From nobody Tue Jun 23 01:15:55 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B44EAC433F5 for ; Mon, 14 Mar 2022 08:48:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231607AbiCNIuB (ORCPT ); Mon, 14 Mar 2022 04:50:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45416 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229613AbiCNIt4 (ORCPT ); Mon, 14 Mar 2022 04:49:56 -0400 Received: from mail-pj1-x102e.google.com (mail-pj1-x102e.google.com [IPv6:2607:f8b0:4864:20::102e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E34BB1400F; Mon, 14 Mar 2022 01:48:45 -0700 (PDT) Received: by mail-pj1-x102e.google.com with SMTP id mm23-20020a17090b359700b001bfceefd8c6so10475014pjb.3; Mon, 14 Mar 2022 01:48:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=WbMnsuV/WjCVgNHAAhwwQM/BhzDcEyxYmkQ9AWEx0BI=; b=bxThCbIpNeDgqjkFCkbfhRkEALDTWe1csTnrolRMgdog4jnm1bH22CRZna/bQgdFKp 1sW1dMPVhsdRfj0VpQqquI3cp7HB6Y1b/3TpsLYBeMYWbTkUjxSgMEFRdl5BOy7bvVub TCtRcbFNERpH4UdbXnj4EkSdge2Rcd+oDdPT6odeNXvies7/Qjs7EioxKajx294l3bIC DkTIHXgGWrYvdfe83M1AqfZR6Z1rZygMxFUTRBG/su6fFwiDz42y+oZ5zPN+vZ3A/7X6 /TTmPVV5mxwz2j+K8ajtR0Tc25GRqSyXIWNpRaavtzvWD+HVT1cUksniqqbEcOlD2XpS 6eaw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=WbMnsuV/WjCVgNHAAhwwQM/BhzDcEyxYmkQ9AWEx0BI=; b=pl8m3bNcJbFbANxbs9hvpX56RgJb/B80payT7h8XOY+Wh4AjDnhTGDGptvDzOMc0IR 2o+PXegqYOOiPo0zQDDe9TFu1vaRejfXCftk7tTpP0VZahGuiLKMxs/FqyZ0pvrk24LM QAWymR0Mwi2LBmS/ajiIlDVzqPNG6zh3gPuy7Sym78gIuDxpdvcHskm15Wvy9418qnif WhdDM1S0+U4KRuitkNSv/tzfJXIIylogHbcrg4w9KePjvrfdTQnIpmoQUHtKPLZBTIEI OgbGIF4XCh0VrugYrHsF26+rwz4B7HccRGw3xoDRHPbRSSaC4YJX+yeib81soH9wnsiZ FznQ== X-Gm-Message-State: AOAM533ceT9xMM6bKm/aojDiU7lCRBFG7l8Vnz7+FbeliPewBe8bw4Er xY8uXWScQf6dLXuxZ66mr24= X-Google-Smtp-Source: ABdhPJyr2nILvPxagHGAFHs0MUGvuIVd6mr/DBlvo6qTVtXtTLACOTTBI/7QXIAYJI7qpOYF+7clWQ== X-Received: by 2002:a17:90a:6508:b0:1be:d59c:1f10 with SMTP id i8-20020a17090a650800b001bed59c1f10mr23905491pjj.229.1647247725315; Mon, 14 Mar 2022 01:48:45 -0700 (PDT) Received: from localhost.localdomain ([2001:288:7001:2708:503c:a5f3:4f9d:fb88]) by smtp.gmail.com with ESMTPSA id y13-20020a056a00180d00b004f733bc57e5sm18798479pfa.192.2022.03.14.01.48.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Mar 2022 01:48:44 -0700 (PDT) From: Jui-Tse Huang To: Jonathan Corbet , Peter Zijlstra , Valentin Schneider , Mauro Carvalho Chehab , Huaixin Chang , Beata Michalska , Chun-Hung Tseng , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Jui-Tse Huang , Ching-Chun Huang , Yiwei Lin Subject: [PATCH v2] docs/scheduler: Introduce the doc of load average Date: Mon, 14 Mar 2022 16:47:59 +0800 Message-Id: <20220314084759.9849-1-juitse.huang@gmail.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" The load average is one of a common as well as easy observed statistic provied by Linux, but still not well documented, which makes the numbers that users observes from the output of top, htop or other system monitoring application are only numbers. This patch gives a discussion on how Linux calculates the load average as well as what metrics are concerned while calculating the load average. The discussion flow is divided into several parts: 1. The expression used to get the load average. 2. Why Linux choose such average method from the other. 2. The meaning of each term in the expression. 3. The metrics, that is, the type of tasks that will be covered in the calculation. 4. A brief explanation over the fixed-point nubmer since the weights defined in the Linux kernel are based on it. Signed-off-by: Jui-Tse Huang Signed-off-by: Yiwei Lin Co-Developed-by: Yiwei Lin --- v1 -> v2: fix typo: '$cat /proc/laodavg' -> '$cat /proc/loadavg' Documentation/scheduler/index.rst | 1 + Documentation/scheduler/load-average.rst | 77 ++++++++++++++++++++++++ 2 files changed, 78 insertions(+) create mode 100644 Documentation/scheduler/load-average.rst diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/in= dex.rst index 88900aabdbf7..bdc779b4190f 100644 --- a/Documentation/scheduler/index.rst +++ b/Documentation/scheduler/index.rst @@ -17,6 +17,7 @@ Linux Scheduler sched-nice-design sched-rt-group sched-stats + load-average =20 text_files =20 diff --git a/Documentation/scheduler/load-average.rst b/Documentation/sched= uler/load-average.rst new file mode 100644 index 000000000000..589320ee1cd7 --- /dev/null +++ b/Documentation/scheduler/load-average.rst @@ -0,0 +1,77 @@ +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Load Average +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Load average is a basic statistic provided by almost all operating systems= that +aims to report the usage of system hardware resources. In Linux kernel, the +load average is calculated via the following expression:: + + / 0 , if t =3D 0 + load_{t} =3D | + \ laod_{t - 1} * exp + active * (1 - exp), otherwise + +The expression represents the exponential moving average of the historical +loading of the system. There are several reasons that Linux kernel chooses +exponential moving average from other similar average equations such as si= mple +moving average or cumulative moving average: + +#. The exponential moving average consumes fixed memory space, while the s= imple + moving average has O(n) space complexity where n is the number of times= lice + within a given interval. +#. The exponential moving average not only applies a higher weight to the = most + recent record but also declines the weight exponentially, which makes t= he + resulting load average reflect the situation of the current system. Nei= ther + the simple moving average nor cumulative moving average has this featur= e. + +In the expression, the load_{t} in the expression indicates the calculated= load +average at the given time t. +The active is the most recent recorded system load. In Linux, the system l= oad +means the number of tasks in the state of TASK_RUNNING or TASK_UNINTERRUPT= IBLE +of the entire system. Tasks with TASK_UNINTERRUPTIBLE state are usually wa= iting +for disk I/O or holding an uninterruptible lock, which is considered as a = part +of system resource, thus, Linux kernel covers them while calculating the l= oad +average. +The exp means the weight applied to the previous report of load average, w= hile +(1 - exp) is the weight applied to the most recently recorded system load. +There are three different weights defined in the Linux kernel, in +include/linux/sched/loadavg.h, to perform statistics in various timescales= :: + + // include/linux/sched/loadavg.h + ... + #define EXP_1 1884 /* 1/exp(5sec/1min) as fixed-point */ + #define EXP_5 2014 /* 1/exp(5sec/5min) */ + #define EXP_15 2037 /* 1/exp(5sec/15min) */ + ... + +According to the expression shown on the top of this page, the weight (exp) +controls how much of the last load load_{t - 1} will take place in the +calculation of current load, while (1 - exp) is the weight applied to the = most +recent record of system load active. + +Due to the security issue, the weights are defined as fixed-point numbers = based +on the unsigned integer rather than floating-pointing numbers. The introdu= ction +of the fixed-point number keeps the FPU away from the calculation process.= Since +the precession of the fixed-point used in the Linux kernel is 11 bits, a +fixed-point can be converted to a floating-point by dividing it by 2048, a= s the +expression shown below:: + + EXP_1 =3D 1884 / 2048 =3D 0.919922 + EXP_5 =3D 2014 / 2048 =3D 0.983398 + EXP_15 =3D 2037 / 2048 =3D 0.994629 + +Which indicates the weights applied to active are:: + + (1 - EXP_1) =3D (1 - 0.919922) =3D 0.080078 + (1 - EXP_5) =3D (1 - 0.983398) =3D 0.016602 + (1 - EXP_15) =3D (1 - 0.994629) =3D 0.005371 + +The load average will be updated every 5 seconds. Each time the scheduler_= tick() +be called, the function calc_global_load_tick() will also be invoked, which +makes the active of each CPU core be calculated and be merged globally, fi= nally, +the load average will be updated with that global active. + +As a user, the load average can be observed via top, htop, or other system +monitor application, or more directly, by the following command:: + + $ cat /proc/loadavg + --=20 2.25.1