Re: Decrease MAX_BACKENDS to 2^16

On 2014-04-26 11:52:44 +0100, Greg Stark wrote:
> On Fri, Apr 25, 2014 at 11:15 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > Since there's absolutely no sensible scenario for setting
> > max_connections that high, I'd like to change the limit to 2^16, so we
> > can use a uint16 in BufferDesc->refcount.
>
> Clearly there's no sensible way to run 64k backends in the current
> architecture.

The current limit is 2^24, I am only proposing to lower it to 2^16.

> But I don't think it's beyond the realm of possibility
> that we'll reduce the overhead in the future with an eye to being able
> to do that. Is it that helpful that it's worth baking in more
> dependencies on that limitation?

I don't think it's realistic that we'll ever have more than 2^16 full
blown backends. We might (I hope!) a builtin pooler, but pooler
connections won't be full backends.
So I really don't see any practical limitation with limiting the max
number of backends to 65k.

What I think it's necessary for is at least:

* Move the buffer content lock inline into to the buffer descriptor,
while still fitting into one cacheline.
* lockless/atomic Pin/Unpin Buffer.

Imo those are significant scalability advantages...

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services