On Tue, 13 Sep 2005 Tom Lane wrote : > "Min Xu (Hsu)" <[EMAIL PROTECTED]> writes: > > ...If this were the case, perhaps first fetch the spin lock with read-only > > permission should have helped. > > But the cmpb instruction in the 8.0 version of TAS would have done that, > and I think we've already established that the cmpb is a loss on most > machines (except maybe single-physical-CPU Xeons). I suggested in my > other message that it might help to grab write permission on the cache > line before actually trying to acquire the spin lock --- but I don't see > how getting read-only permission first will help.
Yes, I agree. What I was trying to say was that if the second scenario you hypothesize were true, i.e. fetching write-able line before actually trying to acquire the spin lock would cause another processor to slow down its execution inside the critical section, then fetching read-only lines would have helped. As you said, however, experimental results shows fetching read-only lines didn't help, which led me wonder whether the second scenario your described was really happening. ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match