Introduce a section on dealing with negative results from a t-test (#156)

* docs: Fixing broken links in ToC

* docs: Add suggestions for dealing with inconsistent test runs.

* docs: adding a paragraph on forcing GC between runs

Author: jdmarshall

README.md

@@ -99,9 +99,9 @@ See the [examples folder](./examples/) for more common usage examples.
   - [Operations Mode](#operations-mode)
   - [Time Mode](#time-mode)
 - [Baseline Comparisons](#baseline-comparisons)
-- [Statistical Significance Testing](#statistical-significance-testing)
-  - [Using with Reporters](#using-with-reporters)
+- [Statistical Significance Testing](#statistical-significance-testing-t-test)
   - [Direct API Usage](#direct-api-usage)
+  - [Fixing Inconclusive Tests](#fixing-inconclusive-tests)
 - [Writing JavaScript Mistakes](#writing-javascript-mistakes)
 
 ## Sponsors
@@ -827,6 +827,8 @@ This helps identify when a benchmark shows a difference due to random variance v
 
 **Note**: Running the entire benchmark suite multiple times may still show variance in absolute numbers due to system-level factors (CPU frequency scaling, thermal throttling, background processes). The t-test helps determine if differences are statistically significant within each benchmark session, but results can vary between separate benchmark runs due to changing system conditions.
 
+See also: [Fixing Inconclusive Tests](doc/Inconclusive.md).
+
 ### Direct API Usage
 
 You can also use the t-test utilities directly for custom analysis:

doc/Inconclusive.md

@@ -0,0 +1,51 @@
+# Fixing Inconclusive Tests
+
+t-tests are looking at the distribution of both sets of results and trying to determine if they overlap in a way that
+makes the average value significant or just noise in the results. A run with a bimodal distribution for instance, caused
+by problems with the machine the tests are running on or the NodeJS runtime doing things in the background. Here are a
+few causes.
+
+## Random Input Data
+
+Variability in the inputs between runs can lead to big changes in the runtime of an algorithm. Particularly with code
+that sorts, filters, or conditionally operates on input data, feeding them certain combinations of data will result in
+wildly different run times from one loop to the next or occasionally from one sample to the next. The Central Limit
+Theorem (that over a long enough time a situation will revert to the mean), does not invalidate the existence of the
+Gambler's Paradox (that it will revert to the mean before I become bankrupt).
+
+It is better to do your fuzzing in fuzz tests and pick representative data for your benchmarks. Partially informed by
+the results of your fuzz tests, and other bug reports.
+
+## Underprovisioned VM, Oversubscribed hardware
+
+For a clean micro benchmark, we generally want to be the only one using the machine at the time. There are a number of
+known issues running benchmarks on machines that are thermally throttling, or on cheap VMs that use best-effort to
+allocate CPU time to the running processes. In particular, docker images with `cpu-shares` are especially poor targets
+for running benchmarks because the quota might expire for one timeslice in the middle of one test or between benchmarks
+in a single Suite. This creates an unfair advantage for the first test, and/or lots of noise in the results. We are
+currently investigating ways to detect this sort of noise, and analyzing if the t-tests are sufficient to do so.
+
+## Epicycles in GC or JIT compilation
+
+If the warmup time is insufficient to get V8 to optimize the code, it may kick in during the middle of a sample, which
+will introduce a bimodal distribution of answers (before, and after). There is currently not a way to adjust the warmup
+time of `bench-node`, but should be added as a feature.
+
+One of the nastiest performance issues to detect in garbage collected code is allocation epicycles. This happens when
+early parts of a calculation create lots of temporary data but not sufficient to cross the incremental or full GC
+threshold, so that the next function in a call sequence routinely gets hit with exceeding the threshold. This is
+especially common in code that generates a JSON or HTML response to a series of calculations - it is the single biggest
+allocation in the sequence, but it gets blamed in the performance report for the lion's share of the CPU time.
+
+If you change the `minTime` up or down, that will alter the number of iterations per sample which may smooth out the
+results. You can also try increasing `minSamples` to get more samples. But also take this as a suggestion that your code
+may have a performance bug that is worth prioritizing.
+
+Forcing GC in the teardown or setup methods between subsequent tests in a single Suite may help with some situations
+when reordering the tests results in differences in runtime, but for the more general case, you may need to review the
+code under test to make it 'play well' both with benchmarking and in production systems.
+
+In production code, particularly where p9# values are used as a fitness test, it is sometimes better to chose the
+algorithm with more consistent runtime over the one with supposedly better average runtime. This can also be true where
+DDOS scenarios are possible - the attacker will always chose the worst, most assymetric request to send to your machine,
+and mean response time will not matter one whit. If `bench-node` is complaining, the problem may not be `bench-node`.