Start and stop programs depending on the system load

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Weight Lifting

Long-running and resource-hungry programs are best deployed when the computer has nothing much to do. The loadwatch tool can make this happen.

The following is a very simple idea. During times when the load on the system is low, a daemon starts programs that have long computing times and therefore could otherwise interfere with user interaction.

In systems built along the lines of Unix, these helpful spirits are referred to as daemons. They putter along as processes in the background and spring into action as soon as a particular event or condition occurs.

The concept of taking advantage of low system load is easy to implement. Thanks to the /proc/ filesystem, important system information is made available to all processes. Even the average user can enter cat /proc/loadavg on a terminal and find out what the current system is or what it was. Listing 1 shows the output the user will receive.

Listing 1

Checking Load Average

$ cat /proc/loadavg
2.83 3.02 2.95 2/580 1462

Entering man 5 proc takes you to an extensive man page where you can find out what the numbers mean by searching for loadavg . The first three entries refer to the load on the system produced by currently executing processes (running : designated with the letter "R") in the last 1, 5, and 15 minutes.

How Does It Work?

The idea behind loadwatch is very simple. The system load gets regularly checked. If something like a threshold event has occurred, then the selected process will be stopped and then reactivated once the load has fallen below a lower threshold (see the "Installation" box for more details).


For now, loadwatch can only been found in the Debian and Ubuntu repositories. For all other distributions, you need to perform the relatively straightforward task of compiling the program yourself from the source code.

You will find two relevant files online [1]. These include loadwatch_1.0+1.1alpha1.orig.tar.gz with the original source code and loadwatch_1.0+1.1alpha1-5.diff.gz , which has an additional patch. This patch modifies the source code yielding the most up-to-date version. For purposes of this project, you should save both files under /tmp/ .

First, decompress the original source code via tar xf loadwatch_1.0+1.1alpha1.orig.tar.gz . This generates the directory loadwatch-1.1/ to which you should then switch.

Then, you should load the patch via the original source code by typing

patch -p1 -i ../loadwatch_1.0+1.1alpha1-5.diff

See Listing 2.

This patch modifies all of the designated files including the man pages, makefile, etc. which is why you should control the output. Because the patch will be located in a higher level directory – in this case /tmp/ – the patch will be appropriately marked via -p1 when you use it in the working directory.

Now you can type ./configure && make && sudo make install to install the program in /usr/local/ in the usual way. It should execute without any difficulties.

Listing 2

Patching the Source Code

$ patch -p1 -i ../loadwatch_1.0+1.1alpha1-5.diff
patching file
patching file loadwatch.c
patching file lw-ctl.c
patching file debian/loadwatch.1
patching file debian/dirs
patching file debian/README.debian
patching file debian/changelog
patching file debian/control
patching file debian/copyright
patching file debian/docs
patching file debian/lw-ctl.1
patching file debian/rules
patching file debian/watch

A second program lw-ctl , makes it possible to externally govern processes controlled by loadwatch . The lw-ctl tool communicates with loadwatch , thereby making it possible to do things like restart programs that have been stopped or stop those that are running.

A Practical Example

The following example will show how loadwatch and lw-ctr work. First, digiKam should be started, and its databases should be updated. Updating is especially important for the pHash fingerprints database, which is used to search for out of focus images and compare them. Updating the databases should not eat up all of the machine resources.

In most cases, it is not enough to do a simple start via loadwatch for programs like digiKam. Listing 3 shows why. The parameters selected here are actually fairly good for this computer. Generating and updating database is done quietly in the background without taking endless amounts of time. The changing states are documented on the terminal (Listing 4).

Listing 3

Starting digiKam

$loadwatch -- digikam
Wed Aug 26 16:00:56 2015: load too high, stopping.

Listing 4

Changing States

Wed Aug 26 17:00:55 2015: load low, continuing.
Wed Aug 26 17:01:15 2015: load too high, stopping.
Wed Aug 26 17:03:45 2015: load low, continuing.
Wed Aug 26 17:03:55 2015: load too high, stopping.
Wed Aug 26 17:04:55 2015: load low, continuing.
digikam(8477)/digikam (core) digikam::PreviewLoadingTask::execute: Cannot extract preview for "32bit_8040963.tif"
digikam(8477)/digikam (core) digikam::PreviewLoadingTask::execute: Cannot extract preview for "32bit_8040963.tif"
Wed Aug 26 17:05:05 2015: load too high, stopping.

Just the initialization of digiKam takes up so much compute time that loadwatch has to switch into wait mode almost immediately. Therefore, it is a good idea to give an upper limit so that digiKam can at least start without immediately being forced to stop. The alternative would be to first execute the start in "RUN" mode via lw-ctr and then switch to "WATCH" mode. See below for details.

Experience teaches that a computer runs fairly well up to a load of 2. Thus, a good upper load limit should be about 1.8, or 90% of 2.0. Additionally, the lower limit should not be set too low, otherwise program execution will be prevented for long periods of time. This is because it takes a certain amount of time before the system load goes down sufficiently. The value for the lower limit is estimated to be 1.6. Controlling this value via lw-ctr is ensured by means of a socket in the /tmp/ directory.

Now the command to be used looks like this:

loadwatch -l 1.6 -h 1.8 -u /tmp/tst.loadwatch -- digikam

As expected, the program will now start easily. At first, the system load will not reach the critical limit because the threshold has been set high enough for the start to proceed without interruption.

The entry for updating the specified databases separately can be found under Maintenance in the Extras menu option (Figure 1). See the "Loadwatch Options" box for additional information.

Loadwatch Options

loadwatch is controlled with a small number of options. The syntax for loadwatch looks as follows:

loadwatch Options -- Program with Options
  • The pseudo-option can usually be elided when there is no danger that the options for the Program will collide with the options for loadwatch .
  • -h <upper threshold> defines the level of the system load at which the Program will be stopped. The upper threshold is given as a decimal number.
  • -l <lower threshold> defines the level of the system load at which the Program will restart. The lower threshold is given as a decimal number.
  • -d <Delay> determines the loadwatch wait interval between two scans of the system load. Delay is given in seconds.
  • -n <Copies> define how many copies are started by the programs controlled by loadwatch . The default setting is one.
  • -u <Socket> defines the socket file for communicating with lw-ctr .
  • -p <PID> makes it possible using the specified PID to retroactively control a program that is currently executing via loadwatch . You should be careful here. This does not always work properly and can at times lead to problems with the system such as blocked input devices. Fortunately, these problems seldom appear and then only with complex programs.
Figure 1: Digikam lets the user specify individual databases to be updated.

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