# overload

### MODULE

overload

### MODULE SUMMARY

An Overload Regulation Process

### DESCRIPTION

`overload` is a process which indirectly regulates CPU usage in the system. The idea is that a main application calls the `request/0` function before starting a major job, and proceeds with the job if the return value is positive; otherwise the job must not be started.

`overload` is part of the `sasl` application, and all configuration parameters are defined there.

A set of two intensities are maintained, the `total intensity` and the `accept intensity`. For that purpose there are two configuration parameters, the `MaxIntensity` and the `Weight` value (both are measured in 1/second).

Then total and accept intensities are calculated as follows. Assume that the time of the current call to `request/0` is `T(n)`, and that the time of the previous call was `T(n-1)`.

- The current `total intensity`, denoted `TI(n)`, is calculated according to the formula,
  `TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) + Weight`,
  where `TI(n-1)` is the previous total intensity.
- The current `accept intensity`, denoted `AI(n)`, is determined by the formula,
  `AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight`,
  where `AI(n-1)` is the previous accept intensity, provided that the value of `exp(-Weight*(T(n) - T(n-1)) * AI(n-1)` is less than `MaxIntensity`; otherwise the value is
  `AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1)`.

The value of configuration parameter `Weight` controls the speed with which the calculations of intensities will react to changes in the underlying input intensity. The inverted value of `Weight`,

`T = 1/Weight`

can be thought of as the "time constant" of the intensity calculation formulas. For example, if `Weight = 0.1`, then a change in the underlying input intensity will be reflected in the `total` and `accept intensities` within approximately 10 seconds.

The overload process defines one alarm, which it sets using `alarm_handler:set_alarm(Alarm)`. `Alarm` is defined as:

`{overload, []}`
  This alarm is set when the current accept intensity exceeds `MaxIntensity`.

A new overload alarm is not set until the current accept intensity has fallen below `MaxIntensity`. To prevent the overload process from generating a lot of set/reset alarms, the alarm is not reset until the current accept intensity has fallen below 75% of `MaxIntensity`, and it is not until then that the alarm can be set again.

### EXPORTS

`request() -> accept | reject`

Returns `accept` or `reject` depending on the current value of the accept intensity.

The application calling this function should be processed with the job in question if the return value is `accept`; otherwise it should not continue with that job.

`get_overload_info() -> OverloadInfo`

Types:

`OverloadInfo = [{total_intensity, TotalIntensity}, {accept_intensity, AcceptIntensity}, {max_intensity, MaxIntensity}, {weight, Weight}, {total_requests, TotalRequests}, {accepted_requests, AcceptedRequests}].`
`TotalIntensity = float() > 0`
`AcceptIntensity = float() > 0`
`MaxIntensity = float() > 0`
`Weight = float() > 0`
`TotalRequests = integer()`
`AcceptedRequests = integer()`

Returns the current total and accept intensities, the configuration parameters, and absolute counts of the total number of requests, and accepted number of requests (since the overload process was started).

### See Also

alarm_handler(3), sasl(3)

### AUTHORS

Martin Björklund - support@erlang.ericsson.se
Peter Högfeldt - support@erlang.ericsson.se

sasl 2.1.3
Copyright © 1991-2006 Ericsson AB