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In UNIX computing, the system load is a measure of the amount of computational 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.
All Unix and Unix-like systems generate a dimensionless 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 14:34:03 up 10:43, 4 users, load average: 0.06, 0.11, 0.09The w and top commands show the same three load average numbers, as do a range of graphical user interface utilities.
In operating systems based on the Linux kernel, this information can be easily accessed by reading the /proc/loadavg file.
To explore this kind of information in dept, according to the Linux's Filesystem Hierarchy Standard, architecture-dependent information are exposed on the file /proc/stat. [1] [2] [3]
An idle computer has a load number of 0 (the idle process is not counted). Each process using or waiting for CPU (the ready queue or run queue) increments the load number by 1. Each process that terminates decrements it 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. [4] This, for example, includes processes blocking due to an NFS server failure or too 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 of 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. [5]
Mathematically speaking, all three values always average all the system load since the system started up. They all decay exponentially, but they decay at different speeds: they decay exponentially by e after 1, 5, and 15 minutes respectively. Hence, the 1-minute load average consists of 63% (more precisely: 1 - 1/e) of the load from the last minute and 37% (1/e) of the average load since start up, excluding the last minute. For the 5- and 15-minute load averages, the same 63%/37% ratio is computed over 5 minutes and 15 minutes, respectively. Therefore, it is not technically accurate that the 1-minute load average only includes the last 60 seconds of activity, as it includes 37% of the activity from the past, but it is correct to state that it includes mostly the last minute.
For single-CPU systems that are CPU bound, one can think of load average as a measure of system utilization during the respective time period. For systems with multiple CPUs, one must divide the load by the number of processors in order to get a comparable measure.
For example, one can interpret a load average of "1.73 0.60 7.98" on a single-CPU system as:
This means that this system (CPU, disk, memory, etc.) could have handled all the work scheduled for the last minute if it were 1.73 times as fast.
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. Linux appears to count each thread separately as adding 1 to the load. [6]
The comparative study of different load indices carried out by Ferrari et al. [7] 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, are both very likely to have utilizations close to 100%, although they obviously differ.[ original research? ]
On Linux systems, the load-average is not calculated on each clock tick, but driven by a variable value that is based on the HZ frequency setting and tested on each clock tick. This setting defines the kernel clock tick rate in Hertz (times per second), and it defaults to 100 for 10ms ticks. Kernel activities use this number of ticks to time themselves. Specifically, the timer.c::calc_load() function, which calculates the load average, runs every LOAD_FREQ = (5*HZ+1) ticks, or about every five seconds:
unsignedlongavenrun[3];staticinlinevoidcalc_load(unsignedlongticks){unsignedlongactive_tasks;/* fixed-point */staticintcount=LOAD_FREQ;count-=ticks;if(count<0){count+=LOAD_FREQ;active_tasks=count_active_tasks();CALC_LOAD(avenrun[0],EXP_1,active_tasks);CALC_LOAD(avenrun[1],EXP_5,active_tasks);CALC_LOAD(avenrun[2],EXP_15,active_tasks);}}The avenrun array contains 1-minute, 5-minute and 15-minute average. The CALC_LOAD macro and its associated values are defined in sched.h:
#define FSHIFT 11 /* nr of bits of precision */#define FIXED_1 (1<<FSHIFT) /* 1.0 as fixed-point */#define LOAD_FREQ (5*HZ+1) /* 5 sec intervals */#define EXP_1 1884 /* 1/exp(5sec/1min) as fixed-point */#define EXP_5 2014 /* 1/exp(5sec/5min) */#define EXP_15 2037 /* 1/exp(5sec/15min) */#define CALC_LOAD(load,exp,n) \ load *= exp; \ load += n*(FIXED_1-exp); \ load >>= FSHIFT;The "sampled" calculation of load averages is a somewhat common behavior; FreeBSD, too, only refreshes the value every five seconds. The interval is usually taken to not be exact so that they do not collect processes that are scheduled to fire at a certain moment. [8]
A post on the Linux mailing list considers its +1 tick insufficient to avoid Moire artifacts from such collection, and suggests an interval of 4.61 seconds instead. [9] This change is common among Android system kernels, although the exact expression used assumes an HZ of 100. [10]
Other commands for assessing system performance include:
uptime – the system reliability and load average top – for an overall system view vmstat – vmstat reports information about runnable or blocked processes, memory, paging, block I/O, traps, and CPU. htop – interactive process viewerdool (formerly dstat), [11] atop – helps correlate all existing resource data for processes, memory, paging, block I/O, traps, and CPU activity. iftop – interactive network traffic viewer per interfacenethogs – interactive network traffic viewer per processiotop – interactive I/O viewer [12] iostat – for storage I/O statistics netstat – for network statistics mpstat – for CPU statisticstload – load average graph for terminal xload – load average graph for X/proc/loadavg – text file containing load averageIn computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processes to share a single central processing unit (CPU), and is an essential feature of a multiprogramming or multitasking operating system. In a traditional CPU, each process - a program in execution - utilizes the various CPU registers to store data and hold the current state of the running process. However, in a multitasking operating system, the operating system switches between processes or threads to allow the execution of multiple processes simultaneously. For every switch, the operating system must save the state of the currently running process, followed by loading the next process state, which will run on the CPU. This sequence of operations that stores the state of the running process and the loading of the following running process is called a context switch.
Mach is a kernel developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian to support operating system research, primarily distributed and parallel computing. Mach is often considered one of the earliest examples of a microkernel. However, not all versions of Mach are microkernels. Mach's derivatives are the basis of the operating system kernel in GNU Hurd and of Apple's XNU kernel used in macOS, iOS, iPadOS, tvOS, and watchOS.
OS-9 is a family of real-time, process-based, multitasking, multi-user operating systems, developed in the 1980s, originally by Microware Systems Corporation for the Motorola 6809 microprocessor. It was purchased by Radisys Corp in 2001, and was purchased again in 2013 by its current owner Microware LP.
In computing, a process is the instance of a computer program that is being executed by one or many threads. There are many different process models, some of which are light weight, but almost all processes are rooted in an operating system (OS) process which comprises the program code, assigned system resources, physical and logical access permissions, and data structures to initiate, control and coordinate execution activity. Depending on the OS, a process may be made up of multiple threads of execution that execute instructions concurrently.
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In computer science, the event loop is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by making a request to some internal or external "event provider", then calls the relevant event handler. The event loop is also sometimes referred to as the message dispatcher, message loop, message pump, or run loop.
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...Dag Wieers ceased development of Dstat...