Limit maximum parallel degree to 1024.

robertmhaas · robertmhaas · commit c7ea68ff8dfa · 2016-05-06T14:50:54.000-04:00
This new limit affects both the max_parallel_degree GUC and the parallel_degree reloption. There may some day be a use case for using more than 1024 CPUs for a single query, but that's surely not the case right now. Not only do not very many people have that many CPUs, but the code hasn't been tested at that kind of scale and is very unlikely to perform well, or even work at all, without a lot more work. The issue addressed by commit 06bd458 is probably just one problem of many. The idea of a more reasonable limit here was suggested by Tom Lane; the value of 1024 was suggested by Amit Kapila.
diff --git a/src/backend/access/common/reloptions.c b/src/backend/access/common/reloptions.c
@@ -275,7 +275,7 @@ static relopt_int intRelOpts[] =
 			RELOPT_KIND_HEAP,
 			AccessExclusiveLock
 		},
-		-1, 0, MAX_BACKENDS
+		-1, 0, 1024
 	},
 
 	/* list terminator */
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
@@ -2662,7 +2662,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_degree,
-		2, 0, MAX_BACKENDS,
+		2, 0, 1024,
 		NULL, NULL, NULL
 	},
 
