On 18/03/2016 00:56, Tom Lane wrote:
> Julien Rouhaud <julien.rouh...@dalibo.com> writes:
>> Shouldn't we also check "parallel_degree < max_worker_process" ?
>> There's no need to compute any further than that. I think the best fix
>> would be to add a CheckHook or AssignHook on max_parallel_degree GUC to
>> make sure it's not more than max_worker_process.
> Please, let's not go there.  Interdependent checks on GUC values are far
> harder to get right than you think.  It's far better to design the GUC
> specifications so that it doesn't matter.
> For an example whereof I speak, check the sordid history of commit
> ee1e5662d8d83307 ("Auto-tune effective_cache size to be 4x shared
> buffers"), which eventually got reverted after a huge amount of thrashing
> trying to make it work consistently.  Admittedly, that was trying to make
> the default value of GUC X depend on GUC Y, but I think checking whether
> X <= Y would have many of the same problems.  The core issue is you don't
> know which one's going to get set first.

Oh, I wasn't aware of that, thanks for the pointer.

> In this particular case I think it'd be fine to document that the
> effective amount of parallelism is Min(parallel_degree,max_worker_process).
>                       regards, tom lane

I just saw that it's already documented that way. I attach a patch that
makes sure we don't try to compute a parallel_degree beyond this limit
(if you think it's worth it), and a missing description and "change
requires restart" for the max_worker_processes parameter in

Julien Rouhaud
http://dalibo.com - http://dalibo.org
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 4f60b85..2886219 100644
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -23,6 +23,7 @@
 #include "catalog/pg_operator.h"
 #include "catalog/pg_proc.h"
 #include "foreign/fdwapi.h"
+#include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
@@ -678,7 +679,7 @@ create_parallel_paths(PlannerInfo *root, RelOptInfo *rel)
 	 * need something here for now.
 	while (rel->pages > parallel_threshold * 3 &&
-		   parallel_degree < max_parallel_degree)
+		   parallel_degree < Min(max_parallel_degree, max_worker_processes))
 		parallel_threshold *= 3;
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 773b4e8..00368bb 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -163,7 +163,8 @@
 # - Asynchronous Behavior -
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
-#max_worker_processes = 8
+#max_worker_processes = 8       # max number of background workers
+                    #(change requires restart)
 #max_parallel_degree = 0		# max number of worker processes per node
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to