Teach radix tree to embed values at runtime

Previously, the decision to store values in leaves or within the child
pointer was made at compile time, with variable length values using
leaves by necessity. This commit allows introspecting the length of
variable length values at runtime for that decision. This requires
the ability to tell whether the last-level child pointer is actually
a value, so we use a pointer tag in the lowest level bit.

Use this in TID store. This entails adding a byte to the header to
reserve space for the tag. Commit f35bd9b stores up to three offsets
within the header with no bitmap, and now the header can be embedded
as above. This reduces worst-case memory usage when TIDs are sparse.

Reviewed (in an earlier version) by Masahiko Sawada

Discussion: https://postgr.es/m/CANWCAZYw+_KAaUNruhJfE=h6WgtBKeDG32St8vBJBEY82bGVRQ@mail.gmail.com
Discussion: https://postgr.es/m/CAD21AoBci3Hujzijubomo1tdwH3XtQ9F89cTNQ4bsQijOmqnEw@mail.gmail.com

Author: j-naylor

src/backend/access/common/tidstore.c

@@ -35,27 +35,58 @@
 #define WORDS_PER_PAGE(n) ((n) / BITS_PER_BITMAPWORD + 1)
 
 /* number of offsets we can store in the header of a BlocktableEntry */
-#define NUM_FULL_OFFSETS ((sizeof(bitmapword) - sizeof(uint16)) / sizeof(OffsetNumber))
+#define NUM_FULL_OFFSETS ((sizeof(uintptr_t) - sizeof(uint8) - sizeof(int8)) / sizeof(OffsetNumber))
 
 /*
  * This is named similarly to PagetableEntry in tidbitmap.c
  * because the two have a similar function.
  */
 typedef struct BlocktableEntry
 {
-	uint16		nwords;
-
-	/*
-	 * We can store a small number of offsets here to avoid wasting space with
-	 * a sparse bitmap.
-	 */
-	OffsetNumber full_offsets[NUM_FULL_OFFSETS];
+	union
+	{
+		struct
+		{
+#ifndef WORDS_BIGENDIAN
+			/*
+			 * We need to position this member so that the backing radix tree
+			 * can use the lowest bit for a pointer tag. In particular, it
+			 * must be placed within 'header' so that it corresponds to the
+			 * lowest byte in 'ptr'. We position 'nwords' along with it to
+			 * avoid struct padding.
+			 */
+			uint8		flags;
+
+			int8		nwords;
+#endif
+
+			/*
+			 * We can store a small number of offsets here to avoid wasting
+			 * space with a sparse bitmap.
+			 */
+			OffsetNumber full_offsets[NUM_FULL_OFFSETS];
+
+#ifdef WORDS_BIGENDIAN
+			int8		nwords;
+			uint8		flags;
+#endif
+		};
+		uintptr_t	ptr;
+	}			header;
 
 	bitmapword	words[FLEXIBLE_ARRAY_MEMBER];
 } BlocktableEntry;
+
+/*
+ * The type of 'nwords' limits the max number of words in the 'words' array.
+ * This computes the max offset we can actually store in the bitmap. In
+ * practice, it's almost always the same as MaxOffsetNumber.
+ */
+#define MAX_OFFSET_IN_BITMAP Min(BITS_PER_BITMAPWORD * PG_INT8_MAX - 1, MaxOffsetNumber)
+
 #define MaxBlocktableEntrySize \
 	offsetof(BlocktableEntry, words) + \
-		(sizeof(bitmapword) * WORDS_PER_PAGE(MaxOffsetNumber))
+		(sizeof(bitmapword) * WORDS_PER_PAGE(MAX_OFFSET_IN_BITMAP))
 
 #define RT_PREFIX local_ts
 #define RT_SCOPE static
@@ -64,7 +95,8 @@ typedef struct BlocktableEntry
 #define RT_VALUE_TYPE BlocktableEntry
 #define RT_VARLEN_VALUE_SIZE(page) \
 	(offsetof(BlocktableEntry, words) + \
-	sizeof(bitmapword) * (page)->nwords)
+	sizeof(bitmapword) * (page)->header.nwords)
+#define RT_RUNTIME_EMBEDDABLE_VALUE
 #include "lib/radixtree.h"
 
 #define RT_PREFIX shared_ts
@@ -75,7 +107,8 @@ typedef struct BlocktableEntry
 #define RT_VALUE_TYPE BlocktableEntry
 #define RT_VARLEN_VALUE_SIZE(page) \
 	(offsetof(BlocktableEntry, words) + \
-	sizeof(bitmapword) * (page)->nwords)
+	sizeof(bitmapword) * (page)->header.nwords)
+#define RT_RUNTIME_EMBEDDABLE_VALUE
 #include "lib/radixtree.h"
 
 /* Per-backend state for a TidStore */
@@ -350,13 +383,13 @@ TidStoreSetBlockOffsets(TidStore *ts, BlockNumber blkno, OffsetNumber *offsets,
 			OffsetNumber off = offsets[i];
 
 			/* safety check to ensure we don't overrun bit array bounds */
-			if (!OffsetNumberIsValid(off))
+			if (off == InvalidOffsetNumber || off > MAX_OFFSET_IN_BITMAP)
 				elog(ERROR, "tuple offset out of range: %u", off);
 
-			page->full_offsets[i] = off;
+			page->header.full_offsets[i] = off;
 		}
 
-		page->nwords = 0;
+		page->header.nwords = 0;
 	}
 	else
 	{
@@ -371,7 +404,7 @@ TidStoreSetBlockOffsets(TidStore *ts, BlockNumber blkno, OffsetNumber *offsets,
 				OffsetNumber off = offsets[idx];
 
 				/* safety check to ensure we don't overrun bit array bounds */
-				if (!OffsetNumberIsValid(off))
+				if (off == InvalidOffsetNumber || off > MAX_OFFSET_IN_BITMAP)
 					elog(ERROR, "tuple offset out of range: %u", off);
 
 				if (off >= next_word_threshold)
@@ -385,8 +418,8 @@ TidStoreSetBlockOffsets(TidStore *ts, BlockNumber blkno, OffsetNumber *offsets,
 			page->words[wordnum] = word;
 		}
 
-		page->nwords = wordnum;
-		Assert(page->nwords == WORDS_PER_PAGE(offsets[num_offsets - 1]));
+		page->header.nwords = wordnum;
+		Assert(page->header.nwords == WORDS_PER_PAGE(offsets[num_offsets - 1]));
 	}
 
 	if (TidStoreIsShared(ts))
@@ -414,12 +447,12 @@ TidStoreIsMember(TidStore *ts, ItemPointer tid)
 	if (page == NULL)
 		return false;
 
-	if (page->nwords == 0)
+	if (page->header.nwords == 0)
 	{
 		/* we have offsets in the header */
 		for (int i = 0; i < NUM_FULL_OFFSETS; i++)
 		{
-			if (page->full_offsets[i] == off)
+			if (page->header.full_offsets[i] == off)
 				return true;
 		}
 		return false;
@@ -430,7 +463,7 @@ TidStoreIsMember(TidStore *ts, ItemPointer tid)
 		bitnum = BITNUM(off);
 
 		/* no bitmap for the off */
-		if (wordnum >= page->nwords)
+		if (wordnum >= page->header.nwords)
 			return false;
 
 		return (page->words[wordnum] & ((bitmapword) 1 << bitnum)) != 0;
@@ -554,18 +587,18 @@ tidstore_iter_extract_tids(TidStoreIter *iter, BlockNumber blkno,
 	result->num_offsets = 0;
 	result->blkno = blkno;
 
-	if (page->nwords == 0)
+	if (page->header.nwords == 0)
 	{
 		/* we have offsets in the header */
 		for (int i = 0; i < NUM_FULL_OFFSETS; i++)
 		{
-			if (page->full_offsets[i] != InvalidOffsetNumber)
-				result->offsets[result->num_offsets++] = page->full_offsets[i];
+			if (page->header.full_offsets[i] != InvalidOffsetNumber)
+				result->offsets[result->num_offsets++] = page->header.full_offsets[i];
 		}
 	}
 	else
 	{
-		for (wordnum = 0; wordnum < page->nwords; wordnum++)
+		for (wordnum = 0; wordnum < page->header.nwords; wordnum++)
 		{
 			bitmapword	w = page->words[wordnum];
 			int			off = wordnum * BITS_PER_BITMAPWORD;

src/include/lib/radixtree.h

@@ -105,6 +105,10 @@
  *   involving a pointer to the value type, to calculate size.
  *     NOTE: implies that the value is in fact variable-length,
  *     so do not set for fixed-length values.
+ * - RT_RUNTIME_EMBEDDABLE_VALUE - for variable length values, allows
+ *   storing the value in a child pointer slot, rather than as a single-
+ *   value leaf, if small enough. This requires that the value, when
+ *   read as a child pointer, can be tagged in the lowest bit.
  *
  * Optional parameters:
  * - RT_SHMEM - if defined, the radix tree is created in the DSA area
@@ -437,7 +441,13 @@ static inline bool
 RT_VALUE_IS_EMBEDDABLE(RT_VALUE_TYPE * value_p)
 {
 #ifdef RT_VARLEN_VALUE_SIZE
+
+#ifdef RT_RUNTIME_EMBEDDABLE_VALUE
+	return RT_GET_VALUE_SIZE(value_p) <= sizeof(RT_PTR_ALLOC);
+#else
 	return false;
+#endif
+
 #else
 	return RT_GET_VALUE_SIZE(value_p) <= sizeof(RT_PTR_ALLOC);
 #endif
@@ -451,7 +461,19 @@ static inline bool
 RT_CHILDPTR_IS_VALUE(RT_PTR_ALLOC child)
 {
 #ifdef RT_VARLEN_VALUE_SIZE
+
+#ifdef RT_RUNTIME_EMBEDDABLE_VALUE
+	/* check for pointer tag */
+#ifdef RT_SHMEM
+	return child & 1;
+#else
+	return ((uintptr_t) child) & 1;
+#endif
+
+#else
 	return false;
+#endif
+
 #else
 	return sizeof(RT_VALUE_TYPE) <= sizeof(RT_PTR_ALLOC);
 #endif
@@ -1729,6 +1751,15 @@ RT_SET(RT_RADIX_TREE * tree, uint64 key, RT_VALUE_TYPE * value_p)
 	{
 		/* store value directly in child pointer slot */
 		memcpy(slot, value_p, value_sz);
+
+#ifdef RT_RUNTIME_EMBEDDABLE_VALUE
+		/* tag child pointer */
+#ifdef RT_SHMEM
+		*slot |= 1;
+#else
+		*((uintptr_t *) slot) |= 1;
+#endif
+#endif
 	}
 	else
 	{
@@ -2888,6 +2919,7 @@ RT_DUMP_NODE(RT_NODE * node)
 #undef RT_DEFINE
 #undef RT_VALUE_TYPE
 #undef RT_VARLEN_VALUE_SIZE
+#undef RT_RUNTIME_EMBEDDABLE_VALUE
 #undef RT_SHMEM
 #undef RT_USE_DELETE
 #undef RT_DEBUG