Re: [HACKERS] SERIALIZABLE with parallel query

On Fri, Feb 23, 2018 at 8:48 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Fri, Feb 23, 2018 at 3:29 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> On Thu, Feb 22, 2018 at 10:35 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Thu, Feb 22, 2018 at 7:54 AM, Thomas Munro
>>>> PS I noticed that for BecomeLockGroupMember() we say "If we can't
>>>> join the lock group, the leader has gone away, so just exit quietly"
>>>> but for various other similar things we spew errors (most commonly
>>>> seen one being "ERROR: could not map dynamic shared memory segment").
>>>> Intentional?
>>>
>>> I suppose I thought that if we failed to map the dynamic shared memory
>>> segment, it might be down to any one of several causes; whereas if we
>>> fail to join the lock group, it must be because the leader has already
>>> exited. There might be a flaw in that thinking, though.
>>>
>>
>> By the way, in which case leader can exit early? As of now, we do
>> wait for workers to end both before the query is finished or in error
>> cases.
>
> create table foo as select generate_series(1, 10)::int a;
> alter table foo set (parallel_workers = 2);
> set parallel_setup_cost = 0;
> set parallel_tuple_cost = 0;
> select count(a / 0) from foo;
>
> That reliably gives me:
> ERROR: division by zero [from leader]
> ERROR: could not map dynamic shared memory segment [from workers]
>
> I thought this was coming from resource manager cleanup, but you're
> right: that happens after we wait for all workers to finish. Perhaps
> this is a race within DestroyParallelContext() itself: when it is
> called by AtEOXact_Parallel() during an abort, it asks the postmaster
> to SIGTERM the workers, then it immediately detaches from the DSM
> segment, and then it waits for the worker to start up.
>

I guess you mean to say worker waits to shutdown/exit. Why would it
wait for startup at that stage?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com