UNIX的平均负载 第一部分 ：如何工作

[pax:~]% uptime
9:40am  up 9 days, 10:36,  4 users,  load average: 0.02, 0.01, 0.00

[pax:~]% procinfo
Linux 2.0.36 (root@pax) (gcc 2.7.2.3) #1 Wed Jul 25 21:40:16 EST 2001 [pax]

Memory:      Total        Used        Free      Shared     Buffers      Cached
Mem:         95564       90252        5312       31412       33104       26412
Swap:        68508           0       68508

Bootup: Sun Jul 21 15:21:15 2002    Load average: 0.15 0.03 0.01 2/58 8557
...

 [pax:~]% w

  9:40am  up 9 days, 10:35,  4 users,  load average: 0.02, 0.01, 0.00
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU  WHAT
mir      ttyp0    :0.0             Fri10pm  3days  0.09s  0.09s  bash
neil     ttyp2    12-35-86-1.ea.co  9:40am  0.00s  0.29s  0.15s  w
...

  4:09am  up 12:48,  1 user,  load average: 0.02, 0.27, 0.17

58 processes: 57 sleeping, 1 running, 0 zombie, 0 stopped
CPU states:  0.5% user,  0.9% system,  0.0% nice, 98.5% idle
Mem:   95564K av,  78704K used,  16860K free,  32836K shrd,  40132K buff
Swap:  68508K av,      0K used,  68508K free                 14508K cched

  PID USER     PRI  NI  SIZE  RSS SHARE STAT  LIB %CPU %MEM   TIME COMMAND
 5909 neil      13   0   720  720   552 R       0  1.5  0.7   0:01 top
    1 root       0   0   396  396   328 S       0  0.0  0.4   0:02 init
    2 root       0   0     0    0     0 SW      0  0.0  0.0   0:00 kflushd
    3 root     -12 -12     0    0     0 SW<     0  0.0  0.0   0:00 kswapd
...

[pax:~]% man "load average"
No manual entry for load average

...
DESCRIPTION
       uptime  gives a one line display of the following informa-
       tion.  The current time, how long the system has been run-
       ning, how many users are currently logged on, and the sys-
       tem load averages for the past 1, 5, and 15 minutes.
...

uptime产生一条信息，包括当前时间，系统连续运行时间，当前登录用户
数量和过去1、5及15分钟内的系统平均负载。

The load average tries to measure the number of active processes at any time. As a measure of CPU utilization, the load average is simplistic, poorly defined, but far from useless.

平均负载试图衡量任何时间内的活动进程数量。在被当作CPU利用率的衡量标准时，虽然平均负载数是简单的和缺乏定义的，但远不是无用的。

... High load averages usually mean that the system is being used heavily and the response time is correspondingly slow.

What's high? ... Ideally, you'd like a load average under, say, 3, ... Ultimately, 'high' means high enough so that you don't need uptime to tell you that the system is overloaded.

高平均负载通常表示系统任务繁重，因而响应时间变慢。

高平均负载是多少？一般来说，要根据系统来判断。理想情况下，总是希望平均负载低一点，3就比较合适。基本上来说，这里的“高”是指足够高，而不需要命令uptime来指出系统已经超载。

The load average is the sum of the run queue length and the number of jobs currently running on the CPUs. In Solaris 2.0 and 2.2 the load average did not include the running jobs but this bug was fixed in Solaris 2.3.

平均负载是系统等待运行队列的长度与当前所有CPU中正在运行的工作的和。在So laris 2.0 及 2.2中，平均负载并不包含正在执行的工作，但这个缺陷在Solaris 2.3中得到了修复。

...different systems will behave differently under the same load average. ... running a single cpu-bound background job .... can bring response to a crawl even though the load avg remains quite low.

……不同的系统在相同平均负载下的表现不同。……运行一个绑定CPU的后台工作……虽然平均负载很低，也能将反应拖得很慢。

600  * Nr of active tasks - counted in fixed-point numbers
601  */
602 static unsigned long count_active_tasks(void)
603 {
604         struct task_struct *p;
605         unsigned long nr = 0;
606
607         read_lock(&tasklist_lock);
608         for_each_task(p) {
609                 if ((p->state == TASK_RUNNING ||
610                      (p->state & TASK_UNINTERRUPTIBLE)))
611                         nr += FIXED_1;
612         }
613         read_unlock(&tasklist_lock);
614         return nr;
615 }

unsigned long avenrun[3];
624
625 static inline void calc_load(unsigned long ticks)
626 {
627         unsigned long active_tasks; /* fixed-point */
628         static int count = LOAD_FREQ;
629
630         count -= ticks;
631         if (count < 0) {
632                 count += LOAD_FREQ;
633                 active_tasks = count_active_tasks();
634                 CALC_LOAD(avenrun[0], EXP_1, active_tasks);
635                 CALC_LOAD(avenrun[1], EXP_5, active_tasks);
636                 CALC_LOAD(avenrun[2], EXP_15, active_tasks);
637         }
638 }

  1 HZ    =   100 ticks
  5 HZ    =   500 ticks
  1 tick  =    10 milliseconds
500 ticks =  5000 milliseconds (or 5 seconds)

58 extern unsigned long avenrun[];         /* Load averages */
59
60 #define FSHIFT          11              /* nr of bits of precision */
61 #define FIXED_1         (1<
62 #define LOAD_FREQ       (5*HZ)          /* 5 sec intervals */
63 #define EXP_1           1884            /* 1/exp(5sec/1min) as fixed-point */
64 #define EXP_5           2014            /* 1/exp(5sec/5min) */
65 #define EXP_15          2037            /* 1/exp(5sec/15min) */
66
67 #define CALC_LOAD(load,exp,n) \
68         load *= exp; \
69         load += n*(FIXED_1-exp); \
70         load >>= FSHIFT;

/*
49  * These are the constant used to fake the fixed-point load-average
50  * counting. Some notes:
51  *  - 11 bit fractions expand to 22 bits by the multiplies: this gives
52  *    a load-average precision of 10 bits integer + 11 bits fractional
53  *  - if you want to count load-averages more often, you need more
54  *    precision, or rounding will get you. With 2-second counting freq,
55  *    the EXP_n values would be 1981, 2034 and 2043 if still using only
56  *    11 bit fractions.
57  */