Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Thu, Mar 12, 2020 at 5:28 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Mar 12, 2020 at 11:15 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > I have fixed this in the attached patch set.
> >
>
> I have modified your
> v4-0003-Conflict-Extension-Page-lock-in-group-member patch. The
> modifications are (a) Change src/backend/storage/lmgr/README to
> reflect new behaviour, (b) Introduce a new macro LOCK_LOCKTAG which
> slightly simplifies the code, (c) moved the deadlock.c check a few
> lines up and (d) changed a few comments.
>
> It might be better if we can move the checks related to extension and
> page lock in a separate API or macro. What do you think?
>
I think moving them inside a macro is a good idea. Also, I think we
should move all the Assert related code inside some debugging macro
similar to this:
#ifdef LOCK_DEBUG
....
#endif

+ /*
+ * The relation extension or page lock can never participate in actual
+ * deadlock cycle. See Asserts in LockAcquireExtended. So, there is
+ * no advantage in checking wait edges from it.
+ */
+ if ((LOCK_LOCKTAG(*lock) == LOCKTAG_RELATION_EXTEND) ||
+ (LOCK_LOCKTAG(*lock) == LOCKTAG_PAGE))
+ return false;
+
Since this is true, we can also avoid these kind of locks in
ExpandConstraints, right? It'll certainly reduce some complexity in
topological sort.

/*
+ * The relation extension or page lock conflict even between the group
+ * members.
+ */
+ if ((LOCK_LOCKTAG(*lock) == LOCKTAG_RELATION_EXTEND) ||
+ (LOCK_LOCKTAG(*lock) == LOCKTAG_PAGE))
+ {
+ PROCLOCK_PRINT("LockCheckConflicts: conflicting (group)",
+ proclock);
+ return true;
+ }
This check includes the heavyweight locks that conflict even under
same parallel group. It also has another property that they can never
participate in deadlock cycles. And, the number of locks under this
category is likely to increase in future with new parallel features.
Hence, it could be used in multiple places. Should we move the
condition inside a macro and just call it from here?

--
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com