Re: [WIP] shared locks

On Mon, Apr 18, 2005 at 09:53:38PM -0400, Bruce Momjian wrote:
> Tom Lane wrote:
> > Alvaro Herrera <alvherre(at)dcc(dot)uchile(dot)cl> writes:
> > > The idea is that a tuple's Xmax can either be a real TransactionId
> > > (which is used normally like current CVS tip), or, if the infomask has
> > > HEAP_XMAX_SHARED_LOCK, a MultiXactId.
> >
> > Interesting idea. Would it be possible to invoke this mechanism only
> > when actually needed --- that is, the first locker of a given tuple
> > puts his plain TransactionId into Xmax (and also sets an infomask bit
> > indicating his intent to have a shared rather than exclusive lock),
> > and then the second locker to come along replaces the TransactionId
> > with a MultiTransactionId including himself and the first locker?
> >
> > This requires 2 infomask bits: 1 for shared vs exclusive lock and one
> > for whether the Xmax contains a TransactionId or MultiTransactionId.
> > But we have them available, and I think I like keeping those concepts
> > separate anyway. (Who's to say we wouldn't want to allow a
> > MultiTransactionId to hold an exclusive lock, someday?)
> >
> > The advantage of course would be substantially less overhead in the very
> > common case where there's no actual contention for the tuple.
>
> Yes, that is certainly possible. Alvaro felt he wanted something
> simpler and that the two-bit case would add complexity, but I agree it
> would reduce overhead in the most common case.

I had thought it would make things more complicated. Now that I know
how the whole thing works I can handle the extra complexity, which is not
much really. Also I wasn't sure if we wanted to waste two infomask
bits on this :-)

> > > MultiXactIds are implemented using two SLRU areas and a couple of
> > > variables in ShmemVariableCache. We also XLog groups of them just like
> > > we do for Oids.
> >
> > So no need for expansible shmem storage? Might be the way to go.

Right. I have stashed some info (like next MultiXactId to assign, the
first MultiXactId this transaction was assigned, etc) in
ShmemVariableCache and PGPROC, but I'm now thinking in storing it
in a [fixed size] shmem area private to multixact.c; this way I don't
have to lock SInvalLock.

BTW, I had to use three additional LWLocks: two for SLRU and one for
MultiXactId generation, which also covers the ShmemVariableCache
variables. I hope that's OK.

--
Alvaro Herrera (<alvherre[(at)]dcc(dot)uchile(dot)cl>)
"No es bueno caminar con un hombre muerto"