WP8 benchmark: use small merge batch size in pipelined mode

This now gets real parallel speedups on the WP8 benchmark in pipelined
mode.

On my laptop, we get:
* non-pipelined mode:                86.5k
* before: pipelined mode (2 cores):  92.2k
* after:  pipelined mode (2 cores): 120.0k

In part this is because pipelined mode on 1 core is a regression: 70.1k
because it has to do strictly more work, and it avoids doing any
batching which normally improves performance.

Author: dcoutts

bench/macro/lsm-tree-bench-wp8.hs

@@ -185,10 +185,13 @@ mkTableConfigRun GlobalOpts{diskCachePolicy} conf = conf {
       LSM.confDiskCachePolicy = diskCachePolicy
     }
 
-mkOverrideDiskCachePolicy :: GlobalOpts -> LSM.TableConfigOverride
-mkOverrideDiskCachePolicy GlobalOpts{diskCachePolicy} =
+mkOverrideDiskCachePolicy :: GlobalOpts -> RunOpts -> LSM.TableConfigOverride
+mkOverrideDiskCachePolicy GlobalOpts{diskCachePolicy} RunOpts {pipelined} =
   LSM.noTableConfigOverride {
-    LSM.overrideDiskCachePolicy = Just diskCachePolicy
+    LSM.overrideDiskCachePolicy = Just diskCachePolicy,
+    LSM.overrideMergeBatchSize  = if pipelined
+                                    then Just (LSM.MergeBatchSize 1)
+                                    else Nothing
   }
 
 mkTracer :: GlobalOpts -> Tracer IO LSM.LSMTreeTrace
@@ -587,7 +590,7 @@ doRun gopts opts = do
         tbl <- if check opts
                 then let conf = mkTableConfigRun gopts benchTableConfig
                       in LSM.newTableWith @IO @K @V @B conf session
-                else let conf = mkOverrideDiskCachePolicy gopts
+                else let conf = mkOverrideDiskCachePolicy gopts opts
                       in LSM.openTableFromSnapshotWith @IO @K @V @B conf session name label
 
         -- In checking mode, compare each output against a pure reference.