doc: dev guide: benchmarking

doc/developer-guide.md

@@ -304,7 +304,109 @@ $ make ghci-dev
 ```
 -->
 
-### Run the tests
+### Run benchmarks
+
+Benchmarks are standard performance measurements,
+which we define using `bench` declarations in cabal files.
+There is [one in hledger.cabal](https://github.com/simonmichael/hledger/blob/master/hledger/hledger.cabal#L228),
+with related code and data files in [hledger/bench/](https://github.com/simonmichael/hledger/tree/master/hledger/bench).
+
+To run the standard hledger benchmark, use `stack bench hledger`. 
+This installs haskell dependencies (but not system dependencies) and rebuilds as needed,
+then runs [hledger/bench/bench.hs](https://github.com/simonmichael/hledger/blob/master/hledger/bench/bench.hs),
+which by default shows quick elapsed-time measurements for several operations on a standard data file:
+
+```shell
+$ stack bench hledger
+NOTE: the bench command is functionally equivalent to 'build --bench'
+...
+hledger-0.26.98: benchmarks
+Running 1 benchmarks...
+Benchmark bench: RUNNING...
+Benchmarking hledger in /Users/simon/src/hledger/hledger with timeit
+read bench/10000x1000x10.journal        [1.63s]
+print                                   [1.41s]
+register                                [1.83s]
+balance                                 [0.24s]
+stats                                   [0.22s]
+Total: 5.33s
+Benchmark bench: FINISH
+```
+
+bench.hs has some other modes, which you can use by compiling and running it directly.
+`--criterion` reports more detailed and dependable measurements, but takes longer:
+
+```shell
+$ cd hledger; stack exec -- ghc -ibench bench/bench && bench/bench --criterion
+...
+Linking bench/bench ...
+Benchmarking hledger in /Users/simon/src/hledger/hledger with criterion
+benchmarking read bench/10000x1000x10.journal
+time                 1.414 s    (1.234 s .. 1.674 s)
+                     0.996 R²   (0.989 R² .. 1.000 R²)
+mean                 1.461 s    (1.422 s .. 1.497 s)
+std dev              59.69 ms   (0.0 s .. 62.16 ms)
+variance introduced by outliers: 19% (moderately inflated)
+
+benchmarking print
+time                 1.323 s    (1.279 s .. 1.385 s)
+                     1.000 R²   (0.999 R² .. 1.000 R²)
+mean                 1.305 s    (1.285 s .. 1.316 s)
+std dev              17.20 ms   (0.0 s .. 19.14 ms)
+variance introduced by outliers: 19% (moderately inflated)
+
+benchmarking register
+time                 1.995 s    (1.883 s .. 2.146 s)
+                     0.999 R²   (0.998 R² .. NaN R²)
+mean                 1.978 s    (1.951 s .. 1.995 s)
+std dev              25.09 ms   (0.0 s .. 28.26 ms)
+variance introduced by outliers: 19% (moderately inflated)
+
+benchmarking balance
+time                 251.3 ms   (237.6 ms .. 272.4 ms)
+                     0.998 R²   (0.997 R² .. 1.000 R²)
+mean                 260.4 ms   (254.3 ms .. 266.5 ms)
+std dev              7.609 ms   (3.192 ms .. 9.638 ms)
+variance introduced by outliers: 16% (moderately inflated)
+
+benchmarking stats
+time                 325.5 ms   (299.1 ms .. 347.2 ms)
+                     0.997 R²   (0.985 R² .. 1.000 R²)
+mean                 329.2 ms   (321.5 ms .. 339.6 ms)
+std dev              11.08 ms   (2.646 ms .. 14.82 ms)
+variance introduced by outliers: 16% (moderately inflated)
+```
+
+`--simplebench` shows a table of elapsed-time measurements for the commands defined in [bench/default.bench](https://github.com/simonmichael/hledger/blob/master/hledger/bench/default.bench).
+It can also show the results for multiple h/ledger executables side by side, if you tweak the bench.hs code.
+Unlike the other modes, it does not link with the hledger code directly, but runs the "hledger" executable found in $PATH (so ensure that's the one you intend to test).
+
+```shell
+$ cd hledger; stack exec -- ghc -ibench bench/bench && bench/bench --simplebench
+Benchmarking /Users/simon/.local/bin/hledger in /Users/simon/src/hledger/hledger with simplebench and shell
+Using bench/default.bench
+Running 4 tests 1 times with 1 executables at 2015-08-23 16:58:59.128112 UTC:
+1: hledger -f bench/10000x1000x10.journal print	[3.27s]
+1: hledger -f bench/10000x1000x10.journal register	[3.65s]
+1: hledger -f bench/10000x1000x10.journal balance	[2.06s]
+1: hledger -f bench/10000x1000x10.journal stats	[2.13s]
+
+Summary (best iteration):
+
++-----------------------------------------++---------+
+|                                         || hledger |
++=========================================++=========+
+| -f bench/10000x1000x10.journal print    ||    3.27 |
+| -f bench/10000x1000x10.journal register ||    3.65 |
+| -f bench/10000x1000x10.journal balance  ||    2.06 |
+| -f bench/10000x1000x10.journal stats    ||    2.13 |
++-----------------------------------------++---------+
+```
+
+For quick, fine-grained performance measurements eg when troubleshooting or optimising, I use [dev.hs](https://github.com/simonmichael/hledger/blob/master/dev.hs).
+
+
+### Run tests
 
 This command will install haskell dependencies (you might need to
 install additional system dependencies yourself) and run the package
@@ -336,7 +438,6 @@ Test haddock doc generation:
 $ make haddocktest
 ```
 
-
 ### Add a test
 
 - identify what to test