Re: Proposal of tunable fix for scalability of 8.4

On Wed, 18 Mar 2009, Simon Riggs wrote:
> I agree with that, apart from the "granting no more" bit.
>
> The most useful behaviour is just to have two modes:
> * exclusive-lock held - all other x locks welcome, s locks queue
> * shared-lock held - all other s locks welcome, x locks queue

The problem with making all other locks welcome is that there is a
possibility of starvation. Imagine a case where there is a constant stream
of shared locks - the exclusive locks may never actually get hold of the
lock under the "all other shared locks welcome" strategy. Likewise with
the reverse.

Taking a snapshot and queueing all newer locks forces fairness in the
locking strategy, and avoids one of the sides getting starved.

Matthew

--
 I've run DOOM more in the last few days than I have the last few
 months.  I just love debugging ;-)  -- Linus Torvalds