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Don't Use the Same Histogram Logging Call in Multiple Places These macros cache a pointer to the histogram object for efficiency, though this comes at the cost of increased binary size: 130 bytes/macro usage sounds small but quickly adds up. called in a loop or logged multiple times per second), use the macros in histogram_macros.h instead. However, when recording metrics on the critical path (e.g. These functions take a lock and perform a map lookup, but the overhead is generally insignificant. Prefer the helper functions defined in histogram_functions.h. To create a new category, the CL must be reviewed by Coding (Emitting to Histograms) If you're tempted to do so, please look through the existing categories to see whether any matches the metric(s) that you are adding. It should be quite rare to introduce new top-level categories into the existing taxonomy. Thus, histogram names should be in the form Category.Name or, etc., where each category organizes related histograms. Histograms are taxonomized into categories, using dot (. Often, this will require providing context in order for the data to be interpretable.įor enumerated histograms in particular, that often means including a bucket that can be used as a baseline for understanding the data recorded to other buckets: see the enumerated histogram section. When defining a new metric, think ahead about how you will analyze the data. If you simply add up the two histograms to get a total duration histogram, you’re implicitly assuming the two histograms' values are independent, which may not be the case.ĭirectly measure what you care about don't try to derive it from other data. However, histograms are pre-aggregated client-side, which means that there‘s no way to recover which emissions should be paired up. If we knew which emissions came from which calls, we could pair them up and derive the total time for the function. You might assume that you can simply sum the histograms for those two functions to get the total time, but that results in misleading data. Do not assume you can calculate what you want from other histograms, as most ways of doing this are incorrect.įor example, suppose you want to measure the runtime of a function that just calls two subfunctions, each of which is instrumented with histogram logging. Measure exactly what you want, whether that's the time used for a function call, the number of bytes transmitted to fetch a page, the number of items in a list, etc. Add Histogram and Documentation in the Same Changelistĭefining Useful Metrics Directly Measure What You Want.Don't Use Same String in Multiple Places.Use Fixed Strings When Using Histogram Macros.Don't Use the Same Histogram Logging Call in Multiple Places.