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help / color / mirror / Atom feedFrom: Joel Jacobson <[email protected]>
To: Tom Lane <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Wed, 15 Oct 2025 05:22:59 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Tue, Oct 14, 2025, at 23:19, Tom Lane wrote:
> "Joel Jacobson" <[email protected]> writes:
>> Having investigated this, the "direct advancement" approach seems
>> correct to me.
>
>> (I understand the exclusive lock in PreCommit_Notify on NotifyQueueLock
>> is of course needed because there are other operations that don't
>> acquire the heavyweight-lock, that take shared/exclusive lock on
>> NotifyQueueLock to read/modify QUEUE_HEAD, so the exclusive lock on
>> NotifyQueueLock in PreCommit_Notify is needed, since it modifies the
>> QUEUE_HEAD.)
>
> Right. What the heavyweight lock buys for us in this context is that
> we can be sure no other would-be notifier can insert any messages
> in between ours, even though we may take and release NotifyQueueLock
> several times to allow readers to sneak in. That in turn means that
> it's safe to advance readers over that whole set of messages if we
> know we didn't wake them up for any of those messages.
Right.
> There is a false-positive possibility if a reader was previously
> signaled but hasn't yet awoken: we will think that maybe we signaled
> it and hence not advance its pointer. This is an error in the safe
> direction however, and it will advance its pointer when it does
> wake up.
I've added a comment on this in SignalBackends.
> A potential complaint is that we are doubling down on the need for
> that heavyweight lock, despite the upthread discussion about maybe
> getting rid of it for better scalability. However, this patch
> only requires holding a lock across all the insertions, not holding
> it through commit which I think is the true scalability blockage.
>
> If we did want to get rid of that lock, we'd only need to stop
> releasing NotifyQueueLock at insertion page boundary crossings,
> which I suspect isn't really that useful anyway.
Right. So if the upthread discussion would get rid of the heavyweight
lock we would just need to hold the exclusive lock across all
insertions. Good to know the two efforts are not conflicting.
> (In connection
> with that though, I think you ought to capture both the "before" and
> "after" pointers within that lock interval, not expend another lock
> acquisition later.)
Fixed.
> It would be good if the patch's comments made these points ...
I've added a comment inside PreCommit_Notify on how it would suffice to
hold the exclusive lock across all insertions, for the purpose of
setting the "before" and "after" pointers, if the heavyweight lock would
be removed.
> also, the comments above struct AsyncQueueControl need to be
> updated, because changing some other backend's queue pos is
> not legal under any of the stated rules.
Fixed.
>> Given all the experiments since my earlier message, here is a fresh,
>> self-contained write-up:
>
> I'm getting itchy about removing the local listenChannels list,
> because what you've done is to replace it with a shared data
> structure that can't be accessed without a good deal of locking
> overhead. That seems like it could easily be a net loss.
I agree, I also prefer the local listenChannels list.
I've changed it back.
> Also, I really do not like this implementation of
> GetPendingNotifyChannels, as it looks like O(N^2) effort.
> The potentially large length of the list it builds is scary too,
> considering the comments that SignalBackends had better not fail.
> If we have to do it that way it'd be better to collect the list
> during PreCommit_Notify.
I agree. I've removed GetPendingNotifyChannels and added a local list,
named pendingNotifyChannels instead, collected during PreCommit_Notify.
> The "Avoid needing to wake listening backends" loop should probably
> be combined with the loop after it; I don't quite see the point of
> iterating over all the listening backends twice.
I agree. Fixed.
> Also, why is the
> second loop only paying attention to backends in the same DB?
Fixed. (We're already sure it's the same DB, since that's part of the
hash key. I've removed the redundant check.)
> I don't love adding queueHeadBeforeWrite and queueHeadAfterWrite into
> the pendingNotifies data structure, as they have no visible connection
> to that. In particular, we will have multiple NotificationList
> structs when there's nested transactions, and it's certainly
> meaningless to have such fields in more than one place. Probably
> just making them independent static variables is the best way.
Fixed.
> The overall layout of what the patch injects where needs another
> look. I don't like inserting code before typedefs and static
> variables within a module: that's not our normal layout style.
Fixed.
/Joel
Attachments:
[application/octet-stream] 0001-optimize_listen_notify-v17.patch (7.8K, 2-0001-optimize_listen_notify-v17.patch)
download | inline diff:
From 600fef1d835a512a34cb8118e0681832cbae5120 Mon Sep 17 00:00:00 2001
From: Joel Jacobson <[email protected]>
Date: Wed, 8 Oct 2025 09:30:54 +0200
Subject: [PATCH 1/2] Improve LISTEN/NOTIFY test coverage
This adds isolation tests to cover previously untested code paths:
* Check simple NOTIFY reparenting when parent has no action
* Check LISTEN reparenting in subtransaction
* Check LISTEN merge path when both outer and inner transactions have actions
* Check LISTEN abort path (ROLLBACK TO SAVEPOINT discards pending actions)
* Check notification_match function (triggered by hash table duplicate detection)
This also adds a test to prepare for the next patch:
* Check ChannelHashAddListener array growth
---
src/test/isolation/expected/async-notify.out | 103 ++++++++++++++++++-
src/test/isolation/specs/async-notify.spec | 52 ++++++++++
2 files changed, 154 insertions(+), 1 deletion(-)
diff --git a/src/test/isolation/expected/async-notify.out b/src/test/isolation/expected/async-notify.out
index 556e1805893..9c19843d2d7 100644
--- a/src/test/isolation/expected/async-notify.out
+++ b/src/test/isolation/expected/async-notify.out
@@ -1,4 +1,4 @@
-Parsed test spec with 3 sessions
+Parsed test spec with 5 sessions
starting permutation: listenc notify1 notify2 notify3 notifyf
step listenc: LISTEN c1; LISTEN c2;
@@ -47,6 +47,105 @@ notifier: NOTIFY "c2" with payload "payload" from notifier
notifier: NOTIFY "c1" with payload "payloads" from notifier
notifier: NOTIFY "c2" with payload "payloads" from notifier
+starting permutation: listenc notifys_simple
+step listenc: LISTEN c1; LISTEN c2;
+step notifys_simple:
+ BEGIN;
+ SAVEPOINT s1;
+ NOTIFY c1, 'simple1';
+ NOTIFY c2, 'simple2';
+ RELEASE SAVEPOINT s1;
+ COMMIT;
+
+notifier: NOTIFY "c1" with payload "simple1" from notifier
+notifier: NOTIFY "c2" with payload "simple2" from notifier
+
+starting permutation: lsbegin lssavepoint lslisten lsrelease lscommit lsnotify
+step lsbegin: BEGIN;
+step lssavepoint: SAVEPOINT s1;
+step lslisten: LISTEN c1; LISTEN c2;
+step lsrelease: RELEASE SAVEPOINT s1;
+step lscommit: COMMIT;
+step lsnotify: NOTIFY c1, 'subxact_test';
+listen_subxact: NOTIFY "c1" with payload "subxact_test" from listen_subxact
+
+starting permutation: lsbegin lslisten_outer lssavepoint lslisten lsrelease lscommit lsnotify
+step lsbegin: BEGIN;
+step lslisten_outer: LISTEN c3;
+step lssavepoint: SAVEPOINT s1;
+step lslisten: LISTEN c1; LISTEN c2;
+step lsrelease: RELEASE SAVEPOINT s1;
+step lscommit: COMMIT;
+step lsnotify: NOTIFY c1, 'subxact_test';
+listen_subxact: NOTIFY "c1" with payload "subxact_test" from listen_subxact
+
+starting permutation: lsbegin lssavepoint lslisten lsrollback lscommit lsnotify_check
+step lsbegin: BEGIN;
+step lssavepoint: SAVEPOINT s1;
+step lslisten: LISTEN c1; LISTEN c2;
+step lsrollback: ROLLBACK TO SAVEPOINT s1;
+step lscommit: COMMIT;
+step lsnotify_check: NOTIFY c1, 'should_not_receive';
+
+starting permutation: listenc notify_many_with_dup
+step listenc: LISTEN c1; LISTEN c2;
+step notify_many_with_dup:
+ BEGIN;
+ SELECT pg_notify('c1', 'msg' || s::text) FROM generate_series(1, 17) s;
+ SELECT pg_notify('c1', 'msg1');
+ COMMIT;
+
+pg_notify
+---------
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+(17 rows)
+
+pg_notify
+---------
+
+(1 row)
+
+notifier: NOTIFY "c1" with payload "msg1" from notifier
+notifier: NOTIFY "c1" with payload "msg2" from notifier
+notifier: NOTIFY "c1" with payload "msg3" from notifier
+notifier: NOTIFY "c1" with payload "msg4" from notifier
+notifier: NOTIFY "c1" with payload "msg5" from notifier
+notifier: NOTIFY "c1" with payload "msg6" from notifier
+notifier: NOTIFY "c1" with payload "msg7" from notifier
+notifier: NOTIFY "c1" with payload "msg8" from notifier
+notifier: NOTIFY "c1" with payload "msg9" from notifier
+notifier: NOTIFY "c1" with payload "msg10" from notifier
+notifier: NOTIFY "c1" with payload "msg11" from notifier
+notifier: NOTIFY "c1" with payload "msg12" from notifier
+notifier: NOTIFY "c1" with payload "msg13" from notifier
+notifier: NOTIFY "c1" with payload "msg14" from notifier
+notifier: NOTIFY "c1" with payload "msg15" from notifier
+notifier: NOTIFY "c1" with payload "msg16" from notifier
+notifier: NOTIFY "c1" with payload "msg17" from notifier
+
+starting permutation: listenc llisten l2listen l3listen lslisten
+step listenc: LISTEN c1; LISTEN c2;
+step llisten: LISTEN c1; LISTEN c2;
+step l2listen: LISTEN c1;
+step l3listen: LISTEN c1;
+step lslisten: LISTEN c1; LISTEN c2;
+
starting permutation: llisten notify1 notify2 notify3 notifyf lcheck
step llisten: LISTEN c1; LISTEN c2;
step notify1: NOTIFY c1;
@@ -95,6 +194,8 @@ listener: NOTIFY "c2" with payload "" from notifier
starting permutation: l2listen l2begin notify1 lbegins llisten lcommit l2commit l2stop
step l2listen: LISTEN c1;
+listener2: NOTIFY "c1" with payload "" from notifier
+listener2: NOTIFY "c1" with payload "" from notifier
step l2begin: BEGIN;
step notify1: NOTIFY c1;
step lbegins: BEGIN ISOLATION LEVEL SERIALIZABLE;
diff --git a/src/test/isolation/specs/async-notify.spec b/src/test/isolation/specs/async-notify.spec
index 0b8cfd91083..942b09d5735 100644
--- a/src/test/isolation/specs/async-notify.spec
+++ b/src/test/isolation/specs/async-notify.spec
@@ -31,6 +31,20 @@ step notifys1 {
ROLLBACK TO SAVEPOINT s2;
COMMIT;
}
+step notifys_simple {
+ BEGIN;
+ SAVEPOINT s1;
+ NOTIFY c1, 'simple1';
+ NOTIFY c2, 'simple2';
+ RELEASE SAVEPOINT s1;
+ COMMIT;
+}
+step notify_many_with_dup {
+ BEGIN;
+ SELECT pg_notify('c1', 'msg' || s::text) FROM generate_series(1, 17) s;
+ SELECT pg_notify('c1', 'msg1');
+ COMMIT;
+}
step usage { SELECT pg_notification_queue_usage() > 0 AS nonzero; }
step bignotify { SELECT count(pg_notify('c1', s::text)) FROM generate_series(1, 1000) s; }
teardown { UNLISTEN *; }
@@ -53,6 +67,26 @@ step l2begin { BEGIN; }
step l2commit { COMMIT; }
step l2stop { UNLISTEN *; }
+# Third listener session for testing array growth.
+
+session listener3
+step l3listen { LISTEN c1; }
+teardown { UNLISTEN *; }
+
+# Session for testing LISTEN in subtransaction with separate steps.
+
+session listen_subxact
+step lsbegin { BEGIN; }
+step lslisten_outer { LISTEN c3; }
+step lssavepoint { SAVEPOINT s1; }
+step lslisten { LISTEN c1; LISTEN c2; }
+step lsrelease { RELEASE SAVEPOINT s1; }
+step lsrollback { ROLLBACK TO SAVEPOINT s1; }
+step lscommit { COMMIT; }
+step lsnotify { NOTIFY c1, 'subxact_test'; }
+step lsnotify_check { NOTIFY c1, 'should_not_receive'; }
+teardown { UNLISTEN *; }
+
# Trivial cases.
permutation listenc notify1 notify2 notify3 notifyf
@@ -60,6 +94,24 @@ permutation listenc notify1 notify2 notify3 notifyf
# Check simple and less-simple deduplication.
permutation listenc notifyd1 notifyd2 notifys1
+# Check simple NOTIFY reparenting when parent has no action.
+permutation listenc notifys_simple
+
+# Check LISTEN reparenting in subtransaction.
+permutation lsbegin lssavepoint lslisten lsrelease lscommit lsnotify
+
+# Check LISTEN merge path when both outer and inner transactions have actions.
+permutation lsbegin lslisten_outer lssavepoint lslisten lsrelease lscommit lsnotify
+
+# Check LISTEN abort path (ROLLBACK TO SAVEPOINT discards pending actions).
+permutation lsbegin lssavepoint lslisten lsrollback lscommit lsnotify_check
+
+# Check notification_match function (triggered by hash table duplicate detection).
+permutation listenc notify_many_with_dup
+
+# Check ChannelHashAddListener array growth.
+permutation listenc llisten l2listen l3listen lslisten
+
# Cross-backend notification delivery. We use a "select 1" to force the
# listener session to check for notifies. In principle we could just wait
# for delivery, but that would require extra support in isolationtester
--
2.50.1
[application/octet-stream] 0002-optimize_listen_notify-v17.patch (30.9K, 3-0002-optimize_listen_notify-v17.patch)
download | inline diff:
From 21e28730b9c2a27d3a2ae97ee31b12e9931f1aab Mon Sep 17 00:00:00 2001
From: Joel Jacobson <[email protected]>
Date: Tue, 14 Oct 2025 08:03:19 +0200
Subject: [PATCH 2/2] Optimize LISTEN/NOTIFY with shared channel map and direct
advancement
This patch reworks the LISTEN/NOTIFY signaling path to avoid the
long-standing inefficiency where every commit wakes all listening
backends in the same database, even those that are listening on
completely different channels.
Problem
-------
At present, SignalBackends has no central knowledge of which backend
listens on which channel. When a backend commits a transaction that
issued NOTIFY, it simply iterates over all registered listeners in the
same database and sends each one a PROCSIG_NOTIFY_INTERRUPT signal.
That behavior is fine when all listeners are on the same channel, but
when many backends are listening on different channels, each NOTIFY
triggers a storm of unnecessary wakeups and context switches. As the
number of idle listeners grows, this often becomes the bottleneck and
throughput drops sharply.
Overview of the solution
------------------------
This patch introduces a lazily-created dynamic shared hash (dshash)
backed by dynamic shared memory (DSA) that maps (dboid, channel) to
arrays of listening backends (ProcNumbers). This allows the sender to
target only those backends actually listening on the channels for which
it has queued notifications.
At commit time:
* AtCommit_Notify updates the shared channelHash to reflect any LISTEN
or UNLISTEN actions performed in the transaction.
* SignalBackends consults this hash to find the backends that are
listening on the channels being notified in the current database, and
signals only those.
Each backend's entry in AsyncQueueControl now includes a wakeupPending
flag to prevent duplicate signals while a previous wakeup is still being
processed.
Direct advancement
------------------
A further optimization avoids signaling idle backends that are not
listening on any of the channels notified within the transaction.
While queuing notifications, PreCommit_Notify records the queue head
position both before and after writing its notifications. Because all
writers are serialized by the existing cluster-wide heavyweight lock on
"database 0", no backend (from any database) can insert entries between
those two points. This guarantees that the region [oldHead, newHead)
contains only the entries written by our commit.
SignalBackends uses this fact to directly advance any backend still
positioned at oldHead up to newHead, avoiding a needless wakeup for
listeners that would otherwise not find any notifies of interest.
Queue health
------------
If a backend has fallen too far behind (lag >= QUEUE_CLEANUP_DELAY
pages), it is signaled to catch up so the global queue tail can advance.
Other notes
-----------
* Maintains dual data structures: a shared channelHash for determining
which backends to signal, and a local per-backend listenChannels list
for fast lock-free lookups during notification processing. This avoids
contention on the shared hash during the high-frequency IsListeningOn
checks that occur for every notification read from the queue.
* Backends remain registered in the global listener list as long as
listenChannels is non-empty.
* Adds LWLock tranche NOTIFY_CHANNEL_HASH and wait event
NotifyChannelHash for visibility.
* No user-visible behavioral changes; this is an internal optimization
only.
---
src/backend/commands/async.c | 528 ++++++++++++++++--
.../utils/activity/wait_event_names.txt | 1 +
src/include/storage/lwlocklist.h | 1 +
src/tools/pgindent/typedefs.list | 2 +
4 files changed, 487 insertions(+), 45 deletions(-)
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index 4bd37d5beb5..cecd4958dea 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -24,8 +24,10 @@
* All notification messages are placed in the queue and later read out
* by listening backends.
*
- * There is no central knowledge of which backend listens on which channel;
- * every backend has its own list of interesting channels.
+ * We also maintain a dynamic shared hash table (dshash) that maps channel
+ * names to the set of backends listening on each channel. This table is
+ * created lazily on the first LISTEN command and grows dynamically as
+ * needed.
*
* Although there is only one queue, notifications are treated as being
* database-local; this is done by including the sender's database OID
@@ -68,16 +70,27 @@
* CommitTransaction() which will then do the actual transaction commit.
*
* After commit we are called another time (AtCommit_Notify()). Here we
- * make any actual updates to the effective listen state (listenChannels).
+ * make any actual updates to the effective listen state (channelHash).
* Then we signal any backends that may be interested in our messages
* (including our own backend, if listening). This is done by
- * SignalBackends(), which scans the list of listening backends and sends a
- * PROCSIG_NOTIFY_INTERRUPT signal to every listening backend (we don't
- * know which backend is listening on which channel so we must signal them
- * all). We can exclude backends that are already up to date, though, and
- * we can also exclude backends that are in other databases (unless they
- * are way behind and should be kicked to make them advance their
- * pointers).
+ * SignalBackends(), which consults the shared channel hash table to
+ * identify listeners for the channels that have pending notifications
+ * in the current database. Each selected backend is marked as having a
+ * wakeup pending to avoid duplicate signals, and a PROCSIG_NOTIFY_INTERRUPT
+ * signal is sent to it.
+ *
+ * When writing notifications, PreCommit_Notify() records the queue head
+ * position both before and after the write. Because all writers serialize
+ * on a cluster-wide heavyweight lock, no backend can insert entries between
+ * these two points. SignalBackends() uses this fact to directly advance any
+ * backend that is still positioned at the old head, avoiding unnecessary
+ * wakeups for idle listeners that have nothing to read.
+ *
+ * To maintain queue health, SignalBackends() also wakes one backend
+ * positioned at the global queue tail to help advance it, and signals
+ * any backend that has fallen too far behind to catch up. These measures
+ * prevent the notification queue from growing indefinitely, while mostly
+ * limiting wakeups to the backends that actually need them.
*
* Finally, after we are out of the transaction altogether and about to go
* idle, we scan the queue for messages that need to be sent to our
@@ -128,6 +141,7 @@
#include <limits.h>
#include <unistd.h>
#include <signal.h>
+#include <string.h>
#include "access/parallel.h"
#include "access/slru.h"
@@ -137,14 +151,17 @@
#include "commands/async.h"
#include "common/hashfn.h"
#include "funcapi.h"
+#include "lib/dshash.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
+#include "storage/dsm_registry.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/procsignal.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
+#include "utils/dsa.h"
#include "utils/guc_hooks.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
@@ -162,6 +179,29 @@
*/
#define NOTIFY_PAYLOAD_MAX_LENGTH (BLCKSZ - NAMEDATALEN - 128)
+/*
+ * Channel hash table definitions
+ *
+ * This hash table maps (database OID, channel name) keys to arrays of
+ * ProcNumbers representing the backends listening on each channel.
+ */
+
+#define INITIAL_LISTENERS_ARRAY_SIZE 4
+
+typedef struct ChannelHashKey
+{
+ Oid dboid;
+ char channel[NAMEDATALEN];
+} ChannelHashKey;
+
+typedef struct ChannelEntry
+{
+ ChannelHashKey key;
+ dsa_pointer listenersArray; /* DSA pointer to ProcNumber array */
+ int numListeners; /* Number of listeners currently stored */
+ int allocatedListeners; /* Allocated size of array */
+} ChannelEntry;
+
/*
* Struct representing an entry in the global notify queue
*
@@ -227,8 +267,8 @@ typedef struct QueuePosition
/*
* Parameter determining how often we try to advance the tail pointer:
* we do that after every QUEUE_CLEANUP_DELAY pages of NOTIFY data. This is
- * also the distance by which a backend in another database needs to be
- * behind before we'll decide we need to wake it up to advance its pointer.
+ * also the distance by which a backend needs to be behind before we'll
+ * decide we need to wake it up to advance its pointer.
*
* Resist the temptation to make this really large. While that would save
* work in some places, it would add cost in others. In particular, this
@@ -246,6 +286,7 @@ typedef struct QueueBackendStatus
Oid dboid; /* backend's database OID, or InvalidOid */
ProcNumber nextListener; /* id of next listener, or INVALID_PROC_NUMBER */
QueuePosition pos; /* backend has read queue up to here */
+ bool wakeupPending; /* signal sent but not yet processed */
} QueueBackendStatus;
/*
@@ -260,9 +301,9 @@ typedef struct QueueBackendStatus
* (since no other backend will inspect it).
*
* When holding NotifyQueueLock in EXCLUSIVE mode, backends can inspect the
- * entries of other backends and also change the head pointer. When holding
- * both NotifyQueueLock and NotifyQueueTailLock in EXCLUSIVE mode, backends
- * can change the tail pointers.
+ * entries of other backends, change the head pointer, and advance other
+ * backends' queue positions. When holding both NotifyQueueLock and
+ * NotifyQueueTailLock in EXCLUSIVE mode, backends can change the tail pointers.
*
* SLRU buffer pool is divided in banks and bank wise SLRU lock is used as
* the control lock for the pg_notify SLRU buffers.
@@ -288,11 +329,16 @@ typedef struct AsyncQueueControl
ProcNumber firstListener; /* id of first listener, or
* INVALID_PROC_NUMBER */
TimestampTz lastQueueFillWarn; /* time of last queue-full msg */
+ dsa_handle channelHashDSA;
+ dshash_table_handle channelHashDSH;
QueueBackendStatus backend[FLEXIBLE_ARRAY_MEMBER];
} AsyncQueueControl;
static AsyncQueueControl *asyncQueueControl;
+static dsa_area *channelDSA = NULL;
+static dshash_table *channelHash = NULL;
+
#define QUEUE_HEAD (asyncQueueControl->head)
#define QUEUE_TAIL (asyncQueueControl->tail)
#define QUEUE_STOP_PAGE (asyncQueueControl->stopPage)
@@ -301,6 +347,7 @@ static AsyncQueueControl *asyncQueueControl;
#define QUEUE_BACKEND_DBOID(i) (asyncQueueControl->backend[i].dboid)
#define QUEUE_NEXT_LISTENER(i) (asyncQueueControl->backend[i].nextListener)
#define QUEUE_BACKEND_POS(i) (asyncQueueControl->backend[i].pos)
+#define QUEUE_BACKEND_WAKEUP_PENDING(i) (asyncQueueControl->backend[i].wakeupPending)
/*
* The SLRU buffer area through which we access the notification queue
@@ -418,6 +465,20 @@ static bool unlistenExitRegistered = false;
/* True if we're currently registered as a listener in asyncQueueControl */
static bool amRegisteredListener = false;
+/*
+ * Queue head positions for direct advancement.
+ * These are captured during PreCommit_Notify while holding the heavyweight
+ * lock on database 0, ensuring no other backend can insert notifications
+ * between them. SignalBackends uses these to advance idle backends.
+ */
+static QueuePosition queueHeadBeforeWrite;
+static QueuePosition queueHeadAfterWrite;
+
+/*
+ * List of channels with pending notifications in the current transaction.
+ */
+static List *pendingNotifyChannels = NIL;
+
/* have we advanced to a page that's a multiple of QUEUE_CLEANUP_DELAY? */
static bool tryAdvanceTail = false;
@@ -457,6 +518,9 @@ static void AddEventToPendingNotifies(Notification *n);
static uint32 notification_hash(const void *key, Size keysize);
static int notification_match(const void *key1, const void *key2, Size keysize);
static void ClearPendingActionsAndNotifies(void);
+static inline void ChannelHashPrepareKey(ChannelHashKey *key, Oid dboid, const char *channel);
+static dshash_hash channelHashFunc(const void *key, size_t size, void *arg);
+static void initChannelHash(void);
/*
* Compute the difference between two queue page numbers.
@@ -478,6 +542,80 @@ asyncQueuePagePrecedes(int64 p, int64 q)
return p < q;
}
+/*
+ * channelHashFunc
+ * Hash function for channel keys.
+ */
+static dshash_hash
+channelHashFunc(const void *key, size_t size, void *arg)
+{
+ const ChannelHashKey *k = (const ChannelHashKey *) key;
+ dshash_hash h;
+
+ h = DatumGetUInt32(hash_uint32(k->dboid));
+ h ^= DatumGetUInt32(hash_any((const unsigned char *) k->channel,
+ strnlen(k->channel, NAMEDATALEN)));
+
+ return h;
+}
+
+/* parameters for the channel hash table */
+static const dshash_parameters channelDSHParams = {
+ sizeof(ChannelHashKey),
+ sizeof(ChannelEntry),
+ dshash_memcmp,
+ channelHashFunc,
+ dshash_memcpy,
+ LWTRANCHE_NOTIFY_CHANNEL_HASH
+};
+
+/*
+ * initChannelHash
+ * Lazy initialization of the channel hash table.
+ */
+static void
+initChannelHash(void)
+{
+ MemoryContext oldcontext;
+
+ /* Quick exit if we already did this */
+ if (asyncQueueControl->channelHashDSH != DSHASH_HANDLE_INVALID &&
+ channelHash != NULL)
+ return;
+
+ /* Otherwise, use a lock to ensure only one process creates the table */
+ LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
+
+ /* Be sure any local memory allocated by DSA routines is persistent */
+ oldcontext = MemoryContextSwitchTo(TopMemoryContext);
+
+ if (asyncQueueControl->channelHashDSH == DSHASH_HANDLE_INVALID)
+ {
+ /* Initialize dynamic shared hash table for channel hash */
+ channelDSA = dsa_create(LWTRANCHE_NOTIFY_CHANNEL_HASH);
+ dsa_pin(channelDSA);
+ dsa_pin_mapping(channelDSA);
+ channelHash = dshash_create(channelDSA, &channelDSHParams, NULL);
+
+ /* Store handles in shared memory for other backends to use */
+ asyncQueueControl->channelHashDSA = dsa_get_handle(channelDSA);
+ asyncQueueControl->channelHashDSH =
+ dshash_get_hash_table_handle(channelHash);
+ }
+ else if (!channelHash)
+ {
+ /* Attach to existing dynamic shared hash table */
+ channelDSA = dsa_attach(asyncQueueControl->channelHashDSA);
+ dsa_pin_mapping(channelDSA);
+ channelHash = dshash_attach(channelDSA, &channelDSHParams,
+ asyncQueueControl->channelHashDSH,
+ NULL);
+ }
+
+ MemoryContextSwitchTo(oldcontext);
+ LWLockRelease(NotifyQueueLock);
+}
+
/*
* Report space needed for our shared memory area
*/
@@ -521,12 +659,16 @@ AsyncShmemInit(void)
QUEUE_STOP_PAGE = 0;
QUEUE_FIRST_LISTENER = INVALID_PROC_NUMBER;
asyncQueueControl->lastQueueFillWarn = 0;
+ asyncQueueControl->channelHashDSA = DSA_HANDLE_INVALID;
+ asyncQueueControl->channelHashDSH = DSHASH_HANDLE_INVALID;
+
for (int i = 0; i < MaxBackends; i++)
{
QUEUE_BACKEND_PID(i) = InvalidPid;
QUEUE_BACKEND_DBOID(i) = InvalidOid;
QUEUE_NEXT_LISTENER(i) = INVALID_PROC_NUMBER;
SET_QUEUE_POS(QUEUE_BACKEND_POS(i), 0, 0);
+ QUEUE_BACKEND_WAKEUP_PENDING(i) = false;
}
}
@@ -894,6 +1036,7 @@ PreCommit_Notify(void)
if (pendingNotifies)
{
ListCell *nextNotify;
+ bool firstIteration = true;
/*
* Make sure that we have an XID assigned to the current transaction.
@@ -922,6 +1065,35 @@ PreCommit_Notify(void)
LockSharedObject(DatabaseRelationId, InvalidOid, 0,
AccessExclusiveLock);
+ /*
+ * Build list of unique channels for SignalBackends().
+ */
+ pendingNotifyChannels = NIL;
+ foreach_ptr(Notification, n, pendingNotifies->events)
+ {
+ char *channel = n->data;
+
+ /* Add if not already in list */
+ if (!list_member_ptr(pendingNotifyChannels, channel))
+ pendingNotifyChannels = lappend(pendingNotifyChannels, channel);
+ }
+
+ /*
+ * For the direct advancement optimization in SignalBackends(), we
+ * need to ensure that no other backend can insert queue entries
+ * between queueHeadBeforeWrite and queueHeadAfterWrite. The
+ * heavyweight lock above provides this guarantee, since it serializes
+ * all writers.
+ *
+ * Note: if the heavyweight lock were ever removed for scalability
+ * reasons, we could achieve the same guarantee by holding
+ * NotifyQueueLock in EXCLUSIVE mode across all our insertions, rather
+ * than releasing and reacquiring it for each page as we do below.
+ */
+
+ /* Initialize queueHeadBeforeWrite to a safe default */
+ SET_QUEUE_POS(queueHeadBeforeWrite, 0, 0);
+
/* Now push the notifications into the queue */
nextNotify = list_head(pendingNotifies->events);
while (nextNotify != NULL)
@@ -939,12 +1111,33 @@ PreCommit_Notify(void)
* point in time we can still roll the transaction back.
*/
LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
+
+ /*
+ * On the first iteration, save the queue head position before we
+ * write any notifications. This is used by SignalBackends() to
+ * identify backends that can be advanced directly without waking
+ * them up.
+ */
+ if (firstIteration)
+ {
+ queueHeadBeforeWrite = QUEUE_HEAD;
+ firstIteration = false;
+ }
+
asyncQueueFillWarning();
if (asyncQueueIsFull())
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("too many notifications in the NOTIFY queue")));
nextNotify = asyncQueueAddEntries(nextNotify);
+
+ /*
+ * Capture the queue head after each batch of entries. On the
+ * last iteration, this gives us the final queue head position for
+ * SignalBackends() to use when advancing idle backends.
+ */
+ queueHeadAfterWrite = QUEUE_HEAD;
+
LWLockRelease(NotifyQueueLock);
}
@@ -1135,6 +1328,10 @@ Exec_ListenPreCommit(void)
static void
Exec_ListenCommit(const char *channel)
{
+ ChannelHashKey key;
+ ChannelEntry *entry;
+ bool found;
+ ProcNumber *listeners;
MemoryContext oldcontext;
/* Do nothing if we are already listening on this channel */
@@ -1152,21 +1349,84 @@ Exec_ListenCommit(const char *channel)
oldcontext = MemoryContextSwitchTo(TopMemoryContext);
listenChannels = lappend(listenChannels, pstrdup(channel));
MemoryContextSwitchTo(oldcontext);
+
+ /* Now update the shared channelHash for SignalBackends() to use */
+ initChannelHash();
+
+ ChannelHashPrepareKey(&key, MyDatabaseId, channel);
+
+ /*
+ * For new entries, we initialize listenersArray to InvalidDsaPointer as a
+ * marker. This handles both the initial creation and potential retry
+ * after OOM.
+ */
+ entry = dshash_find_or_insert(channelHash, &key, &found);
+
+ if (!found)
+ entry->listenersArray = InvalidDsaPointer;
+
+ if (!DsaPointerIsValid(entry->listenersArray))
+ {
+ /* First listener for this channel */
+ entry->listenersArray = dsa_allocate(channelDSA,
+ sizeof(ProcNumber) * INITIAL_LISTENERS_ARRAY_SIZE);
+ entry->numListeners = 0;
+ entry->allocatedListeners = INITIAL_LISTENERS_ARRAY_SIZE;
+ }
+
+ listeners = (ProcNumber *) dsa_get_address(channelDSA,
+ entry->listenersArray);
+
+ for (int i = 0; i < entry->numListeners; i++)
+ {
+ if (listeners[i] == MyProcNumber)
+ {
+ dshash_release_lock(channelHash, entry);
+ return; /* Already registered */
+ }
+ }
+
+ if (entry->numListeners >= entry->allocatedListeners)
+ {
+ int new_size = entry->allocatedListeners * 2;
+ dsa_pointer new_array = dsa_allocate(channelDSA,
+ sizeof(ProcNumber) * new_size);
+ ProcNumber *new_listeners = (ProcNumber *) dsa_get_address(channelDSA,
+ new_array);
+
+ memcpy(new_listeners, listeners,
+ sizeof(ProcNumber) * entry->numListeners);
+
+ dsa_free(channelDSA, entry->listenersArray);
+ entry->listenersArray = new_array;
+ entry->allocatedListeners = new_size;
+ listeners = new_listeners;
+ }
+
+ listeners[entry->numListeners] = MyProcNumber;
+ entry->numListeners++;
+
+ dshash_release_lock(channelHash, entry);
}
/*
* Exec_UnlistenCommit --- subroutine for AtCommit_Notify
*
- * Remove the specified channel name from listenChannels.
+ * Remove the specified channel from the list of channels we are listening on.
*/
static void
Exec_UnlistenCommit(const char *channel)
{
+ ChannelHashKey key;
+ ChannelEntry *entry;
+ ProcNumber *listeners;
ListCell *q;
+ int i;
if (Trace_notify)
elog(DEBUG1, "Exec_UnlistenCommit(%s,%d)", channel, MyProcPid);
+ /* Remove from our local cache */
foreach(q, listenChannels)
{
char *lchan = (char *) lfirst(q);
@@ -1179,6 +1439,46 @@ Exec_UnlistenCommit(const char *channel)
}
}
+ /* Now remove from the shared channelHash */
+ if (channelHash == NULL)
+ return;
+
+ ChannelHashPrepareKey(&key, MyDatabaseId, channel);
+
+ /* Look up the channel with exclusive lock so we can modify it */
+ entry = dshash_find(channelHash, &key, true);
+ if (entry == NULL)
+ return;
+
+ listeners = (ProcNumber *) dsa_get_address(channelDSA,
+ entry->listenersArray);
+
+ for (i = 0; i < entry->numListeners; i++)
+ {
+ if (listeners[i] == MyProcNumber)
+ {
+ entry->numListeners--;
+ if (i < entry->numListeners)
+ memmove(&listeners[i], &listeners[i + 1],
+ sizeof(ProcNumber) * (entry->numListeners - i));
+
+ if (entry->numListeners == 0)
+ {
+ /* Last listener for this channel */
+ dsa_free(channelDSA, entry->listenersArray);
+ dshash_delete_entry(channelHash, entry);
+ }
+ else
+ {
+ dshash_release_lock(channelHash, entry);
+ }
+
+ return;
+ }
+ }
+
+ dshash_release_lock(channelHash, entry);
+
/*
* We do not complain about unlistening something not being listened;
* should we?
@@ -1193,11 +1493,51 @@ Exec_UnlistenCommit(const char *channel)
static void
Exec_UnlistenAllCommit(void)
{
+ dshash_seq_status status;
+ ChannelEntry *entry;
+
if (Trace_notify)
elog(DEBUG1, "Exec_UnlistenAllCommit(%d)", MyProcPid);
+ /* Clear our local cache */
list_free_deep(listenChannels);
listenChannels = NIL;
+
+ /* Now clear from shared channelHash */
+ if (channelHash == NULL)
+ return;
+
+ dshash_seq_init(&status, channelHash, true);
+ while ((entry = dshash_seq_next(&status)) != NULL)
+ {
+ if (entry->key.dboid == MyDatabaseId)
+ {
+ ProcNumber *listeners;
+ int i;
+
+ listeners = (ProcNumber *) dsa_get_address(channelDSA,
+ entry->listenersArray);
+
+ for (i = 0; i < entry->numListeners; i++)
+ {
+ if (listeners[i] == MyProcNumber)
+ {
+ entry->numListeners--;
+ if (i < entry->numListeners)
+ memmove(&listeners[i], &listeners[i + 1],
+ sizeof(ProcNumber) * (entry->numListeners - i));
+
+ if (entry->numListeners == 0)
+ {
+ dsa_free(channelDSA, entry->listenersArray);
+ dshash_delete_current(&status);
+ }
+ break;
+ }
+ }
+ }
+ }
+ dshash_seq_term(&status);
}
/*
@@ -1565,12 +1905,19 @@ asyncQueueFillWarning(void)
/*
* Send signals to listening backends.
*
- * Normally we signal only backends in our own database, since only those
- * backends could be interested in notifies we send. However, if there's
- * notify traffic in our database but no traffic in another database that
- * does have listener(s), those listeners will fall further and further
- * behind. Waken them anyway if they're far enough behind, so that they'll
- * advance their queue position pointers, allowing the global tail to advance.
+ * Normally we signal only backends in our own database, that are
+ * listening on the channels with pending notifies, since only those
+ * backends are interested in notifies we send.
+ *
+ * Backends that are still positioned at the queue head from before our
+ * commit can be safely advanced directly to the new head, since the
+ * queue region we wrote is known to contain only our own notifications.
+ * This avoids unnecessary wakeups when there is nothing of interest to
+ * them.
+ *
+ * In addition, if a backend has fallen too far behind in the queue, we
+ * signal it so that it will advance its position and allow the global
+ * tail pointer to move forward.
*
* Since we know the ProcNumber and the Pid the signaling is quite cheap.
*
@@ -1583,6 +1930,7 @@ SignalBackends(void)
int32 *pids;
ProcNumber *procnos;
int count;
+ ListCell *lc;
/*
* Identify backends that we need to signal. We don't want to send
@@ -1597,36 +1945,111 @@ SignalBackends(void)
count = 0;
LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
- for (ProcNumber i = QUEUE_FIRST_LISTENER; i != INVALID_PROC_NUMBER; i = QUEUE_NEXT_LISTENER(i))
+ foreach(lc, pendingNotifyChannels)
+ {
+ char *channel = (char *) lfirst(lc);
+ ChannelEntry *entry = NULL;
+ ProcNumber *listeners;
+
+ if (channelHash != NULL)
+ {
+ ChannelHashKey key;
+
+ ChannelHashPrepareKey(&key, MyDatabaseId, channel);
+ entry = dshash_find(channelHash, &key, false);
+ }
+
+ if (entry == NULL)
+ continue; /* No listeners registered for this channel */
+
+ listeners = (ProcNumber *) dsa_get_address(channelDSA,
+ entry->listenersArray);
+
+ for (int j = 0; j < entry->numListeners; j++)
+ {
+ ProcNumber i = listeners[j];
+ int32 pid;
+ QueuePosition pos;
+
+ if (QUEUE_BACKEND_WAKEUP_PENDING(i))
+ continue;
+
+ pos = QUEUE_BACKEND_POS(i);
+ pid = QUEUE_BACKEND_PID(i);
+
+ /* Skip if caught up */
+ if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
+ continue;
+
+ Assert(pid != InvalidPid);
+
+ QUEUE_BACKEND_WAKEUP_PENDING(i) = true;
+ pids[count] = pid;
+ procnos[count] = i;
+ count++;
+ }
+
+ dshash_release_lock(channelHash, entry);
+ }
+
+ /*
+ * Direct advancement and lagging backend detection.
+ *
+ * Direct advancement: avoid waking backends still positioned at the old
+ * queue head that aren't interested in our notifications.
+ *
+ * The heavyweight lock on database 0 (held in PreCommit_Notify) ensures
+ * no other backend can insert notifications in the region we just wrote.
+ * Even though we may take and release NotifyQueueLock multiple times
+ * while writing, the heavyweight lock guarantees this region contains
+ * only our messages. Therefore, any backend still positioned at the
+ * queue head from before our write can be safely advanced to the current
+ * queue head without waking it.
+ *
+ * False-positive possibility: if a backend was previously signaled but
+ * hasn't yet awoken, we'll skip advancing it (because wakeupPending is
+ * true). This is safe - the backend will advance its pointer when it
+ * does wake up. The alternative (advancing it anyway) would risk
+ * advancing over notifications from whoever signaled it.
+ *
+ * Lagging backends: we also check if any backend has fallen too far
+ * behind and signal it to catch up, allowing the global tail to advance.
+ */
+ for (ProcNumber i = QUEUE_FIRST_LISTENER;
+ i != INVALID_PROC_NUMBER;
+ i = QUEUE_NEXT_LISTENER(i))
{
- int32 pid = QUEUE_BACKEND_PID(i);
QueuePosition pos;
+ int64 lag;
+ int32 pid;
+
+ if (QUEUE_BACKEND_WAKEUP_PENDING(i))
+ continue;
- Assert(pid != InvalidPid);
pos = QUEUE_BACKEND_POS(i);
- if (QUEUE_BACKEND_DBOID(i) == MyDatabaseId)
+
+ /* Direct advancement for idle backends at the old head */
+ if (pendingNotifies != NULL &&
+ QUEUE_POS_EQUAL(pos, queueHeadBeforeWrite))
{
- /*
- * Always signal listeners in our own database, unless they're
- * already caught up (unlikely, but possible).
- */
- if (QUEUE_POS_EQUAL(pos, QUEUE_HEAD))
- continue;
+ QUEUE_BACKEND_POS(i) = queueHeadAfterWrite;
+ pos = queueHeadAfterWrite;
}
- else
+
+ /* Signal backends that have fallen too far behind */
+ lag = asyncQueuePageDiff(QUEUE_POS_PAGE(QUEUE_HEAD),
+ QUEUE_POS_PAGE(pos));
+
+ if (lag >= QUEUE_CLEANUP_DELAY)
{
- /*
- * Listeners in other databases should be signaled only if they
- * are far behind.
- */
- if (asyncQueuePageDiff(QUEUE_POS_PAGE(QUEUE_HEAD),
- QUEUE_POS_PAGE(pos)) < QUEUE_CLEANUP_DELAY)
- continue;
+ pid = QUEUE_BACKEND_PID(i);
+ Assert(pid != InvalidPid);
+
+ QUEUE_BACKEND_WAKEUP_PENDING(i) = true;
+ pids[count] = pid;
+ procnos[count] = i;
+ count++;
}
- /* OK, need to signal this one */
- pids[count] = pid;
- procnos[count] = i;
- count++;
}
LWLockRelease(NotifyQueueLock);
@@ -1865,6 +2288,7 @@ asyncQueueReadAllNotifications(void)
LWLockAcquire(NotifyQueueLock, LW_SHARED);
/* Assert checks that we have a valid state entry */
Assert(MyProcPid == QUEUE_BACKEND_PID(MyProcNumber));
+ QUEUE_BACKEND_WAKEUP_PENDING(MyProcNumber) = false;
pos = QUEUE_BACKEND_POS(MyProcNumber);
head = QUEUE_HEAD;
LWLockRelease(NotifyQueueLock);
@@ -2373,7 +2797,7 @@ notification_match(const void *key1, const void *key2, Size keysize)
return 1; /* not equal */
}
-/* Clear the pendingActions and pendingNotifies lists. */
+/* Clear the pendingActions, pendingNotifies, and pendingNotifyChannels lists. */
static void
ClearPendingActionsAndNotifies(void)
{
@@ -2385,6 +2809,7 @@ ClearPendingActionsAndNotifies(void)
*/
pendingActions = NULL;
pendingNotifies = NULL;
+ pendingNotifyChannels = NIL;
}
/*
@@ -2395,3 +2820,16 @@ check_notify_buffers(int *newval, void **extra, GucSource source)
{
return check_slru_buffers("notify_buffers", newval);
}
+
+
+/*
+ * ChannelHashPrepareKey
+ * Prepare a channel key for use as a hash key.
+ */
+static inline void
+ChannelHashPrepareKey(ChannelHashKey *key, Oid dboid, const char *channel)
+{
+ memset(key, 0, sizeof(ChannelHashKey));
+ key->dboid = dboid;
+ strlcpy(key->channel, channel, NAMEDATALEN);
+}
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index 7553f6eacef..a4fadbd0767 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -366,6 +366,7 @@ SubtransBuffer "Waiting for I/O on a sub-transaction SLRU buffer."
MultiXactOffsetBuffer "Waiting for I/O on a multixact offset SLRU buffer."
MultiXactMemberBuffer "Waiting for I/O on a multixact member SLRU buffer."
NotifyBuffer "Waiting for I/O on a <command>NOTIFY</command> message SLRU buffer."
+NotifyChannelHash "Waiting to access the <command>NOTIFY</command> channel hash table."
SerialBuffer "Waiting for I/O on a serializable transaction conflict SLRU buffer."
WALInsert "Waiting to insert WAL data into a memory buffer."
BufferContent "Waiting to access a data page in memory."
diff --git a/src/include/storage/lwlocklist.h b/src/include/storage/lwlocklist.h
index 06a1ffd4b08..2768ddf4414 100644
--- a/src/include/storage/lwlocklist.h
+++ b/src/include/storage/lwlocklist.h
@@ -100,6 +100,7 @@ PG_LWLOCKTRANCHE(SUBTRANS_BUFFER, SubtransBuffer)
PG_LWLOCKTRANCHE(MULTIXACTOFFSET_BUFFER, MultiXactOffsetBuffer)
PG_LWLOCKTRANCHE(MULTIXACTMEMBER_BUFFER, MultiXactMemberBuffer)
PG_LWLOCKTRANCHE(NOTIFY_BUFFER, NotifyBuffer)
+PG_LWLOCKTRANCHE(NOTIFY_CHANNEL_HASH, NotifyChannelHash)
PG_LWLOCKTRANCHE(SERIAL_BUFFER, SerialBuffer)
PG_LWLOCKTRANCHE(WAL_INSERT, WALInsert)
PG_LWLOCKTRANCHE(BUFFER_CONTENT, BufferContent)
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 5290b91e83e..5ccdd4043e8 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -412,6 +412,8 @@ CatalogIdMapEntry
CatalogIndexState
ChangeVarNodes_callback
ChangeVarNodes_context
+ChannelEntry
+ChannelHashKey
CheckPoint
CheckPointStmt
CheckpointStatsData
--
2.50.1
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Subject: Re: Optimize LISTEN/NOTIFY
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