Load (computing)
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In UNIX computing, the system load is a measure of the amount of work that a computer system performs. The load average represents the average system load over a period of time. It conventionally appears in the form of three numbers which represent the system load during the last one, five, and fifteen -minute periods.
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[edit] Unix-style load calculation
All Unix and Unix-like systems generate a metric of three "load average" numbers in the kernel. Users can easily query the current result from a Unix shell by running the uptime command:
$ uptime 09:53:15 up 119 days, 19:08, 10 users, load average: 3.73 7.98 0.50
The w and top commands show the same three load average numbers, as do a range of graphical user interface utilities.
An idle computer has a load number of 0 and each process using or waiting for CPU adds to the load number by 1. Most UNIX systems count only processes in the running (on CPU) or runnable (waiting for CPU) states. However, Linux also includes processes in uninterruptible sleep states (usually waiting for disk activity), which can lead to markedly different results if many processes remain blocked in I/O due to a busy or stalled I/O system. This, for example, includes processes blocking due to an NFS server failure or to slow media (e.g., USB 1.x storage devices). Such circumstances can result in an elevated load average, which does not reflect an actual increase in CPU use (but still gives an idea on how long users have to wait).
Systems calculate the load average as the exponentially damped/weighted moving average of the load number. The three values of load average refer to the past one, five, and fifteen minutes of system operation.
For single-CPU systems that are CPU-bound, one can think of load average as a percentage of system utilization during the respective time period. For systems with multiple CPUs, one must divide the number by the number of processors in order to get a comparable percentage.
For example, one can interpret a load average of "1.73 0.50 7.98" on a single-CPU system as:
- during the last minute, the CPU was overloaded by 73% (1 CPU with 1.73 runnable processes, so that 0.73 processes had to wait for a turn)
- during the last 5 minutes, the CPU was underloaded 50% (no processes had to wait for a turn)
- during the last 15 minutes, the CPU was overloaded 698% (1 CPU with 7.98 runnable processes, so that 6.98 processes had to wait for a turn)
This means that this CPU could have handled all of the work scheduled for the last minute if it were 1.73 times as fast, or if there were two (1.73 rounded up) times as many CPUs, but that over the last five minutes it was twice as fast as necessary to prevent runnable processes from waiting their turn.
In a system with four CPUs, a load average of 3.73 would indicate that there were, on average, 3.73 processes ready to run, and each one could be scheduled into a CPU.
On modern UNIX systems, the treatment of threading with respect to load averages varies. Some systems treat threads as processes for the purposes of load average calculation: each thread waiting to run will add 1 to the load. However, other systems, especially systems implementing so-called M:N threading, use different strategies, such as counting the process exactly once for the purpose of load (regardless of the number of threads), or counting only threads currently exposed by the user-thread scheduler to the kernel, which may depend on the level of concurrency set on the process.
Many systems generate the load average by sampling the state of the scheduler periodically, rather than recalculating on all pertinent scheduler events. They adopt this approach for performance reasons, as scheduler events occur frequently, and scheduler efficiency impacts significantly on system efficiency. As a result, sampling error can lead to load averages inaccurately representing actual system behavior. This can pose a particular problem for programs that wake up at a fixed interval that aligns with the load-average sampling, in which case a process may be under- or over-represented in the load average numbers.
[edit] Load average is not CPU utilization
Even though the statements in the previous section might suggest that load average is related to CPU utilization because the section relates CPU to load average, load average does not measure CPU utilization of processes. One reason it does not do this is because load averages computations of processes are in a wrong order to relate to trend information of CPU utilization. In other words, the calculations and numbers directly produced by load averages do not compute numbers in an order from more CPU intensive to less CPU intensive or vice versa, nor do they give computations of numbers that would give another way that would result in direct information about CPU utilization. In summary, the functions of load average give numbers based on load queue of processes. [1] The next section uses the same reference to suggest that load average is not very important or vital to system performance information until or unless the a system's CPU is heavily loaded to around 100%. Then, at levels close to 100%, load average could be very important or significant to determinacy of system performance; however, this would be because average load numbers give direct information about process queue length not CPU utilization -- which is something they do not give direct information about.
[edit] CPU load vs CPU utilization
A comparative study of different load indices carried out by Ferrari et al.[1] reported that CPU load information based upon the CPU queue length does much better in load balancing compared to CPU utilization. The reason CPU queue length did better is probably because, when a host is heavily loaded, its CPU utilization is likely to be close to 100% and it is unable to reflect the exact load level of the utilization. In contrast, CPU queue lengths can directly reflect the amount of load on a CPU. As an example, two systems, one with 3 and the other with 6 processes in the queue, will probably have utilizations close to 100% although they obviously differ.
[edit] See also
Other commands for assessing system performance include:
- uptime for load average
- top for an overall system view
- htop interactive process viewer
- iotop interactive I/O viewer - iotop homepage
- iostat for storage I/O statistics
- netstat for network statistics
- mpstat for CPU statistics
- tload load average graph for terminal
- xload load average graph for X
[edit] External links
- Gunther, Neil. "UNIX Load Average Part 1: How It Works". www.teamquest.com. TeamQuest. http://www.teamquest.com/resources/gunther/ldavg1.shtml. Retrieved 2009-08-12. (including Linux)
- Examining Load Average by Ray Walker
