113 lines
2.8 KiB
ReStructuredText
113 lines
2.8 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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.. include:: ../disclaimer-zh_TW.rst
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:Translator: 胡皓文 Hu Haowen <src.res@email.cn>
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========
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CPU 負載
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========
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Linux通過``/proc/stat``和``/proc/uptime``導出各種信息，用戶空間工具
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如top(1)使用這些信息計算系統花費在某個特定狀態的平均時間。
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例如：
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$ iostat
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Linux 2.6.18.3-exp (linmac) 02/20/2007
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avg-cpu: %user %nice %system %iowait %steal %idle
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10.01 0.00 2.92 5.44 0.00 81.63
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...
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這裡系統認爲在默認採樣周期內有10.01%的時間工作在用戶空間，2.92%的時
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間用在系統空間，總體上有81.63%的時間是空閒的。
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大多數情況下``/proc/stat``的信息幾乎真實反映了系統信息，然而，由於內
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核採集這些數據的方式/時間的特點，有時這些信息根本不可靠。
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那麼這些信息是如何被搜集的呢？每當時間中斷觸發時，內核查看此刻運行的
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進程類型，並增加與此類型/狀態進程對應的計數器的值。這種方法的問題是
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在兩次時間中斷之間系統（進程）能夠在多種狀態之間切換多次，而計數器只
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增加最後一種狀態下的計數。
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舉例
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---
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假設系統有一個進程以如下方式周期性地占用cpu::
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兩個時鐘中斷之間的時間線
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|-----------------------|
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^ ^
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|_ 開始運行 |
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|_ 開始睡眠
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（很快會被喚醒）
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在上面的情況下，根據``/proc/stat``的信息（由於當系統處於空閒狀態時，
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時間中斷經常會發生）系統的負載將會是0
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大家能夠想像內核的這種行爲會發生在許多情況下，這將導致``/proc/stat``
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中存在相當古怪的信息::
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/* gcc -o hog smallhog.c */
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#include <time.h>
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#include <limits.h>
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#include <signal.h>
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#include <sys/time.h>
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#define HIST 10
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static volatile sig_atomic_t stop;
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static void sighandler (int signr)
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{
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(void) signr;
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stop = 1;
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}
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static unsigned long hog (unsigned long niters)
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{
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stop = 0;
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while (!stop && --niters);
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return niters;
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}
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int main (void)
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{
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int i;
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struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
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.it_value = { .tv_sec = 0, .tv_usec = 1 } };
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sigset_t set;
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unsigned long v[HIST];
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double tmp = 0.0;
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unsigned long n;
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signal (SIGALRM, &sighandler);
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setitimer (ITIMER_REAL, &it, NULL);
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hog (ULONG_MAX);
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for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
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for (i = 0; i < HIST; ++i) tmp += v[i];
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tmp /= HIST;
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n = tmp - (tmp / 3.0);
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sigemptyset (&set);
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sigaddset (&set, SIGALRM);
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for (;;) {
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hog (n);
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sigwait (&set, &i);
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}
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return 0;
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}
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參考
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---
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- https://lore.kernel.org/r/loom.20070212T063225-663@post.gmane.org
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- Documentation/filesystems/proc.rst (1.8)
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謝謝
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---
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Con Kolivas, Pavel Machek
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