Index: trunk/ppMerge/src/ppMergeReadChunk.c
===================================================================
--- trunk/ppMerge/src/ppMergeReadChunk.c	(revision 18839)
+++ trunk/ppMerge/src/ppMergeReadChunk.c	(revision 18967)
@@ -1,92 +1,102 @@
 # include "ppMerge.h"
 
-ppMergeFileGroup *ppMergeReadChunk (bool *status, psArray *fileGroups, pmConfig *config, int numChunk) {
+#define THREAD_WAIT 10000               // Microseconds to wait if thread is not available
 
+ppMergeFileGroup *ppMergeReadChunk(bool *status, psArray *fileGroups, pmConfig *config, int numChunk)
+{
     *status = true;
 
     bool mdok;
     bool haveMasks = psMetadataLookupBool(&mdok, config->arguments, "INPUTS.MASKS"); // Do we have masks?
-    bool haveWeights = psMetadataLookupBool(&mdok, config->arguments, "INPUTS.WEIGHTS"); // Do we have weights?
+    bool haveWeights = psMetadataLookupBool(&mdok, config->arguments, "INPUTS.WEIGHTS");// Do we have weights?
     int rows = psMetadataLookupS32(NULL, config->arguments, "ROWS"); // Number of rows to read per chunk
 
     // select an available fileGroup
     while (1) {
-	// check for any fileGroups which can read data
-	for (int j = 0; j < fileGroups->n; j++) {
-	    ppMergeFileGroup *fileGroup = fileGroups->data[j];
-	    if (fileGroup->read) continue;
+        // check for any fileGroups which can read data
+        for (int j = 0; j < fileGroups->n; j++) {
+            ppMergeFileGroup *fileGroup = fileGroups->data[j];
+            if (fileGroup->read) continue;
 
-	    // find max last scan so far
-	    int lastScan = 0;
-	    for (int i = 0; i < fileGroups->n; i++) {
-		ppMergeFileGroup *fileGroup = fileGroups->data[i];
-		lastScan = PS_MAX (fileGroup->lastScan, lastScan);
-	    }
-	    fileGroup->firstScan = lastScan;
-	    fileGroup->lastScan = lastScan + rows;
+            // find max last scan so far
+            int lastScan = 0;
+            for (int i = 0; i < fileGroups->n; i++) {
+                ppMergeFileGroup *fileGroup = fileGroups->data[i];
+                lastScan = PS_MAX(fileGroup->lastScan, lastScan);
+            }
+            fileGroup->firstScan = lastScan;
+            fileGroup->lastScan = lastScan + rows;
 
-	    psArray *readouts = fileGroup->readouts;
+            psArray *readouts = fileGroup->readouts;
 
-	    psTimerStart ("ppMergeReadChunk");
+            psTimerStart ("ppMergeReadChunk");
 
-	    psTrace("ppStack", 2, "Reading data for chunk %d into fileGroup %d....n", numChunk, j);
-	    for (int i = 0; i < readouts->n; i++) {
-		pmReadout *inRO = readouts->data[i]; // Input readout
+            psTrace("ppStack", 2, "Reading data for chunk %d into fileGroup %d....n", numChunk, j);
+            for (int i = 0; i < readouts->n; i++) {
+                pmReadout *inRO = readouts->data[i]; // Input readout
 
-		// override the recorded last scan
-		inRO->thisImageScan  = fileGroup->firstScan;
-		inRO->thisWeightScan = fileGroup->firstScan;
-		inRO->thisMaskScan   = fileGroup->firstScan;
+                // override the recorded last scan
+                inRO->thisImageScan  = fileGroup->firstScan;
+                inRO->thisWeightScan = fileGroup->firstScan;
+                inRO->thisMaskScan   = fileGroup->firstScan;
 
-		// Read a chunk from a file
-		pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT", i); 
+                // Read a chunk from a file
+                pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT", i);
 
-		bool keepReading = false;
-		if (pmReadoutMore(inRO, file->fits, 0, rows, config)) {
-		    keepReading = true;
-		    if (!pmReadoutReadChunk(inRO, file->fits, 0, rows, 0, config)) { 
-			psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT %d", numChunk, i);
-			*status = false;
-			return NULL;
-		    }							
-		}
+                bool keepReading = false;
+                if (pmReadoutMore(inRO, file->fits, 0, rows, config)) {
+                    keepReading = true;
+                    if (!pmReadoutReadChunk(inRO, file->fits, 0, rows, 0, config)) {
+                        psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT %d",
+                                numChunk, i);
+                        *status = false;
+                        return NULL;
+                    }
+                }
 
-		if (haveMasks && pmReadoutMoreMask(inRO, file->fits, 0, rows, config)) {
-		    keepReading = true;
-		    pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT.MASK", i); 
-		    if (!pmReadoutReadChunkMask(inRO, file->fits, 0, rows, 0, config)) { 
-			psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT.MASK %d", numChunk, i);
-			*status = false;
-			return NULL;
-		    }							
-		}
+                if (haveMasks && pmReadoutMoreMask(inRO, file->fits, 0, rows, config)) {
+                    keepReading = true;
+                    pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT.MASK", i);
+                    if (!pmReadoutReadChunkMask(inRO, file->fits, 0, rows, 0, config)) {
+                        psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT.MASK %d",
+                                numChunk, i);
+                        *status = false;
+                        return NULL;
+                    }
+                }
 
-		if (haveWeights && pmReadoutMoreWeight(inRO, file->fits, 0, rows, config)) {
-		    keepReading = true;
-		    pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT.WEIGHT", i); 
-		    if (!pmReadoutReadChunkWeight(inRO, file->fits, 0, rows, 0, config)) {
-			psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT.WEIGHT %d", numChunk, i);
-			*status = false;
-			return NULL;
-		    }							
-		}
-		if (!keepReading) {
-		    return NULL;
-		}
-	    }
+                if (haveWeights && pmReadoutMoreWeight(inRO, file->fits, 0, rows, config)) {
+                    keepReading = true;
+                    pmFPAfile *file = pmFPAfileSelectSingle(config->files, "PPMERGE.INPUT.WEIGHT", i);
+                    if (!pmReadoutReadChunkWeight(inRO, file->fits, 0, rows, 0, config)) {
+                        psError(PS_ERR_IO, false, "Unable to read chunk %d for file PPMERGE.INPUT.WEIGHT %d",
+                                numChunk, i);
+                        *status = false;
+                        return NULL;
+                    }
+                }
+                if (!keepReading) {
+                    return NULL;
+                }
+            }
 
-	    fileGroup->read = fileGroup->busy = true;
-	    return fileGroup;
-	}
+            fileGroup->read = fileGroup->busy = true;
+            return fileGroup;
+        }
 
-	// check for any fileGroups which are done processing
-	bool wait = false;
-	for (int j = 0; j < fileGroups->n; j++) {
-	    ppMergeFileGroup *fileGroup = fileGroups->data[j];
-	    if (!fileGroup->read || fileGroup->busy) continue;
-	    fileGroup->read = false;
-	    wait = true;
-	}
-	if (wait) usleep (10000);
+        // Check for threads that are ready to read
+        bool wait = true;
+        for (int j = 0; j < fileGroups->n; j++) {
+            ppMergeFileGroup *fileGroup = fileGroups->data[j];
+            if (fileGroup->busy) {
+                continue;
+            }
+            fileGroup->read = false;
+            wait = false;
+        }
+        if (wait) {
+            // No threads currently available
+            usleep(THREAD_WAIT);
+        }
     }
     return NULL;