Re: [PATCH] Refactoring of LWLock tranches

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.

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.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company