On 2013-09-25 11:17:51 -0700, Jeff Janes wrote:
> On Wed, Sep 25, 2013 at 10:53 AM, Andres Freund <andres@2ndquadrant.com>wrote:
>
> > On 2013-09-25 00:06:06 -0700, Jeff Janes wrote:
> > > > On 09/20/2013 03:01 PM, Jeff Janes wrote:> 3) Even worse, asking if a
> > > > given transaction has finished yet can be a
> > > > > serious point of system-wide contention, because it takes the
> > > > > ProcArrayLock, once per row which needs to be checked. So you have
> > 20
> > > > > processes all fighting over the ProcArrayLock, each doing so 1000
> > > > times per
> > > > > query.
> >
> > That should be gone in master, we don't use SnapshotNow anymore which
> > had those TransactionIdIsInProgress() calls you're probably referring
> > to. The lookups discussed in this thread now use the statement's
> > snapshot. And all those have their own copy of the currently running
> > transactions.
> See HeapTupleSatisfiesMVCC, near line 943 of tqual.c:
>
> else if (TransactionIdIsInProgress(HeapTupleHeaderGetXmin(tuple)))
> return false;
> else if (TransactionIdDidCommit(HeapTupleHeaderGetXmin(tuple)))
> SetHintBits(tuple, buffer, HEAP_XMIN_COMMITTED,
> HeapTupleHeaderGetXmin(tuple));
>
Hm, sorry, misrembered things a bit there.
> If we guarded that check by moving up line 961 to before 943:
>
> if (XidInMVCCSnapshot(HeapTupleHeaderGetXmin(tuple), snapshot))
> return false; /* treat as still in progress */
>
> Then we could avoid the contention, as that check only refers to local
> memory.
That wouldn't be correct afaics - the current TransactionIdIsInProgress
callsite is only called when no HEAP_XMIN_COMMITTED was set. So you
would have to duplicate it.
> As far as I can tell, the only downside of doing that is that, since hint
> bits might be set later, it is possible some dirty pages will get written
> unhinted and then re-dirtied by the hint bit setting, when more aggressive
> setting would have only one combined dirty write instead. But that seems
> rather hypothetical, and if it really is a problem we should probably
> tackle it directly rather than by barring other optimizations.
I am - as evidenced - too tired to think about this properly, but I
think you might be right here.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
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