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About maxload

In intention it is a Unix shell command which run other task, like nice command. nice tools is static, just run another process with some nice level. maxload goes much further, dynamical change nice level (with the I/O priority) depending of system load level, even pause controled process if it necessary. So maxload is excellent tool to run CPU intensive task as a system overload protection.

Introduction

Some of task are very resource intensive, on the other hand system load is changing over time. The best practice is running intensive task in time when system load is be low. The maxload tool trying solve this problem by continuous monitoring system load and dynamically controls process priority. In example, if you do backups on your server in the night when usually system is not busy the task will by run normaly. But when some of service will begin working and generate heigth system load the maxload will release resources used by his controlled process (not memory). When system load will drop to low level the controlled process will by starts again, even maxload can restore the normal priority for controlled process if it's possible (run as root).

How to use maxload

maxload has simple syntax: maxload [load] [command] [args ...] You need two things to run maxload, max load border [load] and command you want to run [command]. If this command has arguments you type it normaly after command. Example: maxload 1.9 ./backup.sh /home In this case maxload has border 1.9 and run command ./backup.sh /home.

Proper border

The most important things is to set proper border value. This value is depend of the CPU cores number which you have in your system. You can check this by command: top, cat /proc/cpuinfo or nproc.

One CPU core means maximum 1 value of system load, value bigger then 1 means your system is overloads. So if you have two cores the maximum load is 2, for four cores it's 4.

Suppose you have two CPU cores and you have run backup script until system is not overload. If you set border on 2 maxload start work when overload is occurs, and users can experiencing the overload. To avoid this you must set border less then maximum load e.g. 1.9.

Strategy of maxload

maxload changing priority for controlled process during its executing. This change has impact to process scheduling in operating system so it shouldn't happen frequently. Linux kernel calculates load averages from every 5 seconds. So 5 seconds is the highest frequency with maxload work.

Here is the list of maxload frequency work in relative to max border:

  • load <= 10% border (checks every 15 seconds)
  • 11% border <= load <= 25% border (checks every 10 seconds)
  • load > 25% border (checks every 5 seconds)

maxload starts reduce priority for controlled process if system load is between 40-100% of set border and grows.

maxload has four steps to get the lowest priority value for controled process. Here is the list how maxload changes priority for controlled process:

  • LEVEL1: CPU nice 5, I/O priority 4
  • LEVEL2: CPU nice 10, I/O priority 5
  • LEVEL3: CPU nice 15, I/O priority 6
  • LEVEL4: CPU nice 19, I/O priority 7

I/O value is given in best effort (be) scheduler.

If system load exceeds more than set border then maxload paused running controlled process. If system load will by reducing less than set border then maxload starts executing controlled process again.

Back to normal priority

maxload is not only decrease priority but can also increase it when system load is low. This is not main goal of maxload but maxload has it. The point is that the maxload is not check what priority it has received from system when it has start executing. Example, when you run that command as root:

# nice -n 9 maxload 3.5 /root/script.sh

and the next system load is growths to 3.4 and drops back to 0.1, in effect the maxload increase priority (value of nice) equal to 0.

If maxload in this example run as normal user priority of script will by decreasing to 19 and not growing even to system load is drops to 0.0.

maxload increase priority of controled process if system load is between 75-40% of set border and still decreasing. This case required to maxload must by executes as superuser (root).

Negative impact of pause executing process

Pauses in executing process can make the process will not work correctly. Daemon is not good task for running via maxload. Generally linear tasks is good to run via maxload. Good example is the scripts which you run via cron.

Demonstration video

You can see how maxload works on this video: [How works maxload](https://youtu.be/IwpuLnNWhrY)

FAQ

  1. Which value of system load averages is used in maxload: 1, 5, 15 minutes?
    2. 1 minutes.
  2. How maxload pauses and restarts controlled process?
    3. Sends SIGSTOP signal to pause and SIGCONT to restore execution.
  3. How maxload execute controlled process?
    4. By fork() function.