On 04/05/2017 09:05 AM, Kuntal Ghosh wrote:
On Tue, Apr 4, 2017 at 11:22 PM, Tomas Vondra
<tomas.von...@2ndquadrant.com> wrote:
On 04/04/2017 06:52 PM, Robert Haas wrote:

On Mon, Apr 3, 2017 at 6:08 AM, Kuntal Ghosh <kuntalghosh.2...@gmail.com>

On Fri, Mar 31, 2017 at 6:50 PM, Robert Haas <robertmh...@gmail.com>

On Thu, Mar 30, 2017 at 4:35 PM, Kuntal Ghosh
<kuntalghosh.2...@gmail.com> wrote:

2. the server restarts automatically, initialize
BackgroundWorkerData->parallel_register_count and
BackgroundWorkerData->parallel_terminate_count in the shared memory.
After that, it calls ForgetBackgroundWorker and it increments

Hmm.  So this seems like the root of the problem.  Presumably those
things need to be reset AFTER forgetting any background workers from
before the crash.

IMHO, the fix would be not to increase the terminated parallel worker
count whenever ForgetBackgroundWorker is called due to a bgworker
crash. I've attached a patch for the same. PFA.

While I'm not opposed to that approach, I don't think this is a good
way to implement it.  If you want to pass an explicit flag to
ForgetBackgroundWorker telling it whether or not it should performing
the increment, fine.  But with what you've got here, you're
essentially relying on "spooky action at a distance".  It would be
easy for future code changes to break this, not realizing that
somebody's got a hard dependency on 0 having a specific meaning.

I'm probably missing something, but I don't quite understand how these
values actually survive the crash. I mean, what I observed is OOM followed
by a restart, so shouldn't BackgroundWorkerShmemInit() simply reset the
values back to 0? Or do we call ForgetBackgroundWorker() after the crash for
some reason?
AFAICU, during crash recovery, we wait for all non-syslogger children
to exit, then reset shmem(call BackgroundWorkerShmemInit) and perform
StartupDataBase. While starting the startup process we check if any
bgworker is scheduled for a restart. If a bgworker is crashed and not
meant for a restart(parallel worker in our case), we call
ForgetBackgroundWorker() to discard it.

OK, so essentially we reset the counters, getting

    parallel_register_count = 0
    parallel_terminate_count = 0

and then ForgetBackgroundWworker() runs and increments the terminate_count, breaking the logic?

And you're using (parallel_register_count > 0) to detect whether we're still in the init phase or not. Hmmm.

In any case, the comment right before BackgroundWorkerArray says this:

  * These counters can of course overflow, but it's not important here
  * since the subtraction will still give the right number.

which means that this assert

+       Assert(BackgroundWorkerData->parallel_register_count >=
+               BackgroundWorkerData->parallel_terminate_count);

is outright broken, just like any other attempts to rely on simple
comparisons of these two counters, no?

IIUC, the assert statements ensures that register count should always
be greater than or equal to the terminate count.
Whereas, the comment refers to the fact that register count and
terminate count indicate the total number of parallel workers spawned
and terminated respectively since the server has been started. At
every session, we're not resetting the counts, hence they can
overflow. But, their substraction should give you the number of active
parallel worker at any instance.
So, I'm not able to see how the assert is broken w.r.t the aforesaid
comment. Am I missing something here?

The comment says that the counters are allowed to overflow, i.e. after a long uptime you might get these values

    parallel_register_count = UINT_MAX + 1 = 1
    parallel_terminate_count = UINT_MAX

which is fine, because the C handles the overflow during subtraction and so

    parallel_register_count - parallel_terminate_count = 1

But the assert is not doing subtraction, it's comparing the values directly:

    Assert(parallel_register_count >= parallel_terminate_count);

and the (perfectly valid) values trivially violate this comparison.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to