Optimize client memory usage tracking operation while client eviction is disabled (#11348)

## Issue
During the client input/output buffer processing, the memory usage is
incrementally updated to keep track of clients going beyond a certain
threshold `maxmemory-clients` to be evicted. However, this additional
tracking activity leads to unnecessary CPU cycles wasted when no
client-eviction is required. It is applicable in two cases.

* `maxmemory-clients` is set to `0` which equates to no client eviction
  (applicable to all clients)
* `CLIENT NO-EVICT` flag is set to `ON` which equates to a particular
  client not applicable for eviction.  

## Solution
* Disable client memory usage tracking during the read/write flow when
  `maxmemory-clients` is set to `0` or `client no-evict` is `on`.
  The memory usage is tracked only during the `clientCron` i.e. it gets
  periodically updated.
* Cleanup the clients from the memory usage bucket when client eviction
  is disabled.
* When the maxmemory-clients config is enabled or disabled at runtime,
  we immediately update the memory usage buckets for all clients (tested
  scanning 80000 took some 20ms)

Benchmark shown that this can improve performance by about 5% in
certain situations.

Co-authored-by: Oran Agra <oran@redislabs.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/server.h b/src/server.h
index 1ce5ff781..7e383c59b 100644
--- a/src/server.h
+++ b/src/server.h
@@ -1197,7 +1197,7 @@ typedef struct client {
     rax *client_tracking_prefixes; /* A dictionary of prefixes we are already
                                       subscribed to in BCAST mode, in the
                                       context of client side caching. */
-    /* In updateClientMemUsage() we track the memory usage of
+    /* In updateClientMemoryUsage() we track the memory usage of
      * each client and add it to the sum of all the clients of a given type,
      * however we need to remember what was the old contribution of each
      * client, and in which category the client was, in order to remove it
@@ -1551,7 +1551,7 @@ struct redisServer {
     client *current_client;     /* Current client executing the command. */
 
     /* Stuff for client mem eviction */
-    clientMemUsageBucket client_mem_usage_buckets[CLIENT_MEM_USAGE_BUCKETS];
+    clientMemUsageBucket* client_mem_usage_buckets;
 
     rax *clients_timeout_table; /* Radix tree for blocked clients timeouts. */
     int in_nested_call;         /* If > 0, in a nested call of a call */
@@ -2577,8 +2577,8 @@ int handleClientsWithPendingReadsUsingThreads(void);
 int stopThreadedIOIfNeeded(void);
 int clientHasPendingReplies(client *c);
 int islocalClient(client *c);
-int updateClientMemUsage(client *c);
-void updateClientMemUsageBucket(client *c);
+int updateClientMemUsageAndBucket(client *c);
+void removeClientFromMemUsageBucket(client *c, int allow_eviction);
 void unlinkClient(client *c);
 int writeToClient(client *c, int handler_installed);
 void linkClient(client *c);
@@ -3117,6 +3117,8 @@ void initConfigValues();
 void removeConfig(sds name);
 sds getConfigDebugInfo();
 int allowProtectedAction(int config, client *c);
+void initServerClientMemUsageBuckets();
+void freeServerClientMemUsageBuckets();
 
 /* Module Configuration */
 typedef struct ModuleConfig ModuleConfig;