Simplify the code for logical tape read buffers.

Pass the buffer size as argument to LogicalTapeRewindForRead, rather than
setting it earlier with the separate LogicTapeAssignReadBufferSize call.
This way, the buffer size is set closer to where it's actually used, which
makes the code easier to understand.

This makes the calculation for how much memory to use for the buffers less
precise. We now use the same amount of memory for every tape, rounded down
to the nearest BLCKSZ boundary, instead of using one more block for some
tapes, to get the total up to exact amount of memory available. That should
be OK, merging isn't too sensitive to the exact amount of memory used.

Reviewed by Peter Geoghegan

Discussion: <0f607c4b-df23-353e-bf56-c0389d28495f@iki.fi>

Author: hlinnaka

src/backend/utils/sort/logtape.c

@@ -141,14 +141,6 @@ typedef struct LogicalTape
 	long		curBlockNumber; /* this block's logical blk# within tape */
 	int			pos;			/* next read/write position in buffer */
 	int			nbytes;			/* total # of valid bytes in buffer */
-
-	/*
-	 * Desired buffer size to use when reading.  To keep things simple, we use
-	 * a single-block buffer when writing, or when reading a frozen tape.  But
-	 * when we are reading and will only read forwards, we allocate a larger
-	 * buffer, determined by read_buffer_size.
-	 */
-	int			read_buffer_size;
 } LogicalTape;
 
 /*
@@ -609,7 +601,6 @@ LogicalTapeSetCreate(int ntapes)
 		lt->lastBlockBytes = 0;
 		lt->buffer = NULL;
 		lt->buffer_size = 0;
-		lt->read_buffer_size = BLCKSZ;
 		lt->curBlockNumber = 0L;
 		lt->pos = 0;
 		lt->nbytes = 0;
@@ -739,102 +730,133 @@ LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,
 }
 
 /*
- * Rewind logical tape and switch from writing to reading or vice versa.
+ * Rewind logical tape and switch from writing to reading.
  *
- * Unless the tape has been "frozen" in read state, forWrite must be the
- * opposite of the previous tape state.
+ * The tape must currently be in writing state, or "frozen" in read state.
+ *
+ * 'buffer_size' specifies how much memory to use for the read buffer.  It
+ * does not include the memory needed for the indirect blocks.  Regardless
+ * of the argument, the actual amount of memory used is between BLCKSZ and
+ * MaxAllocSize, and is a multiple of BLCKSZ.  The given value is rounded
+ * down and truncated to fit those constraints, if necessary.  If the tape
+ * is frozen, the 'buffer_size' argument is ignored, and a small BLCKSZ byte
+ * buffer is used.
  */
 void
-LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)
+LogicalTapeRewindForRead(LogicalTapeSet *lts, int tapenum, size_t buffer_size)
 {
 	LogicalTape *lt;
 	long		datablocknum;
 
 	Assert(tapenum >= 0 && tapenum < lts->nTapes);
 	lt = &lts->tapes[tapenum];
 
-	if (!forWrite)
+	/*
+	 * Round and cap buffer_size if needed.
+	 */
+	if (lt->frozen)
+		buffer_size = BLCKSZ;
+	else
 	{
-		if (lt->writing)
-		{
-			/*
-			 * Completion of a write phase.  Flush last partial data block,
-			 * flush any partial indirect blocks, rewind for normal
-			 * (destructive) read.
-			 */
-			if (lt->dirty)
-				ltsDumpBuffer(lts, lt);
-			lt->lastBlockBytes = lt->nbytes;
-			lt->writing = false;
-			datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, false);
-		}
-		else
-		{
-			/*
-			 * This is only OK if tape is frozen; we rewind for (another) read
-			 * pass.
-			 */
-			Assert(lt->frozen);
-			datablocknum = ltsRewindFrozenIndirectBlock(lts, lt->indirect);
-		}
+		/* need at least one block */
+		if (buffer_size < BLCKSZ)
+			buffer_size = BLCKSZ;
 
-		/* Allocate a read buffer (unless the tape is empty) */
-		if (lt->buffer)
-			pfree(lt->buffer);
-		lt->buffer = NULL;
-		lt->buffer_size = 0;
-		if (datablocknum != -1L)
-		{
-			lt->buffer = palloc(lt->read_buffer_size);
-			lt->buffer_size = lt->read_buffer_size;
-		}
+		/*
+		 * palloc() larger than MaxAllocSize would fail (a multi-gigabyte
+		 * buffer is unlikely to be helpful, anyway)
+		 */
+		if (buffer_size > MaxAllocSize)
+			buffer_size = MaxAllocSize;
 
-		/* Read the first block, or reset if tape is empty */
-		lt->curBlockNumber = 0L;
-		lt->pos = 0;
-		lt->nbytes = 0;
-		if (datablocknum != -1L)
-			ltsReadFillBuffer(lts, lt, datablocknum);
+		/* round down to BLCKSZ boundary */
+		buffer_size -= buffer_size % BLCKSZ;
+	}
+
+	if (lt->writing)
+	{
+		/*
+		 * Completion of a write phase.  Flush last partial data block, flush
+		 * any partial indirect blocks, rewind for normal (destructive) read.
+		 */
+		if (lt->dirty)
+			ltsDumpBuffer(lts, lt);
+		lt->lastBlockBytes = lt->nbytes;
+		lt->writing = false;
+		datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, false);
 	}
 	else
 	{
 		/*
-		 * Completion of a read phase.  Rewind and prepare for write.
-		 *
-		 * NOTE: we assume the caller has read the tape to the end; otherwise
-		 * untouched data and indirect blocks will not have been freed. We
-		 * could add more code to free any unread blocks, but in current usage
-		 * of this module it'd be useless code.
+		 * This is only OK if tape is frozen; we rewind for (another) read
+		 * pass.
 		 */
-		IndirectBlock *ib,
-				   *nextib;
+		Assert(lt->frozen);
+		datablocknum = ltsRewindFrozenIndirectBlock(lts, lt->indirect);
+	}
 
-		Assert(!lt->writing && !lt->frozen);
-		/* Must truncate the indirect-block hierarchy down to one level. */
-		if (lt->indirect)
-		{
-			for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
-			{
-				nextib = ib->nextup;
-				pfree(ib);
-			}
-			lt->indirect->nextSlot = 0;
-			lt->indirect->nextup = NULL;
-		}
-		lt->writing = true;
-		lt->dirty = false;
-		lt->numFullBlocks = 0L;
-		lt->lastBlockBytes = 0;
-		lt->curBlockNumber = 0L;
-		lt->pos = 0;
-		lt->nbytes = 0;
+	/* Allocate a read buffer (unless the tape is empty) */
+	if (lt->buffer)
+		pfree(lt->buffer);
+	lt->buffer = NULL;
+	lt->buffer_size = 0;
+	if (datablocknum != -1L)
+	{
+		lt->buffer = palloc(buffer_size);
+		lt->buffer_size = buffer_size;
+	}
 
-		if (lt->buffer)
+	/* Read the first block, or reset if tape is empty */
+	lt->curBlockNumber = 0L;
+	lt->pos = 0;
+	lt->nbytes = 0;
+	if (datablocknum != -1L)
+		ltsReadFillBuffer(lts, lt, datablocknum);
+}
+
+/*
+ * Rewind logical tape and switch from reading to writing.
+ *
+ * NOTE: we assume the caller has read the tape to the end; otherwise
+ * untouched data and indirect blocks will not have been freed. We
+ * could add more code to free any unread blocks, but in current usage
+ * of this module it'd be useless code.
+ */
+void
+LogicalTapeRewindForWrite(LogicalTapeSet *lts, int tapenum)
+{
+	LogicalTape *lt;
+	IndirectBlock *ib,
+			   *nextib;
+
+	Assert(tapenum >= 0 && tapenum < lts->nTapes);
+	lt = &lts->tapes[tapenum];
+
+	Assert(!lt->writing && !lt->frozen);
+	/* Must truncate the indirect-block hierarchy down to one level. */
+	if (lt->indirect)
+	{
+		for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
 		{
-			pfree(lt->buffer);
-			lt->buffer = NULL;
-			lt->buffer_size = 0;
+			nextib = ib->nextup;
+			pfree(ib);
 		}
+		lt->indirect->nextSlot = 0;
+		lt->indirect->nextup = NULL;
+	}
+	lt->writing = true;
+	lt->dirty = false;
+	lt->numFullBlocks = 0L;
+	lt->lastBlockBytes = 0;
+	lt->curBlockNumber = 0L;
+	lt->pos = 0;
+	lt->nbytes = 0;
+
+	if (lt->buffer)
+	{
+		pfree(lt->buffer);
+		lt->buffer = NULL;
+		lt->buffer_size = 0;
 	}
 }
 
@@ -1105,28 +1127,3 @@ LogicalTapeSetBlocks(LogicalTapeSet *lts)
 {
 	return lts->nFileBlocks;
 }
-
-/*
- * Set buffer size to use, when reading from given tape.
- */
-void
-LogicalTapeAssignReadBufferSize(LogicalTapeSet *lts, int tapenum, size_t avail_mem)
-{
-	LogicalTape *lt;
-
-	Assert(tapenum >= 0 && tapenum < lts->nTapes);
-	lt = &lts->tapes[tapenum];
-
-	/*
-	 * The buffer size must be a multiple of BLCKSZ in size, so round the
-	 * given value down to nearest BLCKSZ.  Make sure we have at least one
-	 * page. Also, don't go above MaxAllocSize, to avoid erroring out.  A
-	 * multi-gigabyte buffer is unlikely to be helpful, anyway.
-	 */
-	if (avail_mem < BLCKSZ)
-		avail_mem = BLCKSZ;
-	if (avail_mem > MaxAllocSize)
-		avail_mem = MaxAllocSize;
-	avail_mem -= avail_mem % BLCKSZ;
-	lt->read_buffer_size = avail_mem;
-}