Be smarter about freeing tuples during tuplesorts

david-rowley · david-rowley · commit 1083f94dacde · 2022-09-01T11:08:10.000+12:00
During dumptuples() the call to writetuple() would pfree any non-null tuple. This was quite wasteful as this happens just before we perform a reset of the context which stores all of those tuples. It seems to make sense to do a bit of a code refactor to make this work, so here we just get rid of the writetuple function and adjust the WRITETUP macro to call the state's writetup function. The WRITETUP usage in mergeonerun() always has state->slabAllocatorUsed == true, so writetuple() would never free the tuple or do any memory accounting. The only call path that needs memory accounting done is in dumptuples(), so let's just do it manually there. In passing, let's get rid of the state->memtupcount-- code that counts the memtupcount down to 0 one tuple at a time inside the loop. That seems to be a rather inefficient way to set memtupcount to 0, so let's just zero it after the loop instead. Author: David Rowley Discussion: https://postgr.es/m/CAApHDvqZXoDCyrfCzZJR0-xH+7_q+GgitcQiYXUjRani7h4j8Q@mail.gmail.com
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
@@ -395,7 +395,7 @@ struct Sharedsort
 
 #define REMOVEABBREV(state,stup,count)	((*(state)->base.removeabbrev) (state, stup, count))
 #define COMPARETUP(state,a,b)	((*(state)->base.comparetup) (a, b, state))
-#define WRITETUP(state,tape,stup)	(writetuple(state, tape, stup))
+#define WRITETUP(state,tape,stup)	((*(state)->base.writetup) (state, tape, stup))
 #define READTUP(state,stup,tape,len) ((*(state)->base.readtup) (state, stup, tape, len))
 #define FREESTATE(state)	((state)->base.freestate ? (*(state)->base.freestate) (state) : (void) 0)
 #define LACKMEM(state)		((state)->availMem < 0 && !(state)->slabAllocatorUsed)
@@ -453,8 +453,6 @@ struct Sharedsort
 
 
 static void tuplesort_begin_batch(Tuplesortstate *state);
-static void writetuple(Tuplesortstate *state, LogicalTape *tape,
-					   SortTuple *stup);
 static bool consider_abort_common(Tuplesortstate *state);
 static void inittapes(Tuplesortstate *state, bool mergeruns);
 static void inittapestate(Tuplesortstate *state, int maxTapes);
@@ -1339,24 +1337,6 @@ tuplesort_puttuple_common(Tuplesortstate *state, SortTuple *tuple, bool useAbbre
 	MemoryContextSwitchTo(oldcontext);
 }
 
-/*
- * Write a stored tuple onto tape.  Unless the slab allocator is
- * used, after writing the tuple, pfree() the out-of-line data (not the
- * SortTuple struct!), and increase state->availMem by the amount of
- * memory space thereby released.
- */
-static void
-writetuple(Tuplesortstate *state, LogicalTape *tape, SortTuple *stup)
-{
-	state->base.writetup(state, tape, stup);
-
-	if (!state->slabAllocatorUsed && stup->tuple)
-	{
-		FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
-		pfree(stup->tuple);
-	}
-}
-
 static bool
 consider_abort_common(Tuplesortstate *state)
 {
@@ -2260,6 +2240,8 @@ mergeonerun(Tuplesortstate *state)
 	 */
 	beginmerge(state);
 
+	Assert(state->slabAllocatorUsed);
+
 	/*
 	 * Execute merge by repeatedly extracting lowest tuple in heap, writing it
 	 * out, and replacing it with next tuple from same tape (if there is
@@ -2418,10 +2400,20 @@ dumptuples(Tuplesortstate *state, bool alltuples)
 	memtupwrite = state->memtupcount;
 	for (i = 0; i < memtupwrite; i++)
 	{
-		WRITETUP(state, state->destTape, &state->memtuples[i]);
-		state->memtupcount--;
+		SortTuple  *stup = &state->memtuples[i];
+
+		WRITETUP(state, state->destTape, stup);
+
+		/*
+		 * Account for freeing the tuple, but no need to do the actual pfree
+		 * since the tuplecontext is being reset after the loop.
+		 */
+		if (stup->tuple != NULL)
+			FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
 	}
 
+	state->memtupcount = 0;
+
 	/*
 	 * Reset tuple memory.  We've freed all of the tuples that we previously
 	 * allocated.  It's important to avoid fragmentation when there is a stark
