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

On Fri, Mar 13, 2020 at 8:29 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Mar 12, 2020 at 7:50 PM Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
> > 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
> >
> If we move it under some macro, then those Asserts will be only
> enabled when that macro is defined. I think we want there Asserts to
> be enabled always in assert enabled build, these will be like any
> other Asserts in the code. What is the advantage of doing those under
> macro?
>
My concern is related to performance regression. We're using two
static variables in hot-paths only for checking a few asserts. So, I'm
not sure whether we should enable the same by default, specially when
asserts are itself disabled.
-ResetRelExtLockHeldCount()
+ResetRelExtPageLockHeldCount()
{
RelationExtensionLockHeldCount = 0;
+ PageLockHeldCount = 0;
+}
Also, we're calling this method from frequently used functions like
Commit/AbortTransaction. So, it's better these two static variables
share the same cache line and reinitalize them with a single
instruction.

>
> > /*
> > + * 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?
> >
>
> Right, this is what I have suggested upthread. Do you have any
> suggestions for naming such a macro or function? I could think of
> something like LocksConflictAmongGroupMembers or
> LocksNotParticipateInDeadlock. The first one suits more for its usage
> in LockCheckConflicts and the second in the deadlock.c code. So none
> of those sound perfect to me.
>
Actually, I'm not able to come up with a good suggestion. I'm trying
to think of a generic name similar to strong or weak locks but with
the following properties:
a. Locks that don't participate in deadlock detection
b. Locks that conflicts in the same parallel group

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