Chris Thomasson wrote:
"Anthony Williams" <anthony_w.geo@yahoo.com> wrote in message news:zmbl66cr.fsf@yahoo.com...
"Chris Thomasson" <cristom@comcast.net> writes:
"Anthony Williams" <anthony_w.geo@yahoo.com> wrote in message news:d58h7nlv.fsf@yahoo.com...
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If, when the first writer unlocks, the readers compete with the waiting writer, then most of the time some readers will get in before the writer re-registers that it's waiting, and blocks the rest. You could just unblock one thread, and if it's a reader then it unblocks all currently-waiting readers. Rather like a condition variable broadcast....
I could adjust the logic of my algorithm so that it behaves exactly like that...
I implemented a variation of your algorithm that has - active reader count - pending/active writer bit (+semaphore) - blocked reader+writer count (+semaphore) The last active reader unblocks the pending writer, if there is one, else the rest; the writer unblocks the rest on unlock ("thundering herd"). However I'm getting worse performance out of it compared to my other prototype on a dual core Pentium D on Windows. I also tried to separate the blocked readers and blocked writers and only unblock one writer at a time. No change in performance. Maybe I've made a mistake somewhere. :-)