From nobody Tue Jun 23 01:15:54 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 49643C433F5 for ; Mon, 14 Mar 2022 06:03:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236237AbiCNGEk (ORCPT ); Mon, 14 Mar 2022 02:04:40 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44692 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231553AbiCNGEg (ORCPT ); Mon, 14 Mar 2022 02:04:36 -0400 Received: from mail-pg1-x532.google.com (mail-pg1-x532.google.com [IPv6:2607:f8b0:4864:20::532]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5D840E0A1; Sun, 13 Mar 2022 23:03:27 -0700 (PDT) Received: by mail-pg1-x532.google.com with SMTP id z4so12827932pgh.12; Sun, 13 Mar 2022 23:03:27 -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=d8uW8MVx4cu2bnNodD5dVLV/dP3D3LkkTrGrX8U0zMQ=; b=OrwJZ+GOMqgHHcecyi4J1EYX4ljlpHVETDUhA8MuJPldjdHnUNAW44lir2MZ7tGbtC IbV6+vxEtX0KNuSPcQYtNY2WkOkzJVEXBTrTJr2wRDF1ywRrOaRfIA1U+fknjF0Klf+P 6BRzei7PXqpB9YJ0Bn4qLiyisNHANyEyDKU8f0vSXZLd70nuxRnbNdnZaPmF34JkXMhK JCQzBQN7qog72HiFA/Zmu90WG0D3lRRb+2CR6AAq87kzbmqPLvGaxaduinFfAzwieGi1 P9SN8IliT/PHhmCUCEpFnRJ08XdR9z5EDaaUAFKZeZyDoKtVqnPo5CySF4c+1JyvmO6T Jyjg== 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=d8uW8MVx4cu2bnNodD5dVLV/dP3D3LkkTrGrX8U0zMQ=; b=PSwWbLaQvHbp7Qf/iRbHNXOdqPqvxDxrm4HvOAnuud1Xn+rnLOWtADoeL+0zpLHJJf u9DSAFVSpxQ8GJVtibigdtsMomb2799+giOV7DDzb7E4s2+N93tWa5bC+LN0jyGbc+Q/ LoUlz2pyewxN4i4gs611pABlzxz/od6weklBXpmjhXoPixDh0ed+GzIOZLb7EfnfJSe9 OzZvoDWKN9kk8BzBHdZwT6DBCog1M9Jt0onSjQrVg2LKab83jswTHhu0BGoZ5ynPEDz4 0II+zRkjraHJUsbWMa0V2WuEUbs9xMoULWZbMBwCTcDEUKKrKKhvQKCJUZ9NjZ9PerZ6 Xj0w== X-Gm-Message-State: AOAM5325hvgWJxz8bAapXwm6KXApBEDkkFXuk/8G77+furgmU5copGsQ hlWAZ03zp8/oFNBI7rodOAaD03bs0AsLXQ== X-Google-Smtp-Source: ABdhPJz4lT0U5N4b+V4udDfBb/98jw9AIgU+6CZ6/RiKXA9NE0clDlKqPpAYh6G+rwb5qsHBJprCFg== X-Received: by 2002:a05:6a00:1799:b0:4f7:37cd:d03e with SMTP id s25-20020a056a00179900b004f737cdd03emr21921787pfg.10.1647237806621; Sun, 13 Mar 2022 23:03:26 -0700 (PDT) Received: from localhost.localdomain ([2001:288:7001:2708:e222:e75e:e6:3e5e]) by smtp.gmail.com with ESMTPSA id t9-20020a056a0021c900b004f7b425211bsm5341905pfj.36.2022.03.13.23.03.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Mar 2022 23:03:26 -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] docs/scheduler: Introduce the doc of load average Date: Mon, 14 Mar 2022 14:03:04 +0800 Message-Id: <20220314060304.95400-1-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" Load average is one of the common as well as easily observed statistics pro= vided by Linux, but still not well documented, which makes the numbers that users observe from the output of top, htop or other system monitoring application= s are only numbers. This patch discusses how Linux calculates the load average as= well as what is 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 does Linux choose such an average method from the other. 2. The meaning of each term in the expression. 3. The type of tasks that will be covered in the calculation. 4. A brief explanation of the fixed-point number 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 --- 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..1b55f8da4e16 --- /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/laodavg + --=20 2.25.1