Re: [PATCH] Refactoring of LWLock tranches

On Wed, Feb 10, 2016 at 8:51 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, Feb 10, 2016 at 1:32 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > The reason for using centralized way is that we need to request
> > named tranches before initialization of shared memory and as far as
> > I can see, currently there is no way in the subsystems where they can
> > issue such a request, so one possibility is that we introduce new API's
> > like InitBufferLWLocks(), InitLmgrLWLocks(), InitPredicateLWLocks()
> > in respective subsystem and call them in
> > CreateSharedMemoryAndSemaphores() before shared memory
> > initialization. Does by doing that way addresses your concern?
>
> Well, if we're going to have new functions like that, I think the
> place to call them from is PostmasterMain() just before
> process_shared_preload_libraries(). After all, if extensions were
> requesting tranches, they'd do it from
> process_shared_preload_libraries(), so it seems like the right place.
>

Initially I also thought like that, but on further analysis I found that
we also need to request the tranches from InitCommunication() as
that gets called from initdb path.

> However, since the number of locks we need for each of these
> subsystems is fixed at compile time, it seems a bit of a shame to have
> to do something about them at runtime. I wonder if we should just
> hard-code this in CreateLWLocks() instead of trying to use the
> named-tranche facility. That is, where that function does this:
>
> MainLWLockTranche.name = "main";
> MainLWLockTranche.array_base = MainLWLockArray;
> MainLWLockTranche.array_stride = sizeof(LWLockPadded);
> LWLockRegisterTranche(LWTRANCHE_MAIN, &MainLWLockTranche);
>
> ...register four tranches instead. And where it does this:
>
> /* Initialize all fixed LWLocks in main array */
> for (id = 0, lock = MainLWLockArray; id < numLocks; id++, lock++)
> LWLockInitialize(&lock->lock, LWTRANCHE_MAIN);
>
> ...have four loops instead, each initializing with a different tranche
> ID. Then the current method of computing the location of those locks
> would still work just fine; the code changes would be a lot more
> isolated, and we wouldn't have to do runtime save-and-restore of more
> variables on Windows.
>

Sounds sensible, however after changes, CreateLWLocks() started
looking unreadable, so moved initialization and registration of tranches
to separate functions.

Increased number of tranches allocated in LWLockTrancheArray, as
now the LWTRANCHE_FIRST_USER_DEFINED is already greater
than 16.

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