On Wed, Dec 21, 2016 at 3:02 AM, Craig Ringer <craig@2ndquadrant.com> wrote:
> Instead, I've added a new LWLock, ClogTruncationLock, for that
> purpose. vac_truncate_clog() takes it if it decides to attempt clog
> truncation. This lock is held throughout the whole process of clog
> truncation and oldestXid advance, so there's no need for a new
> pendingOldestXid field in ShmemVariableCache. We just take the lock
> then look at oldestXid, knowing that it's guaranteed to correspond to
> an existing clog page that won't go away while we're looking.
> ClogTruncationLock is utterly uncontested so it's going to have no
> meaningful impact compared to all the work we do scanning the clog to
> decide whether we're even going to try truncating it, etc.
That makes everything that happens between when we acquire that lock
and when we release it non-interruptible, which seems undesirable. I
think that extra copy of oldestXid is a nicer approach.
--
Robert Haas
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