Re: Changing shared_buffers without restart - Mailing list pgsql-hackers

On Mon, Jul 14, 2025 at 1:40 PM Dmitry Dolgov <9erthalion6@gmail.com> wrote:
>
> > On Mon, Jul 14, 2025 at 10:25:51AM +0530, Ashutosh Bapat wrote:
> > > Currently it's being addressed via every backend waiting for each other,
> > > but I guess it could be as well managed via handling the freelist, so
> > > that only "available" buffers will be inserted into the lookup table.
> >
> > I didn't get how can that be managed by freelist? Buffers are also
> > allocated through clocksweep, which needs to be managed as well.
>
> The way it is implemented in the patch right now is new buffers are
> added into the freelist right away, when they're initialized by the
> virtue of nextFree. What I have in mind is to do this as the last step,
> when all backends have confirmed shared memory signal was absorbed. This
> would mean that StrategyControll will not return a buffer id from the
> freshly allocated range until everything is done, and no such buffer
> will be inserted into the buffer lookup table.
>
> You're right of course, a buffer id could be returned from the
> ClockSweep and from the custom strategy buffer ring. Buf from what I see
> those are picking a buffer from the set of already utilized buffers,
> meaning that for a buffer to land there it first has to go through
> StrategyControl->firstFreeBuffer, and hence the idea above will be a
> requirement for those as well.

That isn't true. A buffer which was never in the free list can still
be picked up by clock sweep. But you are raising a relevant point
about StrategyControl below

>
> > > Long story short, in the next version of the patch I'll try to
> > > experiment with a simplified design: a simple function to trigger
> > > resizing, launching a coordinator worker, with backends not waiting for
> > > each other and buffers first allocated and then marked as "available to
> > > use".
> >
> > Should all the backends wait between buffer allocation and them being
> > marked as "available"? I assume that marking them as available means
> > "declaring the new NBuffers".
>
> Yep, making buffers available would be equivalent to declaring the new
> NBuffers. What I think is important here is to note, that we use two
> mechanisms for coordination: the shared structure ShmemControl that
> shares the state of operation, and ProcSignal that tells backends to do
> something (change the memory mapping). Declaring the new NBuffers could
> be done via ShmemControl, atomically applying the new value, instead of
> sending a ProcSignal -- this way there is no need for backends to wait,
> but StrategyControl would need to use the ShmemControl instead of local
> copy of NBuffers. Does it make sense to you?

When expanding buffers, letting StrategyControl continue with the old
NBuffers may work. When propagating the new buffer value we have to
reinitialize StrategyControl to use new NBuffers. But when shrinking,
the StrategyControl needs to be initialized with the new NBuffers,
lest it picks a victim from buffers being shrunk. And then if the
operation fails, we have to reinitialize the StrategyControl again
with the old NBuffers.

>
> > What about when shrinking the buffers? Do you plan to make all the
> > backends wait while the coordinator is evicting buffers?
>
> No, it was never planned like that, since it could easily end up with
> coordinator waiting for the backend to unpin a buffer, and the backend
> to wait for a signal from the coordinator.

I agree with the deadlock situation. How do we prevent the backends
from picking or continuing to work with a buffer from buffers being
shrunk then? Each backend then has to do something about their
respective pinned buffers.

--
Best Wishes,
Ashutosh Bapat