Re: ERROR: too many dynamic shared memory segments

On Tue, Nov 28, 2017 at 9:45 AM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> I haven't checked whether this fixes the bug, but if it does, we can
>> avoid introducing an extra branch in BitmapHeapNext.
>
> With my test it's fixing the problem.

I tested it some more and found that, for me, it PARTIALLY fixes the
problem.  I tested like this:

--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -279,7 +279,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
      * parallelism than to fail outright.
      */
     segsize = shm_toc_estimate(&pcxt->estimator);
-    if (pcxt->nworkers > 0)
+    if (pcxt->nworkers > 0 && false)
         pcxt->seg = dsm_create(segsize, DSM_CREATE_NULL_IF_MAXSEGMENTS);
     if (pcxt->seg != NULL)
         pcxt->toc = shm_toc_create(PARALLEL_MAGIC,

That turned out to produce more than one problem.  I find that the
select_parallel test then fails like this:

ERROR:  could not find key 18446744073709486082 in shm TOC at 0x10be98040

The fix for that problem seems to be:

--- a/src/backend/access/transam/parallel.c
+++ b/src/backend/access/transam/parallel.c
@@ -430,7 +430,8 @@ ReinitializeParallelDSM(ParallelContext *pcxt)

     /* Recreate error queues. */
     error_queue_space =
-        shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+        shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, true);
+    Assert(pcxt->nworkers == 0 || error_queue_space != NULL);
     for (i = 0; i < pcxt->nworkers; ++i)
     {
         char       *start;

With that fix in place, I then hit a crash in parallel bitmap heap
scan.  After applying no-pstate.patch, which I just committed and
back-patched to v10, then things look OK.  I'm going to apply the fix
for the error_queue_space problem also.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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