* when reading, and read multiple blocks from the same tape in one go,
* whenever the buffer becomes empty.
*
- * To support the above policy of writing to the lowest free block,
- * ltsGetFreeBlock sorts the list of free block numbers into decreasing
- * order each time it is asked for a block and the list isn't currently
- * sorted. This is an efficient way to handle it because we expect cycles
- * of releasing many blocks followed by re-using many blocks, due to
- * the larger read buffer.
+ * To support the above policy of writing to the lowest free block, the
+ * freelist is a min heap.
*
* Since all the bookkeeping and buffer memory is allocated with palloc(),
* and the underlying file(s) are made with OpenTemporaryFile, all resources
/*
* File size tracking. nBlocksWritten is the size of the underlying file,
* in BLCKSZ blocks. nBlocksAllocated is the number of blocks allocated
- * by ltsGetFreeBlock(), and it is always greater than or equal to
+ * by ltsReleaseBlock(), and it is always greater than or equal to
* nBlocksWritten. Blocks between nBlocksAllocated and nBlocksWritten are
* blocks that have been allocated for a tape, but have not been written
* to the underlying file yet. nHoleBlocks tracks the total number of
* If forgetFreeSpace is true then any freed blocks are simply forgotten
* rather than being remembered in freeBlocks[]. See notes for
* LogicalTapeSetForgetFreeSpace().
- *
- * If blocksSorted is true then the block numbers in freeBlocks are in
- * *decreasing* order, so that removing the last entry gives us the lowest
- * free block. We re-sort the blocks whenever a block is demanded; this
- * should be reasonably efficient given the expected usage pattern.
*/
bool forgetFreeSpace; /* are we remembering free blocks? */
- bool blocksSorted; /* is freeBlocks[] currently in order? */
- long *freeBlocks; /* resizable array */
- int nFreeBlocks; /* # of currently free blocks */
- int freeBlocksLen; /* current allocated length of freeBlocks[] */
+ long *freeBlocks; /* resizable array holding minheap */
+ long nFreeBlocks; /* # of currently free blocks */
+ Size freeBlocksLen; /* current allocated length of freeBlocks[] */
/* The array of logical tapes. */
int nTapes; /* # of logical tapes in set */
return (lt->nbytes > 0);
}
-/*
- * qsort comparator for sorting freeBlocks[] into decreasing order.
- */
-static int
-freeBlocks_cmp(const void *a, const void *b)
+static inline void
+swap_nodes(long *heap, unsigned long a, unsigned long b)
+{
+ unsigned long swap;
+
+ swap = heap[a];
+ heap[a] = heap[b];
+ heap[b] = swap;
+}
+
+static inline unsigned long
+left_offset(unsigned long i)
+{
+ return 2 * i + 1;
+}
+
+static inline unsigned long
+right_offset(unsigned i)
+{
+ return 2 * i + 2;
+}
+
+static inline unsigned long
+parent_offset(unsigned long i)
{
- long ablk = *((const long *) a);
- long bblk = *((const long *) b);
-
- /* can't just subtract because long might be wider than int */
- if (ablk < bblk)
- return 1;
- if (ablk > bblk)
- return -1;
- return 0;
+ return (i - 1) / 2;
}
/*
- * Select a currently unused block for writing to.
+ * Select the lowest currently unused block by taking the first element from
+ * the freelist min heap.
*/
static long
ltsGetFreeBlock(LogicalTapeSet *lts)
{
- /*
- * If there are multiple free blocks, we select the one appearing last in
- * freeBlocks[] (after sorting the array if needed). If there are none,
- * assign the next block at the end of the file.
- */
- if (lts->nFreeBlocks > 0)
+ long *heap = lts->freeBlocks;
+ long blocknum;
+ int heapsize;
+ unsigned long pos;
+
+ /* freelist empty; allocate a new block */
+ if (lts->nFreeBlocks == 0)
+ return lts->nBlocksAllocated++;
+
+ if (lts->nFreeBlocks == 1)
{
- if (!lts->blocksSorted)
- {
- qsort((void *) lts->freeBlocks, lts->nFreeBlocks,
- sizeof(long), freeBlocks_cmp);
- lts->blocksSorted = true;
- }
- return lts->freeBlocks[--lts->nFreeBlocks];
+ lts->nFreeBlocks--;
+ return lts->freeBlocks[0];
}
- else
- return lts->nBlocksAllocated++;
+
+ /* take top of minheap */
+ blocknum = heap[0];
+
+ /* replace with end of minheap array */
+ heap[0] = heap[--lts->nFreeBlocks];
+
+ /* sift down */
+ pos = 0;
+ heapsize = lts->nFreeBlocks;
+ while (true)
+ {
+ unsigned long left = left_offset(pos);
+ unsigned long right = right_offset(pos);
+ unsigned long min_child;
+
+ if (left < heapsize && right < heapsize)
+ min_child = (heap[left] < heap[right]) ? left : right;
+ else if (left < heapsize)
+ min_child = left;
+ else if (right < heapsize)
+ min_child = right;
+ else
+ break;
+
+ if (heap[min_child] >= heap[pos])
+ break;
+
+ swap_nodes(heap, min_child, pos);
+ pos = min_child;
+ }
+
+ return blocknum;
}
/*
static void
ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
{
- int ndx;
+ long *heap;
+ unsigned long pos;
/*
* Do nothing if we're no longer interested in remembering free space.
*/
if (lts->nFreeBlocks >= lts->freeBlocksLen)
{
+ /*
+ * If the freelist becomes very large, just return and leak this free
+ * block.
+ */
+ if (lts->freeBlocksLen * 2 > MaxAllocSize)
+ return;
+
lts->freeBlocksLen *= 2;
lts->freeBlocks = (long *) repalloc(lts->freeBlocks,
lts->freeBlocksLen * sizeof(long));
}
- /*
- * Add blocknum to array, and mark the array unsorted if it's no longer in
- * decreasing order.
- */
- ndx = lts->nFreeBlocks++;
- lts->freeBlocks[ndx] = blocknum;
- if (ndx > 0 && lts->freeBlocks[ndx - 1] < blocknum)
- lts->blocksSorted = false;
+ heap = lts->freeBlocks;
+ pos = lts->nFreeBlocks;
+
+ /* place entry at end of minheap array */
+ heap[pos] = blocknum;
+ lts->nFreeBlocks++;
+
+ /* sift up */
+ while (pos != 0)
+ {
+ unsigned long parent = parent_offset(pos);
+ if (heap[parent] < heap[pos])
+ break;
+
+ swap_nodes(heap, parent, pos);
+ pos = parent;
+ }
}
/*
lts->nBlocksWritten = 0L;
lts->nHoleBlocks = 0L;
lts->forgetFreeSpace = false;
- lts->blocksSorted = true; /* a zero-length array is sorted ... */
lts->freeBlocksLen = 32; /* reasonable initial guess */
lts->freeBlocks = (long *) palloc(lts->freeBlocksLen * sizeof(long));
lts->nFreeBlocks = 0;
projects / users / gsingh / postgres.git / commitdiff