Allow thread pool to be sized with -Dthread_pool_size.

Also use num processors minus one instead of num processors.

core/src/main/java/com/threerings/getdown/data/Application.java

@@ -1224,7 +1224,7 @@ public class Application
         // resources are verified on background threads supplied by the thread pool, and progress
         // is reported by posting runnable actions to the actions queue which is processed by the
         // main (UI) thread
-        Executor exec = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
+        Executor exec = Executors.newFixedThreadPool(SysProps.threadPoolSize());
         final BlockingQueue<Runnable> actions = new LinkedBlockingQueue<Runnable>();
         final int[] completed = new int[1];
 

core/src/main/java/com/threerings/getdown/data/Digest.java

@@ -56,8 +56,7 @@ public class Digest
         throws IOException
     {
         // first compute the digests for all the resources in parallel
-        ExecutorService exec = Executors.newFixedThreadPool(
-            Runtime.getRuntime().availableProcessors());
+        ExecutorService exec = Executors.newFixedThreadPool(SysProps.threadPoolSize());
         final Map<Resource, String> digests = new ConcurrentHashMap<>();
         final BlockingQueue<Object> completed = new LinkedBlockingQueue<>();
         final int fversion = version;

core/src/main/java/com/threerings/getdown/data/SysProps.java

@@ -107,6 +107,12 @@ public class SysProps
         return Integer.getInteger("read_timeout", 30);
     }
 
+    /** Returns the number of threads used to perform digesting and verifying operations in
+      * parallel. Usage: {@code -Dthread_pool_size=N} */
+    public static int threadPoolSize () {
+        return Integer.getInteger("thread_pool_size", Runtime.getRuntime().availableProcessors()-1);
+    }
+
     /** Parses a Java version system property using the supplied regular expression. The numbers
       * extracted from the regexp will be placed in each consecutive hundreds position in the
       * returned value.