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
postgresql.conf.sample.
--
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"
#ifdef OPTIMIZER_DEBUG
@@ -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_degree++;
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
--
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