Re: Latch implementation

On Wed, 2010-09-22 at 13:31 -0700, Ganesh Venkitachalam-1 wrote:
> Hi,
> 
> I've been playing around with measuring the latch implementation in 9.1, 
> and here are the results of a ping-pong test with 2 processes signalling 
> and waiting on the latch. I did three variations (linux 2.6.18, nehalem 
> processor).
> 
> One is the current one.
> 
> The second is built on native semaphors on linux. This one cannot
> implement WaitLatchOrSocket, there's no select involved.

That looks interesting. If we had a need for a latch that would not need
to wait on a socket as well, this would be better. In sync rep, we
certainly do. Thanks for measuring this.

Question is: in that case would we use latches or a PGsemaphore?

If the answer is "latch" then we could just have an additional boolean
option when we request InitLatch() to see what kind of latch we want.

> The third is an implementation based on pipe() and poll. Note: in its 
> current incarnation it's essentially a hack to measure performance, it's 
> not usable in postgres, this assumes all latches are created before any 
> process is forked. We'd need to use mkfifo to sort that out if we really 
> want to go this route, or similar.
> 
> - Current implementation: 1 pingpong is avg 15 usecs
> - Pipe+poll: 9 usecs
> - Semaphore: 6 usecs

Pipe+poll not worth it then.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services