Optimize linear search of WAIT and WAITAOF when unblocking the client (…

…#787)

Currently, if the client enters a blocked state, it will be
added to the server.clients_waiting_acks list. When the client
is unblocked, that is, when unblockClient is called, we will
need to linearly traverse server.clients_waiting_acks to delete
the client, and this search is O(N).

When WAIT (or WAITAOF) is used extensively in some cases, this
O(N) search may be time-consuming. We can remember the list node
and store it in the blockingState struct and it can avoid the
linear search in unblockClientWaitingReplicas.

Signed-off-by: Binbin <binloveplay1314@qq.com>

src/blocked.c

@@ -76,10 +76,13 @@ static void releaseBlockedEntry(client *c, dictEntry *de, int remove_key);
 void initClientBlockingState(client *c) {
     c->bstate.btype = BLOCKED_NONE;
     c->bstate.timeout = 0;
+    c->bstate.unblock_on_nokey = 0;
     c->bstate.keys = dictCreate(&objectKeyHeapPointerValueDictType);
     c->bstate.numreplicas = 0;
+    c->bstate.numlocal = 0;
     c->bstate.reploffset = 0;
-    c->bstate.unblock_on_nokey = 0;
+    c->bstate.client_waiting_acks_list_node = NULL;
+    c->bstate.module_blocked_handle = NULL;
     c->bstate.async_rm_call_handle = NULL;
 }
 
@@ -595,6 +598,11 @@ void blockClientForReplicaAck(client *c, mstime_t timeout, long long offset, lon
     c->bstate.numreplicas = numreplicas;
     c->bstate.numlocal = numlocal;
     listAddNodeHead(server.clients_waiting_acks, c);
+    /* Note that we remember the linked list node where the client is stored,
+     * this way removing the client in unblockClientWaitingReplicas() will not
+     * require a linear scan, but just a constant time operation. */
+    serverAssert(c->bstate.client_waiting_acks_list_node == NULL);
+    c->bstate.client_waiting_acks_list_node = listFirst(server.clients_waiting_acks);
     blockClient(c, BLOCKED_WAIT);
 }
 

src/replication.c

@@ -4358,9 +4358,9 @@ void waitaofCommand(client *c) {
  * waiting for replica acks. Never call it directly, call unblockClient()
  * instead. */
 void unblockClientWaitingReplicas(client *c) {
-    listNode *ln = listSearchKey(server.clients_waiting_acks, c);
-    serverAssert(ln != NULL);
-    listDelNode(server.clients_waiting_acks, ln);
+    serverAssert(c->bstate.client_waiting_acks_list_node);
+    listDelNode(server.clients_waiting_acks, c->bstate.client_waiting_acks_list_node);
+    c->bstate.client_waiting_acks_list_node = NULL;
     updateStatsOnUnblock(c, 0, 0, 0);
 }
 

src/server.h

@@ -1002,21 +1002,23 @@ typedef struct multiState {
 } multiState;
 
 /* This structure holds the blocking operation state for a client.
- * The fields used depend on client->btype. */
+ * The fields used depend on client->bstate.btype. */
 typedef struct blockingState {
     /* Generic fields. */
     blocking_type btype;  /* Type of blocking op if CLIENT_BLOCKED. */
     mstime_t timeout;     /* Blocking operation timeout. If UNIX current time
                            * is > timeout then the operation timed out. */
     int unblock_on_nokey; /* Whether to unblock the client when at least one of the keys
                              is deleted or does not exist anymore */
+
     /* BLOCKED_LIST, BLOCKED_ZSET and BLOCKED_STREAM or any other Keys related blocking */
     dict *keys; /* The keys we are blocked on */
 
     /* BLOCKED_WAIT and BLOCKED_WAITAOF */
-    int numreplicas;      /* Number of replicas we are waiting for ACK. */
-    int numlocal;         /* Indication if WAITAOF is waiting for local fsync. */
-    long long reploffset; /* Replication offset to reach. */
+    int numreplicas;                         /* Number of replicas we are waiting for ACK. */
+    int numlocal;                            /* Indication if WAITAOF is waiting for local fsync. */
+    long long reploffset;                    /* Replication offset to reach. */
+    listNode *client_waiting_acks_list_node; /* list node in server.clients_waiting_acks list. */
 
     /* BLOCKED_MODULE */
     void *module_blocked_handle; /* ValkeyModuleBlockedClient structure.