small tweaks for micro benchmarks

A quick follow-up to #538

Author: mheinzel

bench/micro/Bench/Database/LSMTree/Internal/Lookup.hs

@@ -252,7 +252,10 @@ randomEntry :: StdGen -> (Entry UTxOValue ByteString, StdGen)
 randomEntry g = frequency [
       (20, \g' -> let (!v, !g'') = uniform g' in (Insert v, g''))
     , (1,  \g' -> let (!v, !g'') = uniform g'
-                      (!b, !g''') = randomByteStringR (0, 2000) g''  -- < 2kB
+                      -- The size of the blobs doesn't matter for the benchmark,
+                      -- as it only deals with the blob references. So we make
+                      -- them tiny to not slow down the setup.
+                      (!b, !g''') = randomByteStringR (0, 100) g''
                   in  (InsertWithBlob v b, g'''))
     , (2,  \g' -> let (!v, !g'') = uniform g' in (Mupdate v, g''))
     , (2,  \g' -> (Delete, g'))

bench/micro/Bench/Database/LSMTree/Internal/Merge.hs

@@ -231,11 +231,10 @@ benchMerge conf@Config{name} =
             --    thread `runs` through the environment, too.
             -- 2. It forces the result to normal form, which would traverse the
             --    whole run, so we force to WHNF ourselves and just return `()`.
-
-            -- We make sure to immediately close resulting runs so we don't run
-            -- out of file handles or disk space. However, we don't want it to
-            -- be part of the measurement, as it includes deleting files.
-            -- Therefore, ... TODO
+            -- 3. It doesn't have access to the run we created in the benchmark,
+            --    but the cleanup should not be part of the measurement, as it
+            --    includes deleting files. So we smuggle the reference out using
+            --    an `IORef`.
             Cr.perRunEnvWithCleanup
               ((runs,) <$> newIORef Nothing)
               (releaseRun . snd) $ \(runs', ref) -> do