4.3.5. Can I do anything as a user?

A big environment can slow you down. If you have lots of environment variables set (instead of shell variables), long search paths that are not optimized (errors in setting the path environment variable) and such, the system will need more time to search and read data.

In X, window managers and desktop environments can be real CPU-eaters. A really fancy desktop comes with a price, even when you can download it for free, since most desktops provide add-ons ad infinitum.

Modesty is a virtue if you don't buy a new computer every year.

4.3.5.1. Priority

The priority or importance of a job is defined by it's nice number. A program with a high nice number is friendly to other programs, other users and the system; it is not an important job. The lower the nice number, the more important a job is and the more resources it will take without sharing them.

Making a job nicer by increasing its nice number is only useful for processes that use a lot of CPU time (compilers, math applications and such). Processes that always use a lot of I/O time are automatically rewarded by the system and given a higher priority (a lower nice number), for example keyboard input always gets highest priority on a system.

Defining the priority of a program is done with the nice command.

Most systems also provide the BSD renice command, which allows you to change the niceness of a running command. Again, read the man page for your system-specific information.

       Interactive programs
        

It is NOT a good idea to nice or renice an interactive program or a job running in the foreground.

Use of these commands is usually a task for the system administrator. Read the man page for more info on extra functionality available to the system administrator.

4.3.5.2. CPU resources

On every Linux system, many programs want to use the CPU(s) at the same time, even if you are the only user on the system. Every program needs a certain amount of cycles on the CPU to run. There may be times when there are not enough cycles because the CPU is too busy. The uptime command is wildly inaccurate (it only displays averages, you have to know what is normal), but far from being useless. There are some actions you can undertake if you think your CPU is to blame for the unresponsiveness of your system:

• Run heavy programs when the load is low. This may be the case on your system during the night. See next section for scheduling.
  
• Prevent the system from doing unnecessary work: stop daemons and programs that you don't use, use locate instead of a heavy find , ...
  
• Run big jobs with a low priority
  

If none of these solutions are an option in your particular situation, you may want to upgrade your CPU. On a UNIX machine this is a job for the system admin.

4.3.5.3. Memory resources

When the currently running processes expect more memory than the system has physically available, a Linux system will not crash; it will start paging, or swapping , meaning the process uses the memory on disk or in swap space, moving contents of the physical memory (pieces of running programs or entire programs in the case of swapping) to disk, thus reclaiming the physical memory to handle more processes. This slows the system down enormously since access to disk is much slower than access to memory. The top command can be used to display memory and swap use. Systems using glibc offer the memusage and memusagestat commands to visualize memory usage.

If you find that a lot of memory and swap space are being used, you can try:

• Killing, stopping or renicing those programs that use a big chunk of memory
  
• Adding more memory (and in some cases more swap space) to the system.
  
• Tuning system performance, which is beyond the scope of this document. See the reading list in Appendix A for more.
  

4.3.5.4. I/O resources

While I/O limitations are a

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