshold=30000  -- 34436.4 bit/s -- 127.8 %CPU       halt_poll_threshold=40000  -- 35563.3 bit/s -- 129.6 %CPU       halt_poll_threshold=50000  -- 35787.7 bit/s -- 129.4 %CPU       halt_poll_threshold=100000 -- 35477.7 bit/s -- 130.0 %CPU       halt_poll_threshold=200000 -- 35877.7 bit/s -- 131.0 %CPU       halt_poll_threshold=300000 -- 35730.0 bit/s -- 132.4 %CPU       halt_poll_threshold=500000 -- 34978.4 bit/s -- 134.2 %CPU and think of the default value(200000, for x86) of kvm dynamic poll, I'll set it as the same as kvm dynamic poll. I also test idle VM with diffrent halt_poll_threshold, which doesn't make CPU utilization fluctuated.. >> could I only leave paravirt_poll_threshold_ns parameter (the maximum poll time), >> which is as similar as "adaptive halt-polling" Wanpeng mentioned.. then user can >> turn it off, or find an appropriate threshold for some odd scenario.. > That way lies utter madness. Maybe add it as a debugfs knob, but exposing it to > userspace: NAK. > .. so, I will make these 3 parameters by default in next v4.      paravirt_poll_threshold_ns = 200000      paravirt_poll_shrink = 2      paravirt_poll_grow = 2 neither touch the syscal ABI nor expose it to userspace again. Quan