Subject: [PATCH 1/5] sequential scan for dshash

From: Kyotaro Horiguchi <[email-masked]>
Date: Fri, 29 Jun 2018 16:41:04 +0900

Add sequential scan feature to dshash.
---
 src/backend/lib/dshash.c | 188 ++++++++++++++++++++++++++++++++++++++++++++++-
 src/include/lib/dshash.h |  23 +++++-
 2 files changed, 206 insertions(+), 5 deletions(-)

diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index f095196fb6..1e8c22f94f 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -112,6 +112,7 @@ struct dshash_table
 	size_t		size_log2;		/* log2(number of buckets) */
 	bool		find_locked;	/* Is any partition lock held by 'find'? */
 	bool		find_exclusively_locked;	/* ... exclusively? */
+	bool		seqscan_running;/* now under sequential scan */
 };
 
 /* Given a pointer to an item, find the entry (user data) it holds. */
@@ -127,6 +128,10 @@ struct dshash_table
 #define NUM_SPLITS(size_log2)					\
 	(size_log2 - DSHASH_NUM_PARTITIONS_LOG2)
 
+/* How many buckets are there in a given size? */
+#define NUM_BUCKETS(size_log2)		\
+	(((size_t) 1) << (size_log2))
+
 /* How many buckets are there in each partition at a given size? */
 #define BUCKETS_PER_PARTITION(size_log2)		\
 	(((size_t) 1) << NUM_SPLITS(size_log2))
@@ -153,6 +158,10 @@ struct dshash_table
 #define BUCKET_INDEX_FOR_PARTITION(partition, size_log2)	\
 	((partition) << NUM_SPLITS(size_log2))
 
+/* Choose partition based on bucket index. */
+#define PARTITION_FOR_BUCKET_INDEX(bucket_idx, size_log2)				\
+	((bucket_idx) >> NUM_SPLITS(size_log2))
+
 /* The head of the active bucket for a given hash value (lvalue). */
 #define BUCKET_FOR_HASH(hash_table, hash)								\
 	(hash_table->buckets[												\
@@ -228,6 +237,7 @@ dshash_create(dsa_area *area, const dshash_parameters *params, void *arg)
 
 	hash_table->find_locked = false;
 	hash_table->find_exclusively_locked = false;
+	hash_table->seqscan_running = false;
 
 	/*
 	 * Set up the initial array of buckets.  Our initial size is the same as
@@ -279,6 +289,7 @@ dshash_attach(dsa_area *area, const dshash_parameters *params,
 	hash_table->control = dsa_get_address(area, control);
 	hash_table->find_locked = false;
 	hash_table->find_exclusively_locked = false;
+	hash_table->seqscan_running = false;
 	Assert(hash_table->control->magic == DSHASH_MAGIC);
 
 	/*
@@ -324,7 +335,7 @@ dshash_destroy(dshash_table *hash_table)
 	ensure_valid_bucket_pointers(hash_table);
 
 	/* Free all the entries. */
-	size = ((size_t) 1) << hash_table->size_log2;
+	size = NUM_BUCKETS(hash_table->size_log2);
 	for (i = 0; i < size; ++i)
 	{
 		dsa_pointer item_pointer = hash_table->buckets[i];
@@ -549,9 +560,14 @@ dshash_delete_entry(dshash_table *hash_table, void *entry)
 								LW_EXCLUSIVE));
 
 	delete_item(hash_table, item);
-	hash_table->find_locked = false;
-	hash_table->find_exclusively_locked = false;
-	LWLockRelease(PARTITION_LOCK(hash_table, partition));
+
+	/* We need to keep partition lock while sequential scan */
+	if (!hash_table->seqscan_running)
+	{
+		hash_table->find_locked = false;
+		hash_table->find_exclusively_locked = false;
+		LWLockRelease(PARTITION_LOCK(hash_table, partition));
+	}
 }
 
 /*
@@ -568,6 +584,8 @@ dshash_release_lock(dshash_table *hash_table, void *entry)
 	Assert(LWLockHeldByMeInMode(PARTITION_LOCK(hash_table, partition_index),
 								hash_table->find_exclusively_locked
 								? LW_EXCLUSIVE : LW_SHARED));
+	/* lock is under control of sequential scan */
+	Assert(!hash_table->seqscan_running);
 
 	hash_table->find_locked = false;
 	hash_table->find_exclusively_locked = false;
@@ -592,6 +610,168 @@ dshash_memhash(const void *v, size_t size, void *arg)
 	return tag_hash(v, size);
 }
 
+/*
+ * dshash_seq_init/_next/_term
+ *           Sequentially scan trhough dshash table and return all the
+ *           elements one by one, return NULL when no more.
+ *
+ * dshash_seq_term should be called if and only if the scan is abandoned
+ * before completion; if dshash_seq_next returns NULL then it has already done
+ * the end-of-scan cleanup.
+ *
+ * On returning element, it is locked as is the case with dshash_find.
+ * However, the caller must not release the lock. The lock is released as
+ * necessary in continued scan.
+ *
+ * As opposed to the equivalent for dynanash, the caller is not supposed to
+ * delete the returned element before continuing the scan.
+ *
+ * If consistent is set for dshash_seq_init, the whole hash table is
+ * non-exclusively locked. Otherwise a part of the hash table is locked in the
+ * same mode (partition lock).
+ */
+void
+dshash_seq_init(dshash_seq_status *status, dshash_table *hash_table,
+				bool consistent, bool exclusive)
+{
+	/* allowed at most one scan at once */
+	Assert(!hash_table->seqscan_running);
+
+	status->hash_table = hash_table;
+	status->curbucket = 0;
+	status->nbuckets = 0;
+	status->curitem = NULL;
+	status->pnextitem = InvalidDsaPointer;
+	status->curpartition = -1;
+	status->consistent = consistent;
+	status->exclusive = exclusive;
+	hash_table->seqscan_running = true;
+
+	/*
+	 * Protect all partitions from modification if the caller wants a
+	 * consistent result.
+	 */
+	if (consistent)
+	{
+		int i;
+
+		for (i = 0; i < DSHASH_NUM_PARTITIONS; ++i)
+		{
+			Assert(!LWLockHeldByMe(PARTITION_LOCK(hash_table, i)));
+
+			LWLockAcquire(PARTITION_LOCK(hash_table, i),
+						  exclusive ? LW_EXCLUSIVE : LW_SHARED);
+		}
+		ensure_valid_bucket_pointers(hash_table);
+	}
+}
+
+void *
+dshash_seq_next(dshash_seq_status *status)
+{
+	dsa_pointer next_item_pointer;
+
+	Assert(status->hash_table->seqscan_running);
+	if (status->curitem == NULL)
+	{
+		int partition;
+
+		Assert (status->curbucket == 0);
+		Assert(!status->hash_table->find_locked);
+
+		/* first shot. grab the first item. */
+		if (!status->consistent)
+		{
+			partition =
+				PARTITION_FOR_BUCKET_INDEX(status->curbucket,
+										   status->hash_table->size_log2);
+			LWLockAcquire(PARTITION_LOCK(status->hash_table, partition),
+						  status->exclusive ? LW_EXCLUSIVE : LW_SHARED);
+			status->curpartition = partition;
+
+			/* resize doesn't happen from now until seq scan ends */
+			status->nbuckets =
+				NUM_BUCKETS(status->hash_table->control->size_log2);
+			ensure_valid_bucket_pointers(status->hash_table);
+		}
+
+		next_item_pointer = status->hash_table->buckets[status->curbucket];
+	}
+	else
+		next_item_pointer = status->pnextitem;
+
+	/* Move to the next bucket if we finished the current bucket */
+	while (!DsaPointerIsValid(next_item_pointer))
+	{
+		if (++status->curbucket >= status->nbuckets)
+		{
+			/* all buckets have been scanned. finsih. */
+			dshash_seq_term(status);
+			return NULL;
+		}
+
+		/* Also move parititon lock if needed */
+		if (!status->consistent)
+		{
+			int next_partition =
+				PARTITION_FOR_BUCKET_INDEX(status->curbucket,
+										   status->hash_table->size_log2);
+
+			/* Move lock along with partition for the bucket */
+			if (status->curpartition != next_partition)
+			{
+				/*
+				 * Take lock on the next partition then release the current,
+				 * not in the reverse order. This is required to avoid
+				 * resizing from happening during a sequential scan. Locks are
+				 * taken in partition order so no dead lock happen with other
+				 * seq scans or resizing.
+				 */
+				LWLockAcquire(PARTITION_LOCK(status->hash_table,
+											 next_partition),
+							  status->exclusive ? LW_EXCLUSIVE : LW_SHARED);
+				LWLockRelease(PARTITION_LOCK(status->hash_table,
+											 status->curpartition));
+				status->curpartition = next_partition;
+			}
+		}
+
+		next_item_pointer = status->hash_table->buckets[status->curbucket];
+	}
+
+	status->curitem =
+		dsa_get_address(status->hash_table->area, next_item_pointer);
+	status->hash_table->find_locked = true;
+	status->hash_table->find_exclusively_locked = status->exclusive;
+
+	/*
+	 * This item can be deleted by the caller. Store the next item for the
+	 * next iteration for the occasion.
+	 */
+	status->pnextitem = status->curitem->next;
+
+	return ENTRY_FROM_ITEM(status->curitem);
+}
+
+void
+dshash_seq_term(dshash_seq_status *status)
+{
+	Assert(status->hash_table->seqscan_running);
+	status->hash_table->find_locked = false;
+	status->hash_table->find_exclusively_locked = false;
+	status->hash_table->seqscan_running = false;
+
+	if (status->consistent)
+	{
+		int i;
+
+		for (i = 0; i < DSHASH_NUM_PARTITIONS; ++i)
+			LWLockRelease(PARTITION_LOCK(status->hash_table, i));
+	}
+	else if (status->curpartition >= 0)
+		LWLockRelease(PARTITION_LOCK(status->hash_table, status->curpartition));
+}
+
 /*
  * Print debugging information about the internal state of the hash table to
  * stderr.  The caller must hold no partition locks.
diff --git a/src/include/lib/dshash.h b/src/include/lib/dshash.h
index e5dfd57f0a..b80f3af995 100644
--- a/src/include/lib/dshash.h
+++ b/src/include/lib/dshash.h
@@ -59,6 +59,23 @@ typedef struct dshash_parameters
 struct dshash_table_item;
 typedef struct dshash_table_item dshash_table_item;
 
+/*
+ * Sequential scan state of dshash. The detail is exposed since the storage
+ * size should be known to users but it should be considered as an opaque
+ * type by callers.
+ */
+typedef struct dshash_seq_status
+{
+	dshash_table	   *hash_table;
+	int					curbucket;
+	int					nbuckets;
+	dshash_table_item  *curitem;
+	dsa_pointer			pnextitem;
+	int					curpartition;
+	bool				consistent;
+	bool				exclusive;
+} dshash_seq_status;
+
 /* Creating, sharing and destroying from hash tables. */
 extern dshash_table *dshash_create(dsa_area *area,
 			  const dshash_parameters *params,
@@ -70,7 +87,6 @@ extern dshash_table *dshash_attach(dsa_area *area,
 extern void dshash_detach(dshash_table *hash_table);
 extern dshash_table_handle dshash_get_hash_table_handle(dshash_table *hash_table);
 extern void dshash_destroy(dshash_table *hash_table);
-
 /* Finding, creating, deleting entries. */
 extern void *dshash_find(dshash_table *hash_table,
 			const void *key, bool exclusive);
@@ -80,6 +96,11 @@ extern bool dshash_delete_key(dshash_table *hash_table, const void *key);
 extern void dshash_delete_entry(dshash_table *hash_table, void *entry);
 extern void dshash_release_lock(dshash_table *hash_table, void *entry);
 
+/* seq scan support */
+extern void dshash_seq_init(dshash_seq_status *status, dshash_table *hash_table,
+							bool consistent, bool exclusive);
+extern void *dshash_seq_next(dshash_seq_status *status);
+extern void dshash_seq_term(dshash_seq_status *status);
 /* Convenience hash and compare functions wrapping memcmp and tag_hash. */
 extern int	dshash_memcmp(const void *a, const void *b, size_t size, void *arg);
 extern dshash_hash dshash_memhash(const void *v, size_t size, void *arg);
-- 
2.16.3

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