make async rehash configurable

Author: JohnSully

src/config.cpp

@@ -2982,6 +2982,7 @@ standardConfig configs[] = {
     createSDSConfig("availability-zone", NULL, MODIFIABLE_CONFIG, 0, g_pserver->sdsAvailabilityZone, "", NULL, NULL),
     createIntConfig("overload-protect-percent", NULL, MODIFIABLE_CONFIG, 0, 200, g_pserver->overload_protect_threshold, 0, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("force-eviction-percent", NULL, MODIFIABLE_CONFIG, 0, 100, g_pserver->force_eviction_percent, 0, INTEGER_CONFIG, NULL, NULL),
+    createBoolConfig("enable-async-rehash", NULL, MODIFIABLE_CONFIG, g_pserver->enable_async_rehash, 1, NULL, NULL),
 
 #ifdef USE_OPENSSL
     createIntConfig("tls-port", NULL, MODIFIABLE_CONFIG, 0, 65535, g_pserver->tls_port, 0, INTEGER_CONFIG, NULL, updateTLSPort), /* TCP port. */

src/server.cpp

@@ -2137,7 +2137,7 @@ void databasesCron(bool fMainThread) {
                 ::dict *dict = g_pserver->db[rehash_db]->dictUnsafeKeyOnly();
                 /* Are we async rehashing? And if so is it time to re-calibrate? */
                 /* The recalibration limit is a prime number to ensure balancing across threads */
-                if (rehashes_per_ms > 0 && async_rehashes < 131 && !cserver.active_defrag_enabled && cserver.cthreads > 1 && dictSize(dict) > 2048 && dictIsRehashing(dict) && !g_pserver->loading && aeLockContention() > 1) {
+                if (g_pserver->enable_async_rehash && rehashes_per_ms > 0 && async_rehashes < 131 && !cserver.active_defrag_enabled && cserver.cthreads > 1 && dictSize(dict) > 2048 && dictIsRehashing(dict) && !g_pserver->loading && aeLockContention() > 1) {
                     serverTL->rehashCtl = dictRehashAsyncStart(dict, rehashes_per_ms * ((1000 / g_pserver->hz) / 10));  // Estimate 10% CPU time spent in lock contention
                     if (serverTL->rehashCtl)
                         ++async_rehashes;

src/server.h

@@ -2306,6 +2306,7 @@ struct redisServer {
     rax *errors;                /* Errors table */
     int activerehashing;        /* Incremental rehash in serverCron() */
     int active_defrag_running;  /* Active defragmentation running (holds current scan aggressiveness) */
+    int enable_async_rehash = 1;    /* Should we use the async rehash feature? */
     int cronloops;              /* Number of times the cron function run */
     char runid[CONFIG_RUN_ID_SIZE+1];  /* ID always different at every exec. */
     int sentinel_mode;          /* True if this instance is a Sentinel. */