systemd service using 100% of my CPU when it doesn't if I start it without systemd

Question

I'm using Debian Jessie with the latest updates. I made a systemd service to run a script when my server starts. Here's its configuration:

[Unit]
Description=(my description)

[Service]
ExecStart=/usr/bin/bot
Restart=restart-always

[Install]
WantedBy=multi-user.target

/usr/bin/bot is a script running a Mono executable. It consists of:

#!/bin/bash
(cd /path/to/my/executable && mono bot.exe)

(I replaced the path here, but the one on my script is correct.)

When I run the script /usr/bin/bot normally (simply /usr/bin/bot on my terminal), it is working as expected. top reports it's using between 0 and, say, 20% of my CPU, which is normal. But when I start it with service bot start, top says it's always using at least 100% of my CPU.

In both cases bot is working as expected.

What could explain such a big difference in CPU usage?

Thank you.

Answer 1

Problem is not located in systemd.

Systemd runs process without stdin (=/dev/null). All syscalls to read() are finished immediately (with normal stdin, read() is blocked until new data arrive). In general practice read() is called within unfinished loop making huge CPU usage. To confirm this, try to attach to running process with strace -p <pid>.

Process has to be adapted to run without stdin or use some wrappers like proposed screen command or nohup

Answer 2

I "fixed it" by putting my bot under a screen, like so:

[Unit]
Description=(my description)

[Service]
RemainAfterExit=yes
ExecStart=/usr/bin/screen -dmS bot /usr/bin/bot
Restart=restart-always

[Install]
WantedBy=multi-user.target

I don't know why putting my process in a screen fixes its high CPU usage, but hey, it works.

Answer 3

As per @niziak's solution the input is blocking. In my case solved by Adding StandardInput=tty to the [Service] fixed it. I was already re-directing output to an (unused) console on /dev/tty12.

My [service] section:

[Service]
ExecStart=/usr/local/bin/vtclock -1  -c| -f /tmp/ntpstatus
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartPreventExitStatus=255
Type=simple
User=vtclock
StandardInput=tty
StandardOutput=tty
StandardError=null
TTYPath=/dev/tty12 

(OS = Ubuntu 16.04.3 LTS, my first attempt at a systemd service)

Answer 4

I see this is an old thread, but I leave my scenario here, maybe a few years time some finds it usefull.

I have a discord bot in c# dotnet core.

I made a systemctl daemon for it in my ubuntu server.

I have the git repo stored on te server as well. when I used it in test, I went with the

dotnet run 

it consumed 0,3% of cpu.

I made a publish of it and placed it in my active server folder. From there in the daemon it went to 100% cpu usage in the daemon.

I got mixed between the differrent versions, but I felt that it was runing normally from terminal.

but no.

My rookie mistake was doing

dotnet publish -r linux-x64 --self-contained true

instead of

dotnet publish -r linux-x64 

But for a long time I did not noticed, because the self contained version was runing fine with minimal cpu, until I moved out from the procet bin directory.

Answer 5

If you've developed a console application in C# / .NET Core, don't use this to keep the console running:

Console.Read();

As it will result in 100% CPU usage when you run the application as a systemd service on linux.

Rather use:

while (true) { Console.ReadKey(); }

I had the same problem.