On Thu, 22 May 2014 09:46:43 +0100 Mel Gorman <mgorman@xxxxxxx> wrote: > > > If I'm still on track here, what happens if we switch to wake-all so we > > > can avoid the dangling flag? I doubt if there are many collisions on > > > that hash table? > > > > Wake-all will be ugly and loose a herd of waiters, all racing to > > acquire, all but one of whoem will loose the race. It also looses the > > fairness, its currently a FIFO queue. Wake-all will allow starvation. > > > > And the cost of the thundering herd of waiters may offset any benefit of > reducing the number of calls to page_waitqueue and waker functions. Well, none of this has been demonstrated. As I speculated earlier, hash chain collisions will probably be rare, except for the case where a bunch of processes are waiting on the same page. And in this case, perhaps wake-all is the desired behavior. Take a look at do_read_cache_page(). It does lock_page(), but it doesn't actually *need* to. It checks ->mapping and PG_uptodate and then... unlocks the page! We could have used wait_on_page_locked() there and permitted concurrent threads to run concurrently. btw, I'm struggling a bit to understand why we bother checking ->mapping there as we're about to unlock the page anyway... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
