MVStore: add feature to set the cache concurrency

h2/src/docsrc/html/changelog.html

@@ -91,6 +91,8 @@ Change Log
     when using larger databases and many threads.
     To re-enable, use the file name prefix "cache:".
 </li>
+<li>MVStore: add feature to set the cache concurrency.
+</li>
 <li>LIRS cache: improved hit rate because now added entries get hot if they 
     were in the non-resident part of the cache before.
 </li>

h2/src/main/org/h2/mvstore/MVStore.java

@@ -326,6 +326,10 @@ public class MVStore {
         if (mb > 0) {
             CacheLongKeyLIRS.Config cc = new CacheLongKeyLIRS.Config();
             cc.maxMemory = mb * 1024L * 1024L;
+            o = config.get("cacheConcurrency");
+            if (o != null) {
+                cc.segmentCount = (Integer) o;
+            }
             cache = new CacheLongKeyLIRS<Page>(cc);
             cc.maxMemory /= 4;
             cacheChunkRef = new CacheLongKeyLIRS<PageChildren>(cc);
@@ -2826,6 +2830,16 @@ public class MVStore {
             return set("cacheSize", mb);
         }
 
+        /**
+         * Set the read cache concurrency. The default is 16, meaning 16 segments are used.
+         *
+         * @param concurrency the cache concurrency
+         * @return this
+         */
+        public Builder cacheConcurrency(int concurrency) {
+            return set("cacheConcurrency", concurrency);
+        }
+
         /**
          * Compress data before writing using the LZF algorithm. This will save
          * about 50% of the disk space, but will slow down read and write