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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
This commit is contained in:
Jeff Davis 2020-02-13 09:43:51 -08:00
parent 607f8ce74d
commit 7fdd919ae7
1 changed files with 36 additions and 7 deletions
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43
src/backend/utils/sort/logtape.c
View File

@ -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 */