At Mon, 17 Apr 2017 17:50:58 +0530, Ashutosh Bapat <ashutosh.ba...@enterprisedb.com> wrote in <cafjfprdcww4h0a-zrl-eiaekkpj8o0gr2m1fwz1usesrfrm...@mail.gmail.com> > On Mon, Apr 17, 2017 at 1:53 PM, Kyotaro HORIGUCHI > <horiguchi.kyot...@lab.ntt.co.jp> wrote: > > At Thu, 13 Apr 2017 13:04:12 -0400, Robert Haas <robertmh...@gmail.com> > > wrote in > > <CA+TgmoaxnNmuONgP=bxjojrgbnmpti6ms8oswzbc2yq2ueu...@mail.gmail.com> > >> On Thu, Apr 21, 2016 at 10:53 AM, Robert Haas <robertmh...@gmail.com> > >> wrote: > >> > On Thu, Apr 21, 2016 at 8:44 AM, Michael Paquier > >> > <michael.paqu...@gmail.com> wrote: > >> >> On Thu, Apr 21, 2016 at 5:22 PM, Etsuro Fujita > >> >> <fujita.ets...@lab.ntt.co.jp> wrote: > >> >>> Attached is an updated version of the patch, which modified Michael's > >> >>> version of the patch, as I proposed in [1] (see "Other changes:"). I > >> >>> modified comments for pgfdw_get_result/pgfdw_exec_query also, mainly > >> >>> because > >> >>> words like "non-blocking mode" there seems confusing (note that we have > >> >>> PQsetnonbloking). > >> >> > >> >> OK, so that is what I sent except that the comments mentioning PG_TRY > >> >> are moved to their correct places. That's fine for me. Thanks for > >> >> gathering everything in a single patch and correcting it. > >> > > >> > I have committed this patch. Thanks for working on this. Sorry for the > >> > delay. > >> > >> This 9.6-era patch, as it turns out, has a problem, which is that we > >> now respond to an interrupt by sending a cancel request and a > >> NON-interruptible ABORT TRANSACTION command to the remote side. If > >> the reason that the user is trying to interrupt is that the network > >> connection has been cut, they interrupt the original query only to get > >> stuck in a non-interruptible wait for ABORT TRANSACTION. That is > >> non-optimal. > > > > Agreed. > > > >> It is not exactly clear to me how to fix this. Could we get by with > >> just slamming the remote connection shut, instead of sending an > >> explicit ABORT TRANSACTION? The remote side ought to treat a > >> disconnect as equivalent to an ABORT anyway, I think, but maybe our > >> local state would get confused. (I have not checked.) > >> > >> Thoughts? > > > > Perhaps we will get stuck at query cancellation before ABORT > > TRANSACTION in the case. A connection will be shut down when > > anything wrong (PQstatus(conn) != CONNECTION_OK and so) on > > aborting local transactoin . So I don't think fdw gets confused > > or sholdn't be confused by shutting down there. > > > > The most significant issue I can see is that the same thing > > happens in the case of graceful ABORT TRANSACTION. It could be a > > performance issue. > > > > We could set timeout here but maybe we can just slamming the > > connection down instead of sending a query cancellation. It is > > caused only by timeout or interrupts so I suppose it is not a > > problem *with a few connections*. > > > > > > Things are a bit diffent with hundreds of connections. The > > penalty of reconnection would be very high in the case. > > > > If we are not willing to pay such high penalty, maybe we are to > > manage busy-idle time of each connection and trying graceful > > abort if it is short enough, maybe having a shoft timeout. > > > > Furthermore, if most or all of the hundreds of connections get > > stuck, such timeout will accumulate up like a mountain... > > Even when the transaction is aborted because a user cancels a query, > we do want to preserve the connection, if possible, to avoid
Yes. > reconnection. If the request to cancel the query itself fails, we > should certainly drop the connection. Here's the patch to do that. A problem I think on this would be that we still try to make another connection for canceling and it would stall for several minutes per connection on a packet stall, which should be done in a second on ordinary circumstances. Perhaps we might want here is async canceling with timeout. regards, -- Kyotaro Horiguchi NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
