If R (runqueue) is greater than B (waitqueue) in vmstat, is it I/O bound

cpu usage

If the runqueue is the number of processes waiting for their turn on the CPU + processes currently running, and waitqueue is the number of processes waiting for I/O, then wouldn’t B in the vmstat output being greater than R mean that there is an I/O bound, not CPU bound? I am confused because the link below says the opposite …
From http://nonfunctionaltestingtools.blogspot.com/2013/03/vmstat-output-explained.html?m=1

“If runnable threads (r) divided by the number of CPU is greater than one -> possible CPU bottleneck (The (r) coulmn should be compared with number of CPUs (logical CPUs as in uptime) if we have enough CPUs or we have more threads.) High numbers in the blocked processes column (b) indicates slow disks. (r) should always be higher than (b); if it is not, it usually means you have a CPU bottleneck”

Best Answer

A higher number in b than in r means the CPUs are often idle, so you are right being confused. The document should have read 'means you have an I/O bottleneck'.

Beware that the page says r should never be higher than the number of CPUs, and r=16 on a 12 CPU system is a "serious" problem. This is quite exaggerated. That just means CPUs are fully used and some threads are waiting. Usually no big deal.

Finally, don't confuse threads and processes, like the linked document sometimes does too. The r and b columns show number of threads, not processes. Not all processes are single threaded.

Related Solutions

Shell – Get CPU usage and run a command if it is higher than 80%

I had a similar issue and had this short bash script already done. It is calculating the load average for the last 15 minutes, if you want a different timeframe, it shold be change (to check the load avg for last 5 min, change the awk to print $1).
This will tell you the relative usage of the CPUs :

#!/bin/bash
cores=$(nproc) 
load=$(awk '{print $3}'< /proc/loadavg)
echo | awk -v c="${cores}" -v l="${load}" '{print "relative load is " l*100/c "%"}'

Should run on Ubuntu and Centos.

To get to the point where you check if load is above 80% and 'do something' you should add to this script :

usage=$(echo | awk -v c="${cores}" -v l="${load}" '{print l*100/c}' | awk -F. '{print $1}')
if [[ ${usage} -ge 80 ]]; then
    echo "delete all from install folder and make it read only"
fi

Linux – How to Get Number of Processes Waiting for CPU

The number of runnable processes is given by procs_running in /proc/stat:

awk '/procs_running/ { print $2 }' /proc/stat

Subtract the number of CPU threads available, stopping at 0, and you’ll get the number of scheduling units (processes or threads) waiting to be scheduled. You can determine the number of CPU threads available from /proc/stat too, using the cpu? lines. Overall:

awk '/cpu[^ ]/ { nb = substr($1, 4); if (nb > nbcpus) nbcpus = nb };
/procs_running/ { runqueue = $2 - nbcpus; if (runqueue < 0) runqueue = 0; print runqueue }' /proc/stat

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Related Solutions

Shell – Get CPU usage and run a command if it is higher than 80%

Linux – How to Get Number of Processes Waiting for CPU

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