ENH: commenting cap on the number of streams in MRG, and raising it.

theano/sandbox/rng_mrg.py

@@ -605,7 +605,7 @@ def guess_n_streams(size, warn=True):
     Return a guess at a good number of streams.
 
     :param warn: If True, warn when a guess cannot be made (in which case
-    we return 30 * 256).
+    we return 60 * 256).
     """
     # TODO: a smart way of choosing the number of streams, see #612.
     # Note that this code was moved out of `MRG_RandomStreams` so that it can
@@ -618,14 +618,22 @@ def guess_n_streams(size, warn=True):
             r *= s
         if r > 6:
             r = r/6 # chosen as fastest for rbm_benchmark
-        return min(r, 30 * 256)
+
+        # The purpose of sampling from many streams is to be able to use
+        # the GPU to its full capacity.  It just wastes RAM and stream-initialization time to
+        # allocate more streams than necessary for the GPU.
+        # XXX: This number is chosen to be good for 280 and 480 architectures,
+        #      Better would be to use pycuda to query the number of
+        #      processors on the GPU device,
+        #      rather than guessing 60.
+        return min(r, 60 * 256)
     else:
         if warn:
             warnings.warn((
                     "MRG_RandomStreams Can't determine #streams from "
-                    "size (%s), guessing 30*256") % str(size),
+                    "size (%s), guessing 60*256") % str(size),
                     stacklevel=3)
-        return 30 * 256
+        return 60 * 256
 
 
 class MRG_RandomStreams(object):