100% CPU utilization in a limited rate test

[piknik]

Context

• Version of iperf3: 3.17.1

• Hardware: i7-1165G7

• Operating system (and distribution, if any): Arch Linux

Bug Report

• Expected Behavior

Minimal CPU utilization for a limited rate bandwidth test; 1 packet per second

• Actual Behavior

100% CPU utilization

• Steps to Reproduce

iperf3 -c server -b 7776 -l 972 -t 0
This limits iperf3 to send 1 packet per second of size 972. UDP or TCP doesn't matter.

With strace we observe that a function is called many times (over 1000) in a loop between each packet output:
strace -f -p $(pidof iperf3)
[pid 185030] pselect6(5, [4], [], NULL, {tv_sec=0, tv_nsec=911000}, NULL) = 0 (Timeout)

[davidBar-On]

With strace we observe that a function is called many times (over 1000) in a loop between each packet output:

This is the result of the --pacing_timer interrupt - the default is 1000us and this is the reason for the1000 calls per second to the timer interrupt handler..

The CPU overload is because with the move to multi-threads, the sending function is now running in a loop that does not include any mechanism for waiting if nothing should be sent (in the non-multi-threads version this was done by select()). Submitted PR #1743 with a suggested fix.