On Mon, Mar 14, 2016 at 01:33:08PM +0100, Tomas Vondra wrote: > On 03/14/2016 07:14 AM, Noah Misch wrote: > >On Mon, Mar 14, 2016 at 02:00:03AM +0100, Tomas Vondra wrote: > >>+ * XXX Maybe this should also care about the clock skew, just like the > >>+ * block a few lines down. > > > >Yes, it should. (The problem is large (>~100s), backward clock resets, not > >skew.) A clock reset causing "msg->clock_time < dbentry->stats_timestamp" > >will usually also cause "msg->cutoff_time < dbentry->stats_timestamp". Such > >cases need the correction a few lines down. > > I'll look into that. I have to admit I have a hard time reasoning about the > code handling clock skew, so it might take some time, though.
No hurry; it would be no problem to delay this several months. > >The other thing needed here is to look through and update comments about > >last_statrequests. For example, this loop is dead code due to us writing > >files as soon as we receive one inquiry: > > > > /* > > * Find the last write request for this DB. If it's older than the > > * request's cutoff time, update it; otherwise there's nothing to do. > > * > > * Note that if a request is found, we return early and skip the below > > * check for clock skew. This is okay, since the only way for a DB > > * request to be present in the list is that we have been here since the > > * last write round. > > */ > > slist_foreach(iter, &last_statrequests) ... > > > >I'm okay keeping the dead code for future flexibility, but the comment should > >reflect that. > > Yes, that's another thing that I'd like to look into. Essentially the > problem is that we always process the inquiries one by one, so we never > actually see a list with more than a single element. Correct? Correct. > I think the best way to address that is to peek is to first check how much > data is in the UDP queue, and then fetching all of that before actually > doing the writes. Peeking at the number of requests first (or even some > reasonable hard-coded limit) should should prevent starving the inquirers in > case of a steady stream or inquiries. Now that you mention it, a hard-coded limit sounds good: write the files for pending inquiries whenever the socket empties or every N messages processed, whichever comes first. I don't think the amount of pending UDP data is portably available, and I doubt it's important. Breaking every, say, one thousand messages will make the collector predictably responsive to inquiries, and that is the important property. I would lean toward making this part 9.7-only; it would be a distinct patch from the one previously under discussion. Thanks, nm -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers