On Tue, Nov 29, 2011 at 7:12 AM, Pavan Deolasee
<pavan.deola...@gmail.com> wrote:
> I think that a good idea. We need a representation that needs minimum
> processing to derive the snapshot.
I was looking over the generated code for GetSnapshotData to see if there
is any low hanging fruit for micro-optimization. The assembly mostly looks
pretty tight, but there are 3 function calls to TransactionIdPrecedes and
TransactionIdFollowsOrEquals. All the parameters are known to be normal
xids, so there are duplicated checks for that and a lot of movs for the calling
convention. I wonder if replacing them with special case macros would be
a good idea. In that case the whole check will compile down to one cmp
instruction. I'm running a set of benchmarks now on my laptop, but I guess
the difference will mostly become noticeable on beefier hardware when
ProcArray lock is heavily contended. Attached is a patch, if anyone wishes
to give it a go.
--
Ants Aasma
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 19ff524..59ff996 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1324,7 +1324,7 @@ GetSnapshotData(Snapshot snapshot)
/* Update globalxmin to be the smallest valid xmin */
xid = pgxact->xmin; /* fetch just once */
if (TransactionIdIsNormal(xid) &&
- TransactionIdPrecedes(xid, globalxmin))
+ TransactionIdPrecedesBothNormal(xid, globalxmin))
globalxmin = xid;
/* Fetch xid just once - see GetNewTransactionId */
@@ -1341,11 +1341,11 @@ GetSnapshotData(Snapshot snapshot)
*/
if (TransactionIdIsNormal(xid))
{
- if (TransactionIdFollowsOrEquals(xid, xmax))
+ if (TransactionIdFollowsOrEqualsBothNormal(xid, xmax))
continue;
if (pgxact != MyPgXact)
snapshot->xip[count++] = xid;
- if (TransactionIdPrecedes(xid, xmin))
+ if (TransactionIdPrecedesBothNormal(xid, xmin))
xmin = xid;
}
diff --git a/src/include/access/transam.h b/src/include/access/transam.h
index c038fd9..31401f9 100644
--- a/src/include/access/transam.h
+++ b/src/include/access/transam.h
@@ -44,6 +44,9 @@
#define TransactionIdStore(xid, dest) (*(dest) = (xid))
#define StoreInvalidTransactionId(dest) (*(dest) = InvalidTransactionId)
+#define TransactionIdPrecedesBothNormal(id1, id2) ((int32) (id1 - id2) < 0)
+#define TransactionIdFollowsOrEqualsBothNormal(id1, id2) ((int32) (id1 - id2) >= 0)
+
/* advance a transaction ID variable, handling wraparound correctly */
#define TransactionIdAdvance(dest) \
do { \
--
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