performGC before getting RTS/GC stats

The deprecated getGCStats had some nice documentation: "If you would
like your statistics as recent as possible, first run a performGC"

This is sadly missing from getRTSStats but I believe it still holds.
When regressing allocated over iters, I'd see a believable slope, but an
unbelievable y intercept, something like -300000 when the slope is 150
or so. That's because the first hundred or so measurements were 0 for
bytes allocated, as the RTS didn't bother to run a GC before the RTS
stats were sampled.

Now we do 3 samples:

  1. performGC before the first one, to ensure it's up-to-date.
  2. Do the second one after the action, without a performGC, so we can
     get legit readings on the GC-related stats.
  3. performGC and then sample again, so we can get up-to-date readings
     on other metrics.
  4. Carefully choose whether to diff start stats against the end stats
     per- or post-GC.

Also included is a fix to the ToJSON Measurements instance, which
duplicated the mutator cpu seconds where GC cpu seconds should go.

Author: avieth

Criterion/Measurement.hs

@@ -108,6 +108,7 @@ data GCStatistics = GCStatistics
 -- | Try to get GC statistics, bearing in mind that the GHC runtime
 -- will throw an exception if statistics collection was not enabled
 -- using \"@+RTS -T@\".
+-- If you need guaranteed up-to-date stats, call performGC first.
 getGCStatistics :: IO (Maybe GCStatistics)
 #if MIN_VERSION_base(4,10,0)
 -- Use RTSStats/GCDetails to gather GC stats
@@ -170,16 +171,23 @@ measure :: Benchmarkable        -- ^ Operation to benchmark.
         -> Int64                -- ^ Number of iterations.
         -> IO (Measured, Double)
 measure bm iters = runBenchmarkable bm iters addResults $ \ !n act -> do
+  -- Ensure the stats from getGCStatistics are up-to-date.
+  performGC
   startStats <- getGCStatistics
   startTime <- getTime
   startCpuTime <- getCPUTime
   startCycles <- getCycles
   act
-  endStats <- getGCStatistics
   endTime <- getTime
   endCpuTime <- getCPUTime
   endCycles <- getCycles
-  let !m = applyGCStatistics endStats startStats $ measured {
+  -- From these we can derive GC-related deltas.
+  endStatsPreGC <- getGCStatistics
+  performGC
+  -- From these we can derive all other deltas, and performGC guarantees they
+  -- are up-to-date.
+  endStatsPostGC <- getGCStatistics
+  let !m = applyGCStatistics endStatsPostGC endStatsPreGC startStats $ measured {
              measTime    = max 0 (endTime - startTime)
            , measCpuTime = max 0 (endCpuTime - startCpuTime)
            , measCycles  = max 0 (fromIntegral (endCycles - startCycles))
@@ -313,22 +321,24 @@ measured = Measured {
 -- | Apply the difference between two sets of GC statistics to a
 -- measurement.
 applyGCStatistics :: Maybe GCStatistics
-                  -- ^ Statistics gathered at the __end__ of a run.
+                  -- ^ Statistics gathered at the __end__ of a run, post GC.
+                  -> Maybe GCStatistics
+                  -- ^ Statistics gathered at the __end__ of a run, pre GC.
                   -> Maybe GCStatistics
                   -- ^ Statistics gathered at the __beginning__ of a run.
                   -> Measured
                   -- ^ Value to \"modify\".
                   -> Measured
-applyGCStatistics (Just end) (Just start) m = m {
-    measAllocated          = diff gcStatsBytesAllocated
-  , measNumGcs             = diff gcStatsNumGcs
-  , measBytesCopied        = diff gcStatsBytesCopied
-  , measMutatorWallSeconds = diff gcStatsMutatorWallSeconds
-  , measMutatorCpuSeconds  = diff gcStatsMutatorCpuSeconds
-  , measGcWallSeconds      = diff gcStatsGcWallSeconds
-  , measGcCpuSeconds       = diff gcStatsGcCpuSeconds
-  } where diff f = f end - f start
-applyGCStatistics _ _ m = m
+applyGCStatistics (Just endPostGC) (Just endPreGC) (Just start) m = m {
+    measAllocated          = diff endPostGC start gcStatsBytesAllocated
+  , measNumGcs             = diff endPreGC  start gcStatsNumGcs
+  , measBytesCopied        = diff endPostGC start gcStatsBytesCopied
+  , measMutatorWallSeconds = diff endPostGC start gcStatsMutatorWallSeconds
+  , measMutatorCpuSeconds  = diff endPostGC start gcStatsMutatorCpuSeconds
+  , measGcWallSeconds      = diff endPreGC  start gcStatsGcWallSeconds
+  , measGcCpuSeconds       = diff endPreGC  start gcStatsGcCpuSeconds
+  } where diff end start f = f end - f start
+applyGCStatistics _ _ _ m = m
 
 -- | Convert a number of seconds to a string.  The string will consist
 -- of four decimal places, followed by a short description of the time

Criterion/Types.hs

@@ -212,7 +212,7 @@ instance ToJSON Measured where
       (measTime, measCpuTime, measCycles, measIters,
        i measAllocated, i measNumGcs, i measBytesCopied,
        d measMutatorWallSeconds, d measMutatorCpuSeconds,
-       d measGcWallSeconds, d measMutatorCpuSeconds)
+       d measGcWallSeconds, d measGcCpuSeconds)
       where i = fromInt; d = fromDouble
 
 instance NFData Measured where