On Mon, Oct 27, 2014 at 3:31 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > I happened to be looking at sinvaladt.c and noticed the loop added in > commit b4fbe392f8ff6ff1a66b488eb7197eef9e1770a4: > > /* > * Now that the maxMsgNum change is globally visible, we give everyone > * a swift kick to make sure they read the newly added messages. > * Releasing SInvalWriteLock will enforce a full memory barrier, so > * these (unlocked) changes will be committed to memory before we exit > * the function. > */ > for (i = 0; i < segP->lastBackend; i++) > { > ProcState *stateP = &segP->procState[i]; > > stateP->hasMessages = true; > } > > This seems fairly awful when there are a large number of backends.
We did quite a bit of testing at the time and found that it was a clear improvement over what we had been doing previously. Of course, that's not to say we can't or shouldn't improve it further. > Why could we not remove the hasMessages flags again, and change the > unlocked test > > if (!stateP->hasMessages) > return 0; > > into > > if (stateP->nextMsgNum == segP->maxMsgNum && > !stateP->resetState) > return 0; > > If we are looking at stale shared state, this test might produce a > false positive, but I don't see why it's any less safe than testing a > hasMessages boolean. It was discussed at the time: http://www.postgresql.org/message-id/CA+TgmoY3Q56ZR6i8h+iGhXCw6rCZyvdWJ3RQT=PMVev4-=+n...@mail.gmail.com http://www.postgresql.org/message-id/13077.1311702...@sss.pgh.pa.us -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers