[HACKERS] Re: [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
Tom Lane wrote: 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. 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. 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. Sounds good to me. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
[HACKERS] Re: [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
Heikki Linnakangas wrote: Tom Lane wrote: Also, we have a generic issue that making fresh entries in a hashtable might result in a concurrent hash_seq_search scan visiting existing entries more than once; that's definitely not something any of the existing callers are thinking about. Ouch. Note that we can also miss some entries altogether, which is probably even worse. In case someone is wondering how that can happen, here's an example. We're scanning a bucket that contains four entries, and we split it after returning 1: 1 - 2* - 3 - 4 * denotes the next entry the seq scan has stored. If this is split goes example like this: 1 - 3 2* - 4 The seq scan will continue scanning from 2, then 4, and miss 3 altogether. 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. Should we document the fact that it's not safe to insert new entries to a hash table while scanning it, and fix the few call sites that do that, or does anyone see a better solution? One alternative would be to inhibit bucket splits while a scan is in progress, but then we'd need to take care to clean up after each scan. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
[HACKERS] Re: [BUGS] BUG #3245: PANIC: failed to re-find shared loc k o b j ect
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. 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. We could have two kinds of seq scans, with and without support for concurrent inserts. If you open a scan without that support, it acts just like today, and no extra bookkeeping or clean up by the caller is required. If you need concurrent inserts, we inhibit bucket splits, but it's up to the caller to explicitly close the scan, possibly with PG_TRY/CATCH. I'm not sure if that's simpler in the end, but we could get away without adding generic bookkeeping mechanism. 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. 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. Do we need to support multiple simultaneous seq scans of a hash table? I suppose we do.. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
