Re: dynamic shared memory and locks

On 01/05/2014 07:56 PM, Robert Haas wrote:
> Right now, storing spinlocks in dynamic shared memory *almost* works,
> but there are problems with --disable-spinlocks.  In that
> configuration, we use semaphores to simulate spinlocks.  Every time
> someone calls SpinLockInit(), it's going to allocate a new semaphore
> which will never be returned to the operating system, so you're pretty
> quickly going to run out.  There are a couple of things we could do
> about this:

5. Allocate a fixed number of semaphores, and multiplex them to emulate 
any number of spinlocks. For example, allocate 100 semaphores, and use 
the address of the spinlock modulo 100 to calculate which semaphore to 
use to emulate that spinlock.

That assumes that you never hold more than one spinlock at a time, 
otherwise you can get deadlocks. I think that assumptions holds 
currently, because acquiring two spinlocks at a time would be bad on 
performance grounds anyway.

- Heikki