How to obtain the current number of jiffies since reboot in Linux?

Question

Linux's /proc/PID/stat lists several metrics that are measured in jiffies. Is there a way to get the current # of jiffies since reboot? Jiffies I also assume to be seconds times the USER_HZ value.

Can I get this by summing the first 4 arguments on the cpu line in /proc/stat?

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UPDATE:

$ date +"%s.%N" && grep '^jiffies' timer_list
1262103103.162169230
jiffies: 1007865965
jiffies: 1007865965
jiffies: 1007865965
jiffies: 1007865965
$ date +"%s.%N" && grep '^jiffies' timer_list
1262103108.706475051
jiffies: 1007867351
jiffies: 1007867351
jiffies: 1007867351
jiffies: 1007867351

The delta between these is almost exactly 250 jiffies / second. Appears as though all the jiffies lines per CPU are equivalent (though I guess they might not be if a hot | ondemand CPU was provisioned/replaced). I think this gives me the best place to pull a consistent jiffies counter.

Answer 1

Technically jiffy in computer parlance is the duration of 1 tick of the system timer interrupt. It's not absolute though. For Linux 2.6.13+ on Intel x86 jiffy is 4ms, but can range from 1ms to 10ms depending upon architecture and kernel version.

From the Kernel Timer Systems page:

Historically, the kernel used 100 as the value for HZ, yielding a jiffy interval of 10 ms. With 2.4, the HZ value for i386 was changed to 1000, yeilding a jiffy interval of 1 ms. Recently (2.6.13) the kernel changed HZ for i386 to 250. (1000 was deemed too high).

It lists /proc/timer_list and /proc/timer_stats.

You can activate the timer_stats at boot time, then cat this file to print stats.

Answer 2

No, you only need the first line. The first line aggregates everything else in the other cpu lines.

Example output:

[john@awesome]$cat /proc/stat
cpu  35024984 1771325 94153391 1810948613 2648063 352387 557232
cpu0 13955475 927654 59431476 895791946 1910028 318618 438048
cpu1 21069509 843671 34721915 915156667 738035 33769 119184
intr 1403502159 1138402452 597 0 3 3 0 5 0 1 0 0 0 12315 0 92119425 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 57676632 0 0 0 0 0 0 0 115290726 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
ctxt 21043582666
btime 1252332786
processes 25663823
procs_running 1
procs_blocked 0

What each column means (left to right):

• user: normal processes executing in user mode
• nice: niced processes executing in user mode
• system: processes executing in kernel mode
• idle: idle time
• iowait: waiting for I/O to complete
• irq: servicing interrupts
• softirq: servicing softirqs

as you can see, the first column after cpu (user mode processes) is equal to the 2 numbers beneath it added together.

Answer 3

In later Linux Kernels you can query /proc/timer_list and find out the number of jiffies since reboot from each CPU that's present within the system. They should always match.

$ grep -E "^cpu|^jiff" /proc/timer_list
cpu: 0
jiffies: 4299690231
cpu: 1
jiffies: 4299690231

If you take a look at the code behind timer_list.c the bit that prints the above:

...
...
        P_ns(iowait_sleeptime);
        P(last_jiffies);
        P(next_timer);
        P_ns(idle_expires);
        SEQ_printf(m, "jiffies: %Lu\n",
               (unsigned long long)jiffies);

Notice that it's unsigned long long.

Answer 4

jiffies per second:

awk 'BEGIN {"cat /proc/timer_list | grep '\''^jiffies'\'' | awk '\''{print $2}'\''" | getline a; "cat /proc/uptime | awk '\''{print $1}'\''" | getline b ;printf "%.4f\n", a/b}'

Explanation: It divides jiffies since boot found in /proc/timer_list by the seconds since boot found in /proc/uptime