On Sat, 2005-01-01 at 18:55, Tom Lane wrote: > Simon Riggs <[EMAIL PROTECTED]> writes: > > I think the proposal sounds safe, though I worry about performance. > > There is no performance loss; we are just changing the order in which > we acquire two locks. If there were some risk of blocking for a > measurable time while holding the BufMgrLock, then that would be bad for > concurrent performance --- but in fact the per-buffer lock is guaranteed > free at that point. > > I don't think there's any value in trying to avoid the I/O. This is a > corner case of such rarity that it's only been seen perhaps half a dozen > times in the history of the project. "Optimizing" it is not the proper > concern. The case where the I/O is wasted because someone re-pins the > buffer during the write is far more likely, simply because of the > relative widths of the windows involved; and we can't avoid that.
The deadlock is incredibly rare, I agree. That was not my point. The situation where another backend requests the block immediately before the I/O is fairly common AFAICS, especially since StrategyGetBuffer ignores the BM_DIRTY flag in selecting victims. ISTM making the code deadlock-safe will effect cases where there never would have been a deadlock, slowing both backends down while waiting for the I/O to complete. > Bottom line is that I don't think it's useful to consider this as a > performance issue. What we need is correctness with minimum extra > complication of the logic. I'll step aside and look more closely for 8.1 -- Best Regards, Simon Riggs ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])