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* [PATCH v45 2/7] Add conditional lock feature to dshash
@ 2020-03-13 07:58 Kyotaro Horiguchi <[email protected]>
0 siblings, 0 replies; 3+ messages in thread
From: Kyotaro Horiguchi @ 2020-03-13 07:58 UTC (permalink / raw)
Dshash currently waits for lock unconditionally. It is inconvenient
when we want to avoid being blocked by other processes. This commit
adds alternative functions of dshash_find and dshash_find_or_insert
that allows immediate return on lock failure.
---
src/backend/lib/dshash.c | 98 +++++++++++++++++++++-------------------
src/include/lib/dshash.h | 3 ++
2 files changed, 55 insertions(+), 46 deletions(-)
diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index 520bfa0979..853d78b528 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -383,6 +383,10 @@ dshash_get_hash_table_handle(dshash_table *hash_table)
* the caller must take care to ensure that the entry is not left corrupted.
* The lock mode is either shared or exclusive depending on 'exclusive'.
*
+ * If found is not NULL, *found is set to true if the key is found in the hash
+ * table. If the key is not found, *found is set to false and a pointer to a
+ * newly created entry is returned.
+ *
* The caller must not lock a lock already.
*
* Note that the lock held is in fact an LWLock, so interrupts will be held on
@@ -392,36 +396,7 @@ dshash_get_hash_table_handle(dshash_table *hash_table)
void *
dshash_find(dshash_table *hash_table, const void *key, bool exclusive)
{
- dshash_hash hash;
- size_t partition;
- dshash_table_item *item;
-
- hash = hash_key(hash_table, key);
- partition = PARTITION_FOR_HASH(hash);
-
- Assert(hash_table->control->magic == DSHASH_MAGIC);
- Assert(!hash_table->find_locked);
-
- LWLockAcquire(PARTITION_LOCK(hash_table, partition),
- exclusive ? LW_EXCLUSIVE : LW_SHARED);
- ensure_valid_bucket_pointers(hash_table);
-
- /* Search the active bucket. */
- item = find_in_bucket(hash_table, key, BUCKET_FOR_HASH(hash_table, hash));
-
- if (!item)
- {
- /* Not found. */
- LWLockRelease(PARTITION_LOCK(hash_table, partition));
- return NULL;
- }
- else
- {
- /* The caller will free the lock by calling dshash_release_lock. */
- hash_table->find_locked = true;
- hash_table->find_exclusively_locked = exclusive;
- return ENTRY_FROM_ITEM(item);
- }
+ return dshash_find_extended(hash_table, key, exclusive, false, false, NULL);
}
/*
@@ -439,31 +414,60 @@ dshash_find_or_insert(dshash_table *hash_table,
const void *key,
bool *found)
{
- dshash_hash hash;
- size_t partition_index;
- dshash_partition *partition;
+ return dshash_find_extended(hash_table, key, true, false, true, found);
+}
+
+
+/*
+ * Find the key in the hash table.
+ *
+ * "exclusive" is the lock mode in which the partition for the returned item
+ * is locked. If "nowait" is true, the function immediately returns if
+ * required lock was not acquired. "insert" indicates insert mode. In this
+ * mode new entry is inserted and set *found to false. *found is set to true if
+ * found. "found" must be non-null in this mode.
+ */
+void *
+dshash_find_extended(dshash_table *hash_table, const void *key,
+ bool exclusive, bool nowait, bool insert, bool *found)
+{
+ dshash_hash hash = hash_key(hash_table, key);
+ size_t partidx = PARTITION_FOR_HASH(hash);
+ dshash_partition *partition = &hash_table->control->partitions[partidx];
+ LWLockMode lockmode = exclusive ? LW_EXCLUSIVE : LW_SHARED;
dshash_table_item *item;
- hash = hash_key(hash_table, key);
- partition_index = PARTITION_FOR_HASH(hash);
- partition = &hash_table->control->partitions[partition_index];
-
- Assert(hash_table->control->magic == DSHASH_MAGIC);
- Assert(!hash_table->find_locked);
+ /* must be exclusive when insert allowed */
+ Assert(!insert || (exclusive && found != NULL));
restart:
- LWLockAcquire(PARTITION_LOCK(hash_table, partition_index),
- LW_EXCLUSIVE);
+ if (!nowait)
+ LWLockAcquire(PARTITION_LOCK(hash_table, partidx), lockmode);
+ else if (!LWLockConditionalAcquire(PARTITION_LOCK(hash_table, partidx),
+ lockmode))
+ return NULL;
+
ensure_valid_bucket_pointers(hash_table);
/* Search the active bucket. */
item = find_in_bucket(hash_table, key, BUCKET_FOR_HASH(hash_table, hash));
if (item)
- *found = true;
+ {
+ if (found)
+ *found = true;
+ }
else
{
- *found = false;
+ if (found)
+ *found = false;
+
+ if (!insert)
+ {
+ /* The caller didn't told to add a new entry. */
+ LWLockRelease(PARTITION_LOCK(hash_table, partidx));
+ return NULL;
+ }
/* Check if we are getting too full. */
if (partition->count > MAX_COUNT_PER_PARTITION(hash_table))
@@ -479,7 +483,8 @@ restart:
* Give up our existing lock first, because resizing needs to
* reacquire all the locks in the right order to avoid deadlocks.
*/
- LWLockRelease(PARTITION_LOCK(hash_table, partition_index));
+ LWLockRelease(PARTITION_LOCK(hash_table, partidx));
+
resize(hash_table, hash_table->size_log2 + 1);
goto restart;
@@ -493,12 +498,13 @@ restart:
++partition->count;
}
- /* The caller must release the lock with dshash_release_lock. */
+ /* The caller will free the lock by calling dshash_release_lock. */
hash_table->find_locked = true;
- hash_table->find_exclusively_locked = true;
+ hash_table->find_exclusively_locked = exclusive;
return ENTRY_FROM_ITEM(item);
}
+
/*
* Remove an entry by key. Returns true if the key was found and the
* corresponding entry was removed.
diff --git a/src/include/lib/dshash.h b/src/include/lib/dshash.h
index a6ea377173..5b8114d041 100644
--- a/src/include/lib/dshash.h
+++ b/src/include/lib/dshash.h
@@ -91,6 +91,9 @@ extern void *dshash_find(dshash_table *hash_table,
const void *key, bool exclusive);
extern void *dshash_find_or_insert(dshash_table *hash_table,
const void *key, bool *found);
+extern void *dshash_find_extended(dshash_table *hash_table, const void *key,
+ bool exclusive, bool nowait, bool insert,
+ bool *found);
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);
--
2.27.0
----Next_Part(Fri_Jan__8_10_24_34_2021_185)--
Content-Type: Text/X-Patch; charset=us-ascii
Content-Transfer-Encoding: 7bit
Content-Disposition: inline;
filename="v45-0003-Make-archiver-process-an-auxiliary-process.patch"
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Expanding HOT updates for expression and partial indexes
@ 2025-02-09 06:14 Laurenz Albe <[email protected]>
2025-02-10 12:02 ` Re: Expanding HOT updates for expression and partial indexes Burd, Greg <[email protected]>
0 siblings, 1 reply; 3+ messages in thread
From: Laurenz Albe @ 2025-02-09 06:14 UTC (permalink / raw)
To: Burd, Greg <[email protected]>; pgsql-hackers
On Thu, 2025-02-06 at 22:24 +0000, Burd, Greg wrote:
> Attached find a patch that expands the cases where heap-only tuple (HOT) updates are possible
> without changing the basic semantics of HOT. This is accomplished by examining expression
> indexes for changes to determine if indexes require updating or not. A similar approach is
> taken for partial indexes, the predicate is evaluated and, in some cases, HOT updates are
> allowed.
>
> [...]
>
> Third, there is overhead to this patch, it is no longer a single simple bitmap test to choose
> HOT or not in heap_update(). Sometimes this patch will perform expensive additional checks
> and ultimately not go down the HOT path, new overhead with no benefit. Some expressions are
> more expensive than others to evaluate, there is no logic to adjust for that. The Surjective
> patch/email thread had quite a bit of discussion on this without resolution. I’ve chosen to
> add a GUC that optionally avoids the expression evaluation. I’m open to ideas here as well,
> addition of another GUC or removal of the one I’ve added. I’ve tried to avoid rechecking
> indexes for changes when possible.
I think that the goal of this patch is interesting and desirable.
The greatest concern for me is the performance impact. I think that a switch is warranted,
but I am not sure if it should be a GUC. Wouldn't it be better to have a reloption, so that
this can be configured per table? I am not sure if a global switch is necessary, but I am
not fundamentally against it.
Yours,
Laurenz Albe
^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Expanding HOT updates for expression and partial indexes
2025-02-09 06:14 Re: Expanding HOT updates for expression and partial indexes Laurenz Albe <[email protected]>
@ 2025-02-10 12:02 ` Burd, Greg <[email protected]>
0 siblings, 0 replies; 3+ messages in thread
From: Burd, Greg @ 2025-02-10 12:02 UTC (permalink / raw)
To: Laurenz Albe <[email protected]>; +Cc: pgsql-hackers
On Feb 9, 2025, at 1:14 AM, Laurenz Albe <[email protected]> wrote:
>
> I think that the goal of this patch is interesting and desirable.
Thanks for taking a look at it. Which version did you prefer, v3 or v4?
> The greatest concern for me is the performance impact.
Agreed, I’m still looking for ways to minimize it (suggestions welcome).
> I think that a switch is warranted, but I am not sure if it should be a GUC.
> Wouldn't it be better to have a reloption, so that this can be configured per table?
I can remove the GUC in favor of a reloption, that makes sense to me.
> Yours,
> Laurenz Albe
best,
-greg
^ permalink raw reply [nested|flat] 3+ messages in thread
end of thread, other threads:[~2025-02-10 12:02 UTC | newest]
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2020-03-13 07:58 [PATCH v45 2/7] Add conditional lock feature to dshash Kyotaro Horiguchi <[email protected]>
2025-02-09 06:14 Re: Expanding HOT updates for expression and partial indexes Laurenz Albe <[email protected]>
2025-02-10 12:02 ` Re: Expanding HOT updates for expression and partial indexes Burd, Greg <[email protected]>
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