CODEWITHSUNDEEP
linuxsystem
nicky_zs
nicky_zs

Reputation: 3773

Could someone explain what is nice time in cpu stat?

I know the difference between user time and sys time. However, I'm not quite sure about the nice time. I used to know that nice time is the time used in nice pr, but when I did an experiment, I found that the nice time didn't grow up after I reniced a 100%-CPU-using program(infinite loop doing add in Java) to 19. So, I'm confused...

Btw, I'm going to write a C++ program to monitor the CPU usage. All I can do now, is to read /proc/stat twice and get the difference. But, I don't know how to calculate the Totle time.

total = user + sys + idle or

total = user + sys + nice + idle or even

total = user + sys + nice + idle + iowait + ... (the entire line).

Which is right?

Upvotes: 3

Views: 1354

Answers (1)

user2845360
user2845360

Reputation:

Mpstat(1) reads /proc/stat. Diving into kernel source tree I found a file kernel/sched/cputime.c, taken from Linux 3.11.7 sources, which seems to include relevant bits which update things reflected in /proc/stat:

/*
 * Account user cpu time to a process.
 * @p: the process that the cpu time gets accounted to
 * @cputime: the cpu time spent in user space since the last update
 * @cputime_scaled: cputime scaled by cpu frequency
 */
void account_user_time(struct task_struct *p, cputime_t cputime,
                       cputime_t cputime_scaled)
{
        int index;

        /* Add user time to process. */
        p->utime += cputime;
        p->utimescaled += cputime_scaled;
        account_group_user_time(p, cputime);

        index = (TASK_NICE(p) > 0) ? CPUTIME_NICE : CPUTIME_USER;

        /* Add user time to cpustat. */
        task_group_account_field(p, index, (__force u64) cputime);

        /* Account for user time used */
        acct_account_cputime(p);
}

This hints that time spent running niced tasks is not included into column displaying time spent running user mode tasks in general (the line index= seems relevant to this).

Upvotes: 1

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