SERVER-4800 make unsorted parallel cursors do round-robin when pulling data, minimize timeouts

Signed-off-by: Tad Marshall <tad@10gen.com>

2 files changed, 11 insertions, 1 deletions

diff --git a/client/parallel.cpp b/client/parallel.cpp
index d7975a6849b..df2f3fcfc5b 100644
--- a/client/parallel.cpp
+++ b/client/parallel.cpp
@@ -365,6 +365,7 @@ namespace mongo {
 
     void ParallelSortClusteredCursor::_finishCons() {
         _numServers = _servers.size();
+        _lastFrom = 0;
         _cursors = 0;
 
         if ( ! _sortKey.isEmpty() && ! _fields.isEmpty() ) {
@@ -693,7 +694,13 @@ namespace mongo {
         BSONObj best = BSONObj();
         int bestFrom = -1;
 
-        for ( int i=0; i<_numServers; i++) {
+        for( int j = 0; j < _numServers; j++ ){
+
+            // Iterate _numServers times, starting one past the last server we used.
+            // This means we actually start at server #1, not #0, but shouldn't matter
+
+            int i = ( j + _lastFrom + 1 ) % _numServers;
+
             if ( ! _cursors[i].more() )
                 continue;
 
@@ -714,6 +721,8 @@ namespace mongo {
             bestFrom = i;
         }
 
+        _lastFrom = bestFrom;
+
         uassert( 10019 ,  "no more elements" , ! best.isEmpty() );
         _cursors[bestFrom].next();
 
diff --git a/client/parallel.h b/client/parallel.h
index 869bff95a4a..cbf64826596 100644
--- a/client/parallel.h
+++ b/client/parallel.h
@@ -241,6 +241,7 @@ namespace mongo {
         virtual void _explain( map< string,list<BSONObj> >& out );
 
         int _numServers;
+        int _lastFrom;
         set<ServerAndQuery> _servers;
         BSONObj _sortKey;