NAME¶

spausedd — Utility to detect and log scheduler pause

SYNOPSIS¶

DESCRIPTION¶

The spausedd utility is used for detecting and logging scheduler pause. This means, when process should have been scheduled, but it was not. It's also able to use steal time (time spent in other operating systems when running in a virtualized environment) so it is (to some extend) able to detect if problem is on the VM or host side. spausedd is able to read information about steal time ether from kernel or (if compiled in) also use VMGuestLib. Internally spausedd works as following pseudocode:

repeat:
    store current monotonic time
    store current steal time
    sleep for (timeout / 3)
    set time_diff to (current monotonic time - stored monotonic time)
    if time_diff > timeout:
        display error
        set steal_time_diff to (current steal time - stored steal time)
        if (steal_time_diff / time_diff) * 100 > steal_threshold:
            display steal time error

spausedd arguments are as follows:

-d

Display debug messages (specify twice to display also trace messages).

-D

Run on background (daemonize).

-f

Run on foreground (do not demonize - default).

-h

Show help.

-p

Do not set RR scheduler.

-P

Do not move process to root cgroup.

-m steal_threshold

Set steal threshold percent. (default is 10 if kernel information is used and 100 if VMGuestLib is used).

-t timeout

Set timeout value in milliseconds (default 200).

If spausedd receives a SIGUSR1 signal, the current statistics are show.

EXAMPLES¶

To generate CPU load yes(1) together with chrt(1) is used in following examples:

If chrt fails it may help to use cgexec(1) like:

First example is physical or virtual machine with 4 CPU threads so yes(1) was executed 4 times. In a while spausedd should start logging messages similar to:

This means that spausedd didn't got time to run for longer time than default timeout. It's also visible that steal time was 0% so spausedd is running ether on physical machine or VM where host machine is not overloaded (VM was scheduled on time).

Second example is a host machine with 2 CPU threads running one VM. VM is running an instance of spausedd. Two instancies of yes(1) was executed on the host machine. After a while spausedd should start logging messages similar to:

This means that spausedd didn't got the time to run for almost one second. Also because steal time is high, it means that spausedd was not scheduled because VM wasn't scheduled by host machine.

DIAGNOSTICS¶

The spausedd utility exits 0 on success, and >0 if an error occurs.

AUTHORS¶

The spausedd utility was written by Jan Friesse ⟨jfriesse@redhat.com⟩.

BUGS¶

• OS is not updating steal time as often as monotonic clock. This means that steal time difference can be (and very often is) bigger than monotonic clock difference, so steal time percentage can be bigger than 100%. It's happening very often for highly overloaded host machine when spausedd is called with small timeout. This problem is even bigger when VMGuestLib is used.
• VMGuestLib seems to randomly block.