Re: [HACKERS] [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
I wrote: Heikki Linnakangas [EMAIL PROTECTED] writes: We could have two kinds of seq scans, with and without support for concurrent inserts. Yeah, I considered that too, but it just seems too error-prone. We could maybe make it trustworthy by having hash_seq_search complain if it noticed there had been any concurrent insertions --- but then you're putting new overhead into hash_seq_search, which kind of defeats the argument for it (and hash_seq_search is a bit of a bottleneck, so extra cycles there matter). I just finished looking through the uses of hash_seq_search, and realized that there is one place where it would be a bit painful to convert to the insertion-safe approach I'm proposing; namely nodeAgg.c. The places where the hashtable iteration is started and used are scattered, and we don't really track whether the iteration is done or not, so it's hard to be sure where to cancel the iteration. It could probably be made to work but it seems like it'd be fragile. I still don't want to introduce more checking overhead into hash_seq_search, though, so what I'm now thinking about is a new dynahash primitive named something like hash_freeze, which'd mark a hashtable as disallowing insertions. If the hashtable is frozen before hash_seq_init then we don't add it to the central list of scans, and therefore there is no cleanup to do at the end. nodeAgg can use this mode since it doesn't modify its hashtable anymore after beginning its readout scan. BTW, we didn't really get into details, but for the insertion-safe case I'm envisioning adding a routine hash_seq_term, which you would need to call if and only if you abandon a hash_seq_search scan without running it to completion (if you do the complete scan, hash_seq_search will automatically call hash_seq_term before returning NULL). All but a very small number of places run their searches to completion and therefore won't require any source code changes with this API. Thoughts? regards, tom lane ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
Heikki Linnakangas [EMAIL PROTECTED] writes: Tom Lane wrote: I still don't want to introduce more checking overhead into hash_seq_search, though, so what I'm now thinking about is a new dynahash primitive named something like hash_freeze, which'd mark a hashtable as disallowing insertions. If the hashtable is frozen before hash_seq_init then we don't add it to the central list of scans, and therefore there is no cleanup to do at the end. nodeAgg can use this mode since it doesn't modify its hashtable anymore after beginning its readout scan. This plan includes having the list of hash tables that mustn't be expanded? And the list would be cleaned up at the end of transaction, to avoid leaks. Right, all that's still the same. This is just a way to exempt certain scans from that machinery, by allowing the caller to declare he doesn't need to modify the hashtable anymore. AFAICS that covers our needs, at least for the present. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
Heikki Linnakangas [EMAIL PROTECTED] writes: I briefly went through all callers of hash_seq_init. The only place where we explicitly rely on being able to add entries to a hash table while scanning it is in tbm_lossify. There's more complex loops in portalmem.c and relcache.c, which I think are safe, but would need to look closer. There's also the pg_prepared_statement set-returning-function that keeps a scan open across calls, which seems error-prone. The pending-fsync stuff in md.c is also expecting to be able to add entries during a scan. I don't think we can go in the direction of forbidding insertions during a scan --- as the case at hand shows, it's just not always obvious that that could happen, and finding/fixing such a problem is nigh impossible. (We were darn fortunate to be able to reproduce this one.) Plus we have a couple of places where it's really necessary to be able to do it, anyway. The only answer I can see that seems reasonably robust is to change dynahash.c so that it tracks whether any seq_search scans are open on a hashtable, and doesn't carry out any splits while one is. This wouldn't cost anything noticeable in performance, assuming that not very many splits are postponed. The PITA aspect of it is that we'd need to add bookkeeping mechanisms to ensure that the count of active scans gets cleaned up on error exit. It's not like we've not got lots of those, though. Possibly we could simplify matters a bit by not worrying about cleaning up leaked counts at subtransaction abort, ie, the list of open scans would only get forced to empty at top transaction end. This carries a slightly higher risk of meaningful performance degradation, but in practice I doubt it's a big problem. If we agreed that then we'd not need ResourceOwner support --- it could be handled like LWLock counts. pg_prepared_statement is simply broken --- what if the next-to-scan statement is deleted between calls? It'll have to be changed. Comments? regards, tom lane ---(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
Re: [HACKERS] [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
Heikki Linnakangas [EMAIL PROTECTED] writes: Tom Lane wrote: The pending-fsync stuff in md.c is also expecting to be able to add entries during a scan. No, mdsync starts the scan from scratch after calling AbsorbFsyncRequests. That was last month ;-). It doesn't restart any more. We could have two kinds of seq scans, with and without support for concurrent inserts. Yeah, I considered that too, but it just seems too error-prone. We could maybe make it trustworthy by having hash_seq_search complain if it noticed there had been any concurrent insertions --- but then you're putting new overhead into hash_seq_search, which kind of defeats the argument for it (and hash_seq_search is a bit of a bottleneck, so extra cycles there matter). Hmm. Unlike lwlocks, hash tables can live in different memory contexts, so we can't just have list of open scans similar to held_lwlocks array. I had first thought about adding a scan counter to the hashtable control struct, but the prospect of hash tables being deallocated while the central list still has references to them seems pretty scary --- we could find ourselves clobbering some other data structure entirely when we go to decrement the count. What seems better now is to have an array or list of HTAB pointers, one for each active scan (so the same hashtable might appear in the list multiple times). When we are considering whether to split, we have to look through the list to see if our table is listed. The list is unlikely to be long so this shouldn't affect performance. If a hash table is deallocated while we still think it has an active scan, nothing very bad happens. The absolute worst possible consequence is if some new hash table gets allocated at exactly the same spot; we'd inhibit splits on it, which still doesn't break correctness though it might kill performance. In any case we can have checking code that complains about leaked scan pointers at transaction end, so any such bug shouldn't survive long. For shared hash tables, this design only works for scans being done by the same backend doing insertion; but locking considerations would probably require that no other backend inserts while we scan anyway (you'd need something much more complicated than shared/exclusive locks to manage it otherwise). regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
