In that case, there can be some starvation of writers. If all the shareds are woken up but the exclusives are left in the front of the queued, no starvation can occur.
On 03/11/09 22:01, Scott Carey wrote:
Re: Proposal of tunable fix for scalability of 8.4 On 3/11/09 3:27 PM, "Kevin Grittner" <Kevin.Grittner@xxxxxxxxxxxx> wrote:Well I am waking up not just shared but shared and exclusives.. However i like your idea of waking up the next N waiters where N matches the number of cpus available. In my case it is 64 so yes this works well since the idea being of all the 64 waiters running right now one will be able to lock the next lock immediately and hence there are no cycles wasted where nobody gets a lock which is often the case when you say wake up only 1 waiter and hope that the process is on the CPU (which in my case it is 64 processes) and it is able to acquire the lock.. The probability of acquiring the lock within the next few cycles is much less for only 1 waiter than giving chance to 64 such processes and then let them fight based on who is already on CPU and acquire the lock. That way the period where nobody has a lock is reduced and that helps to cut out "artifact" idle time on the system.
If you want to make this more fair, instead of freeing all shared locks, limit the count to some number, such as the number of CPU cores. Perhaps rather than wake-up-all-waiters=true, the parameter can be an integer representing how many shared locks can be freed at once if an exclusive lock is encountered.
That was a bit of confusion on my part with respect to what the change was doing. Thanks for clarification.
As soon as I get more "cycles" I will try variations of it but it would help if others can try it out in their own environments to see if it helps their instances.
-Jignesh
