HPCC-15404 Optimize roxie standalone queries

Start worker threads on demand.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>

roxie/ccd/ccdqueue.cpp

@@ -726,7 +726,8 @@ class RoxieQueue : public CInterface, implements IThreadFactory
     CriticalSection qcrit;
     unsigned headRegionSize;
     unsigned numWorkers;
-
+    unsigned started;
+    std::atomic<unsigned> idle;
 public:
     IMPLEMENT_IINTERFACE;
 
@@ -735,6 +736,8 @@ public:
         headRegionSize = _headRegionSize;
         numWorkers = _numWorkers;
         workers.setown(createThreadPool("RoxieWorkers", this, NULL, numWorkers));
+        started = 0;
+        idle = 0;
     }
 
     virtual IPooledThread *createNew();
@@ -742,8 +745,7 @@ public:
 
     void start()
     {
-        for (unsigned i = 0; i < numWorkers; i++)
-            workers->start(this);
+        // We start on demand now...
     }
 
     IPooledThreadIterator *running()
@@ -776,6 +778,11 @@ public:
             queueLength++;
             if (queueLength>maxQueueLength)
                 maxQueueLength = queueLength;
+            if (idle==0 && started < numWorkers)
+            {
+                workers->start(this);
+                started++;
+            }
         }
         available.signal();
     }
@@ -866,7 +873,9 @@ public:
 
     void wait()
     {
+        idle++;
         available.wait();
+        idle--;
     }
 
     void signal(unsigned num)