Logical Tape Set: lazily allocate read buffer.

The write buffer was already lazily-allocated, so this is more
symmetric. It also means that a freshly-rewound tape (whether for
reading or writing) is not consuming memory for the buffer.

Discussion: https://postgr.es/m/97c46a59c27f3c38e486ca170fcbc618d97ab049.camel%40j-davis.com

diff --git a/src/backend/utils/sort/logtape.c b/src/backend/utils/sort/logtape.c
index 33495f8b4b33922a538a4bc59fa2a8ba71f18837..fd7624c23128b8b153856830a6fb50b24c8888f9 100644 (file)
--- a/src/backend/utils/sort/logtape.c
+++ b/src/backend/utils/sort/logtape.c
@@ -201,6 +201,7 @@ static long ltsGetFreeBlock(LogicalTapeSet *lts);
 static void ltsReleaseBlock(LogicalTapeSet *lts, long blocknum);
 static void ltsConcatWorkerTapes(LogicalTapeSet *lts, TapeShare *shared,
                                 SharedFileSet *fileset);
+static void ltsInitReadBuffer(LogicalTapeSet *lts, LogicalTape *lt);
 
 
 /*
@@ -535,6 +536,27 @@ ltsConcatWorkerTapes(LogicalTapeSet *lts, TapeShare *shared,
    lts->nHoleBlocks = lts->nBlocksAllocated - nphysicalblocks;
 }
 
+/*
+ * Lazily allocate and initialize the read buffer. This avoids waste when many
+ * tapes are open at once, but not all are active between rewinding and
+ * reading.
+ */
+static void
+ltsInitReadBuffer(LogicalTapeSet *lts, LogicalTape *lt)
+{
+   if (lt->firstBlockNumber != -1L)
+   {
+       Assert(lt->buffer_size > 0);
+       lt->buffer = palloc(lt->buffer_size);
+   }
+
+   /* Read the first block, or reset if tape is empty */
+   lt->nextBlockNumber = lt->firstBlockNumber;
+   lt->pos = 0;
+   lt->nbytes = 0;
+   ltsReadFillBuffer(lts, lt);
+}
+
 /*
  * Create a set of logical tapes in a temporary underlying file.
  *
@@ -821,15 +843,9 @@ LogicalTapeRewindForRead(LogicalTapeSet *lts, int tapenum, size_t buffer_size)
    lt->buffer_size = 0;
    if (lt->firstBlockNumber != -1L)
    {
-       lt->buffer = palloc(buffer_size);
+       /* the buffer is lazily allocated, but set the size here */
        lt->buffer_size = buffer_size;
    }
-
-   /* Read the first block, or reset if tape is empty */
-   lt->nextBlockNumber = lt->firstBlockNumber;
-   lt->pos = 0;
-   lt->nbytes = 0;
-   ltsReadFillBuffer(lts, lt);
 }
 
 /*
@@ -878,6 +894,9 @@ LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
    lt = &lts->tapes[tapenum];
    Assert(!lt->writing);
 
+   if (lt->buffer == NULL)
+       ltsInitReadBuffer(lts, lt);
+
    while (size > 0)
    {
        if (lt->pos >= lt->nbytes)
@@ -1015,6 +1034,9 @@ LogicalTapeBackspace(LogicalTapeSet *lts, int tapenum, size_t size)
    Assert(lt->frozen);
    Assert(lt->buffer_size == BLCKSZ);
 
+   if (lt->buffer == NULL)
+       ltsInitReadBuffer(lts, lt);
+
    /*
     * Easy case for seek within current block.
     */
@@ -1087,6 +1109,9 @@ LogicalTapeSeek(LogicalTapeSet *lts, int tapenum,
    Assert(offset >= 0 && offset <= TapeBlockPayloadSize);
    Assert(lt->buffer_size == BLCKSZ);
 
+   if (lt->buffer == NULL)
+       ltsInitReadBuffer(lts, lt);
+
    if (blocknum != lt->curBlockNumber)
    {
        ltsReadBlock(lts, blocknum, (void *) lt->buffer);
@@ -1114,6 +1139,10 @@ LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
 
    Assert(tapenum >= 0 && tapenum < lts->nTapes);
    lt = &lts->tapes[tapenum];
+
+   if (lt->buffer == NULL)
+       ltsInitReadBuffer(lts, lt);
+
    Assert(lt->offsetBlockNumber == 0L);
 
    /* With a larger buffer, 'pos' wouldn't be the same as offset within page */