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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).
total intensity, denoted
TI(n), is calculated according to the formula,
TI(n) = exp(-Weight*(T(n) - T(n-1)) * TI(n-1) + Weight,
TI(n-1) is the previous total intensity.
accept intensity, denoted
AI(n), is determined by the formula,
AI(n) = exp(-Weight*(T(n) - T(n-1)) * AI(n-1) + Weight,
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, []}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.
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
OverloadInfo = [{total_intensity, TotalIntensity},
{accept_intensity, AcceptIntensity}, {max_intensity,
MaxIntensity}, {weight, Weight}, {total_requests,
TotalRequests}, {accepted_requests, AcceptedRequests}].TotalIntensity = float() > 0AcceptIntensity = float() > 0MaxIntensity = float() > 0Weight = float() > 0TotalRequests = 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).
alarm_handler(3), sasl(3)