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53 lines
2.2 KiB
53 lines
2.2 KiB
4 months ago
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# ART Metrics
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This directory contains most of ART's metrics framework. Some portions that
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rely on the runtime can be found in the `runtime/metrics` directory.
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## Declaring Metrics
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ART's internal metrics are listed in the `ART_METRICS` macro in `metrics.h`.
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Each metric has a `METRIC` entry which takes a name for the metric, a type
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(such as counter or histogram), and any additional arguments that are needed.
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### Counters
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METRIC(MyCounter, MetricsCounter)
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Counters store a single value that can be added to. This is useful for counting
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events, counting the total amount of time spent in a section of code, and other
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uses.
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### Accumulators
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METRIC(MyAccumulator, MetricsAccumulator, type, accumulator_function)
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Example:
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METRIC(MaximumTestMetric, MetricsAccumulator, int64_t, std::max<int64_t>)
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Accumulators are a generalization of counters that takes an accumulator
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function that is used to combine the new value with the old value. Common
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choices are the min and max function. To be valid, the accumulator function
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must be monotonic in its first argument. That is, if
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`x_new == accumulator_function(x_old, y)` then `x_new ⪯ x_old` for some
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ordering relation `⪯` (e.g. less-than-or-equal or greater-than-or-equal).
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### Histograms
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METRIC(MyHistogram, MetricsHistogram, num_buckets, minimum_value, maximum_value)
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Histograms divide a range into several buckets and count how many times a value
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falls within each bucket. They are useful for seeing the overall distribution
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for different events.
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The `num_buckets` parameter affects memory usage for the histogram and data
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usage for exported metrics. It is recommended to keep this below 16. The
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`minimum_value` and `maximum_value` parameters are needed because we need to
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know what range the fixed number of buckets cover. We could keep track of the
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observed ranges and try to rescale the buckets or allocate new buckets, but
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this would make incrementing them more expensive than just some index
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arithmetic and an add. Values outside the range get clamped to the nearest
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bucket (basically, the two buckets on either side are infinitely long). If we
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see those buckets being way taller than the others, it means we should consider
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expanding the range.
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