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

@@ -180,16 +180,23 @@ mkTableConfigSetup GlobalOpts{diskCachePolicy} SetupOpts{bloomFilterAlloc} conf
     , LSM.confBloomFilterAlloc = bloomFilterAlloc
     }
 
-mkTableConfigRun :: GlobalOpts -> LSM.TableConfig -> LSM.TableConfig
-mkTableConfigRun GlobalOpts{diskCachePolicy} conf = conf {
-      LSM.confDiskCachePolicy = diskCachePolicy
+mkTableConfigRun :: GlobalOpts -> RunOpts -> LSM.TableConfig -> LSM.TableConfig
+mkTableConfigRun GlobalOpts{diskCachePolicy} RunOpts {pipelined} conf =
+    conf {
+      LSM.confDiskCachePolicy = diskCachePolicy,
+      LSM.confMergeBatchSize  = if pipelined
+                                  then LSM.MergeBatchSize 1
+                                  else LSM.confMergeBatchSize conf
     }
 
-mkOverrideDiskCachePolicy :: GlobalOpts -> LSM.TableConfigOverride
-mkOverrideDiskCachePolicy GlobalOpts{diskCachePolicy} =
-  LSM.noTableConfigOverride {
-    LSM.overrideDiskCachePolicy = Just diskCachePolicy
-  }
+mkTableConfigOverride :: GlobalOpts -> RunOpts -> LSM.TableConfigOverride
+mkTableConfigOverride GlobalOpts{diskCachePolicy} RunOpts {pipelined} =
+    LSM.noTableConfigOverride {
+      LSM.overrideDiskCachePolicy = Just diskCachePolicy,
+      LSM.overrideMergeBatchSize  = if pipelined
+                                      then Just (LSM.MergeBatchSize 1)
+                                      else Nothing
+    }
 
 mkTracer :: GlobalOpts -> Tracer IO LSM.LSMTreeTrace
 mkTracer gopts
@@ -585,9 +592,9 @@ doRun gopts opts = do
         -- reference version starts with empty (as it's not practical or
         -- necessary for testing to load the whole snapshot).
         tbl <- if check opts
-                then let conf = mkTableConfigRun gopts benchTableConfig
+                then let conf = mkTableConfigRun gopts opts benchTableConfig
                       in LSM.newTableWith @IO @K @V @B conf session
-                else let conf = mkOverrideDiskCachePolicy gopts
+                else let conf = mkTableConfigOverride gopts opts
                       in LSM.openTableFromSnapshotWith @IO @K @V @B conf session name label
 
         -- In checking mode, compare each output against a pure reference.