Re: Let's make PostgreSQL multi-threaded

On 05/06/2023 12:26, Ranier Vilela wrote:
> First, it is not clear what are the benefits of architecture change?
>
> Performance?

I doubt it makes much performance difference, at least not initially. It
might help a little with backend startup time, and maybe some other
things. And might reduce the overhead of context switches and TLB cache
misses.

In the long run, a single-process architecture makes it easier to have
shared catalog caches, plan cache, etc. which can improve performance.
And it can make it easier to launch helper threads for things where
worker processes would be too heavy-weight. But those benefits will
require more work, they won't happen just by replacing processes with
threads.

The ease of developing things like that is my motivation.

> Development becomes much more complicated and error-prone.

I don't agree with that.

We currently bend over backwards to make all allocations fixed-sized in
shared memory. You learn to live with that, but a lot of things would be
simpler if you could allocate and free in shared memory more freely.
It's no panacea, you still need to be careful with locking and
concurrency. But a lot simpler.

We have built dynamic shared memory etc. over the years to work around
the limitations of shared memory. But it's still a lot more complicated.

Code that doesn't need to communicate with other processes/threads is
simple to write in either model.

> There are still many low-hanging fruit to be had that can improve
> performance.
> And the code can gradually and safely remove multithreading barriers.
>
> 1. gradual reduction of global variables
> 2. introduction of local context structures
> 3. shrink current structures (to fit in 32, 64 boundaries)
>
> 4. scope reduction

Right, the reason I started this thread is to explicitly note that it is
a worthy goal. If it's not, the above steps would be pointless. But if
we agree that it is a worthy goal, we can start to incrementally work
towards it.

--
Heikki Linnakangas
Neon (https://neon.tech)