On Wed, Sep 25, 2013 at 10:53 AM, Andres Freund <and...@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));
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.
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.
Cheers,
Jeff
