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If we monitor with setImmediate, we miss big blocking loops. For example, suppose we have 1000 1ms loops then a single bad 1000ms loop. setImmediate will only be called at the right time 1/1000 of the time (it has to be the loop just before the bad one). So this monitoring method gives a good average if the std dev is low, but doesn't pick up spikes. Instead, we can monitor the skew from the expected time between setIntervals. In the case above, with a setInterval for 1000ms, we will pick up a skew proportional to the amount of time that it overlaps the bad loop. So 50% change of picking up skew > 500ms, and thus getting a good sense of any spikes.
16 lines
473 B
CoffeeScript
16 lines
473 B
CoffeeScript
module.exports = EventLoopMonitor =
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monitor: (logger, interval = 1000, log_threshold = 100) ->
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Metrics = require "./metrics"
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previous = Date.now()
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intervalId = setInterval () ->
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now = Date.now()
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offset = now - previous - interval
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if offset > log_threshold
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logger.warn {offset: offset}, "slow event loop"
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previous = now
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Metrics.timing("event-loop-millsec", offset)
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, interval
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Metrics.registerDestructor () ->
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clearInterval(intervalId)
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