Incremental eviction processing (redis#7653)

Rather than blindly evicting until maxmemory limit is achieved, this
update adds a time limit to eviction.  While over the maxmemory limit,
eviction will process before each command AND as a timeProc when no
commands are running.

This will reduce the latency impact on many cases, especially pathological
cases like massive used memory increase during dict rehashing.

There is a risk that some other edge cases (like massive pipelined use
of MGET) could cause Redis memory usage to keep growing despite the
eviction attempts, so a new maxmemory-eviction-tenacity config is
introduced to let users mitigate that.

README.md

@@ -354,7 +354,7 @@ Inside server.c you can find code that handles other vital things of the Redis s
 
 * `call()` is used in order to call a given command in the context of a given client.
 * `activeExpireCycle()` handles eviction of keys with a time to live set via the `EXPIRE` command.
-* `freeMemoryIfNeeded()` is called when a new write command should be performed but Redis is out of memory according to the `maxmemory` directive.
+* `performEvictions()` is called when a new write command should be performed but Redis is out of memory according to the `maxmemory` directive.
 * The global variable `redisCommandTable` defines all the Redis commands, specifying the name of the command, the function implementing the command, the number of arguments required, and other properties of each command.
 
 networking.c

redis.conf

@@ -912,6 +912,14 @@ acllog-max-len 128
 #
 # maxmemory-samples 5
 
+# Eviction processing is designed to function well with the default setting.
+# If there is an unusually large amount of write traffic, this value may need to
+# be increased.  Decreasing this value may reduce latency at the risk of 
+# eviction processing effectiveness
+#   0 = minimum latency, 10 = default, 100 = process without regard to latency
+#
+# maxmemory-eviction-tenacity 10
+
 # Starting from Redis 5, by default a replica will ignore its maxmemory setting
 # (unless it is promoted to master after a failover or manually). It means
 # that the eviction of keys will be just handled by the master, sending the

src/config.c

@@ -2139,7 +2139,7 @@ static int updateMaxmemory(long long val, long long prev, char **err) {
         if ((unsigned long long)val < used) {
             serverLog(LL_WARNING,"WARNING: the new maxmemory value set via CONFIG SET (%llu) is smaller than the current memory usage (%zu). This will result in key eviction and/or the inability to accept new write commands depending on the maxmemory-policy.", server.maxmemory, used);
         }
-        freeMemoryIfNeededAndSafe();
+        performEvictions();
     }
     return 1;
 }
@@ -2327,6 +2327,7 @@ standardConfig configs[] = {
     createIntConfig("replica-priority", "slave-priority", MODIFIABLE_CONFIG, 0, INT_MAX, server.slave_priority, 100, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("repl-diskless-sync-delay", NULL, MODIFIABLE_CONFIG, 0, INT_MAX, server.repl_diskless_sync_delay, 5, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("maxmemory-samples", NULL, MODIFIABLE_CONFIG, 1, INT_MAX, server.maxmemory_samples, 5, INTEGER_CONFIG, NULL, NULL),
+    createIntConfig("maxmemory-eviction-tenacity", NULL, MODIFIABLE_CONFIG, 0, 100, server.maxmemory_eviction_tenacity, 10, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("timeout", NULL, MODIFIABLE_CONFIG, 0, INT_MAX, server.maxidletime, 0, INTEGER_CONFIG, NULL, NULL), /* Default client timeout: infinite */
     createIntConfig("replica-announce-port", "slave-announce-port", MODIFIABLE_CONFIG, 0, 65535, server.slave_announce_port, 0, INTEGER_CONFIG, NULL, NULL),
     createIntConfig("tcp-backlog", NULL, IMMUTABLE_CONFIG, 0, INT_MAX, server.tcp_backlog, 511, INTEGER_CONFIG, NULL, NULL), /* TCP listen backlog. */

src/evict.c

@@ -33,13 +33,14 @@
 #include "server.h"
 #include "bio.h"
 #include "atomicvar.h"
+#include <math.h>
 
 /* ----------------------------------------------------------------------------
  * Data structures
  * --------------------------------------------------------------------------*/
 
 /* To improve the quality of the LRU approximation we take a set of keys
- * that are good candidate for eviction across freeMemoryIfNeeded() calls.
+ * that are good candidate for eviction across performEvictions() calls.
  *
  * Entries inside the eviction pool are taken ordered by idle time, putting
  * greater idle times to the right (ascending order).
@@ -97,25 +98,7 @@ unsigned long long estimateObjectIdleTime(robj *o) {
     }
 }
 
-/* freeMemoryIfNeeded() gets called when 'maxmemory' is set on the config
- * file to limit the max memory used by the server, before processing a
- * command.
- *
- * The goal of the function is to free enough memory to keep Redis under the
- * configured memory limit.
- *
- * The function starts calculating how many bytes should be freed to keep
- * Redis under the limit, and enters a loop selecting the best keys to
- * evict accordingly to the configured policy.
- *
- * If all the bytes needed to return back under the limit were freed the
- * function returns C_OK, otherwise C_ERR is returned, and the caller
- * should block the execution of commands that will result in more memory
- * used by the server.
- *
- * ------------------------------------------------------------------------
- *
- * LRU approximation algorithm
+/* LRU approximation algorithm
  *
  * Redis uses an approximation of the LRU algorithm that runs in constant
  * memory. Every time there is a key to expire, we sample N keys (with
@@ -150,7 +133,7 @@ void evictionPoolAlloc(void) {
     EvictionPoolLRU = ep;
 }
 
-/* This is an helper function for freeMemoryIfNeeded(), it is used in order
+/* This is an helper function for performEvictions(), it is used in order
  * to populate the evictionPool with a few entries every time we want to
  * expire a key. Keys with idle time smaller than one of the current
  * keys are added. Keys are always added if there are free entries.
@@ -341,11 +324,6 @@ unsigned long LFUDecrAndReturn(robj *o) {
     return counter;
 }
 
-/* ----------------------------------------------------------------------------
- * The external API for eviction: freeMemoryIfNeeded() is called by the
- * server when there is data to add in order to make space if needed.
- * --------------------------------------------------------------------------*/
-
 /* We don't want to count AOF buffers and slaves output buffers as
  * used memory: the eviction should use mostly data size. This function
  * returns the sum of AOF and slaves buffer. */
@@ -434,39 +412,113 @@ int getMaxmemoryState(size_t *total, size_t *logical, size_t *tofree, float *lev
     return C_ERR;
 }
 
-/* This function is periodically called to see if there is memory to free
- * according to the current "maxmemory" settings. In case we are over the
- * memory limit, the function will try to free some memory to return back
- * under the limit.
- *
- * The function returns C_OK if we are under the memory limit or if we
- * were over the limit, but the attempt to free memory was successful.
- * Otherwise if we are over the memory limit, but not enough memory
- * was freed to return back under the limit, the function returns C_ERR. */
-int freeMemoryIfNeeded(void) {
-    int keys_freed = 0;
+
+/* The evictionTimeProc is started when "maxmemory" has been breached and
+ * could not immediately be resolved.  This will spin the event loop with short
+ * eviction cycles until the "maxmemory" condition has resolved or there are no
+ * more evictable items.  */
+static int isEvictionProcRunning = 0;
+static int evictionTimeProc(
+        struct aeEventLoop *eventLoop, long long id, void *clientData) {
+    UNUSED(eventLoop);
+    UNUSED(id);
+    UNUSED(clientData);
+
+    if (performEvictions() == EVICT_RUNNING) return 0;  /* keep evicting */
+
+    /* For EVICT_OK - things are good, no need to keep evicting.
+     * For EVICT_FAIL - there is nothing left to evict.  */
+    isEvictionProcRunning = 0;
+    return AE_NOMORE;
+}
+
+/* Check if it's safe to perform evictions.
+ *   Returns 1 if evictions can be performed
+ *   Returns 0 if eviction processing should be skipped
+ */
+static int isSafeToPerformEvictions(void) {
+    /* - There must be no script in timeout condition.
+     * - Nor we are loading data right now.  */
+    if (server.lua_timedout || server.loading) return 0;
+
     /* By default replicas should ignore maxmemory
      * and just be masters exact copies. */
-    if (server.masterhost && server.repl_slave_ignore_maxmemory) return C_OK;
+    if (server.masterhost && server.repl_slave_ignore_maxmemory) return 0;
+
+    /* When clients are paused the dataset should be static not just from the
+     * POV of clients not being able to write, but also from the POV of
+     * expires and evictions of keys not being performed. */
+    if (clientsArePaused()) return 0;
+
+    return 1;
+}
+
+/* Algorithm for converting tenacity (0-100) to a time limit.  */
+static unsigned long evictionTimeLimitUs() {
+    serverAssert(server.maxmemory_eviction_tenacity >= 0);
+    serverAssert(server.maxmemory_eviction_tenacity <= 100);
+
+    if (server.maxmemory_eviction_tenacity <= 10) {
+        /* A linear progression from 0..500us */
+        return 50uL * server.maxmemory_eviction_tenacity;
+    }
 
+    if (server.maxmemory_eviction_tenacity < 100) {
+        /* A 15% geometric progression, resulting in a limit of ~2 min at tenacity==99  */
+        return (unsigned long)(500.0 * pow(1.15, server.maxmemory_eviction_tenacity - 10.0));
+    }
+
+    return ULONG_MAX;   /* No limit to eviction time */
+}
+
+/* Check that memory usage is within the current "maxmemory" limit.  If over
+ * "maxmemory", attempt to free memory by evicting data (if it's safe to do so).
+ *
+ * It's possible for Redis to suddenly be significantly over the "maxmemory"
+ * setting.  This can happen if there is a large allocation (like a hash table
+ * resize) or even if the "maxmemory" setting is manually adjusted.  Because of
+ * this, it's important to evict for a managed period of time - otherwise Redis
+ * would become unresponsive while evicting.
+ *
+ * The goal of this function is to improve the memory situation - not to
+ * immediately resolve it.  In the case that some items have been evicted but
+ * the "maxmemory" limit has not been achieved, an aeTimeProc will be started
+ * which will continue to evict items until memory limits are achieved or
+ * nothing more is evictable.
+ *
+ * This should be called before execution of commands.  If EVICT_FAIL is
+ * returned, commands which will result in increased memory usage should be
+ * rejected.
+ *
+ * Returns:
+ *   EVICT_OK       - memory is OK or it's not possible to perform evictions now
+ *   EVICT_RUNNING  - memory is over the limit, but eviction is still processing
+ *   EVICT_FAIL     - memory is over the limit, and there's nothing to evict
+ * */
+int performEvictions(void) {
+    if (!isSafeToPerformEvictions()) return EVICT_OK;
+
+    int keys_freed = 0;
     size_t mem_reported, mem_tofree, mem_freed;
-    mstime_t latency, eviction_latency, lazyfree_latency;
+    mstime_t latency, eviction_latency;
     long long delta;
     int slaves = listLength(server.slaves);
-    int result = C_ERR;
+    int result = EVICT_FAIL;
 
-    /* When clients are paused the dataset should be static not just from the
-     * POV of clients not being able to write, but also from the POV of
-     * expires and evictions of keys not being performed. */
-    if (clientsArePaused()) return C_OK;
     if (getMaxmemoryState(&mem_reported,NULL,&mem_tofree,NULL) == C_OK)
-        return C_OK;
+        return EVICT_OK;
+
+    if (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)
+        return EVICT_FAIL;  /* We need to free memory, but policy forbids. */
+
+    unsigned long eviction_time_limit_us = evictionTimeLimitUs();
 
     mem_freed = 0;
 
     latencyStartMonitor(latency);
-    if (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)
-        goto cant_free; /* We need to free memory, but policy forbids. */
+
+    monotime evictionTimer;
+    elapsedStart(&evictionTimer);
 
     while (mem_freed < mem_tofree) {
         int j, k, i;
@@ -581,60 +633,61 @@ int freeMemoryIfNeeded(void) {
             decrRefCount(keyobj);
             keys_freed++;
 
-            /* When the memory to free starts to be big enough, we may
-             * start spending so much time here that is impossible to
-             * deliver data to the slaves fast enough, so we force the
-             * transmission here inside the loop. */
-            if (slaves) flushSlavesOutputBuffers();
-
-            /* Normally our stop condition is the ability to release
-             * a fixed, pre-computed amount of memory. However when we
-             * are deleting objects in another thread, it's better to
-             * check, from time to time, if we already reached our target
-             * memory, since the "mem_freed" amount is computed only
-             * across the dbAsyncDelete() call, while the thread can
-             * release the memory all the time. */
-            if (server.lazyfree_lazy_eviction && !(keys_freed % 16)) {
-                if (getMaxmemoryState(NULL,NULL,NULL,NULL) == C_OK) {
-                    /* Let's satisfy our stop condition. */
-                    mem_freed = mem_tofree;
+            if (keys_freed % 16 == 0) {
+                /* When the memory to free starts to be big enough, we may
+                 * start spending so much time here that is impossible to
+                 * deliver data to the replicas fast enough, so we force the
+                 * transmission here inside the loop. */
+                if (slaves) flushSlavesOutputBuffers();
+
+                /* Normally our stop condition is the ability to release
+                 * a fixed, pre-computed amount of memory. However when we
+                 * are deleting objects in another thread, it's better to
+                 * check, from time to time, if we already reached our target
+                 * memory, since the "mem_freed" amount is computed only
+                 * across the dbAsyncDelete() call, while the thread can
+                 * release the memory all the time. */
+                if (server.lazyfree_lazy_eviction) {
+                    if (getMaxmemoryState(NULL,NULL,NULL,NULL) == C_OK) {
+                        break;
+                    }
+                }
+
+                /* After some time, exit the loop early - even if memory limit
+                 * hasn't been reached.  If we suddenly need to free a lot of
+                 * memory, don't want to spend too much time here.  */
+                if (elapsedUs(evictionTimer) > eviction_time_limit_us) {
+                    // We still need to free memory - start eviction timer proc
+                    if (!isEvictionProcRunning) {
+                        isEvictionProcRunning = 1;
+                        aeCreateTimeEvent(server.el, 0,
+                                evictionTimeProc, NULL, NULL);
+                    }
+                    break;
                 }
             }
         } else {
             goto cant_free; /* nothing to free... */
         }
     }
-    result = C_OK;
+    /* at this point, the memory is OK, or we have reached the time limit */
+    result = (isEvictionProcRunning) ? EVICT_RUNNING : EVICT_OK;
 
 cant_free:
-    /* We are here if we are not able to reclaim memory. There is only one
-     * last thing we can try: check if the lazyfree thread has jobs in queue
-     * and wait... */
-    if (result != C_OK) {
-        latencyStartMonitor(lazyfree_latency);
-        while(bioPendingJobsOfType(BIO_LAZY_FREE)) {
+    if (result == EVICT_FAIL) {
+        /* At this point, we have run out of evictable items.  It's possible
+         * that some items are being freed in the lazyfree thread.  Perform a
+         * short wait here if such jobs exist, but don't wait long.  */
+        if (bioPendingJobsOfType(BIO_LAZY_FREE)) {
+            usleep(eviction_time_limit_us);
             if (getMaxmemoryState(NULL,NULL,NULL,NULL) == C_OK) {
-                result = C_OK;
-                break;
+                result = EVICT_OK;
             }
-            usleep(1000);
         }
-        latencyEndMonitor(lazyfree_latency);
-        latencyAddSampleIfNeeded("eviction-lazyfree",lazyfree_latency);
     }
+
     latencyEndMonitor(latency);
     latencyAddSampleIfNeeded("eviction-cycle",latency);
     return result;
 }
 
-/* This is a wrapper for freeMemoryIfNeeded() that only really calls the
- * function if right now there are the conditions to do so safely:
- *
- * - There must be no script in timeout condition.
- * - Nor we are loading data right now.
- *
- */
-int freeMemoryIfNeededAndSafe(void) {
-    if (server.lua_timedout || server.loading) return C_OK;
-    return freeMemoryIfNeeded();
-}

src/networking.c

@@ -1830,8 +1830,8 @@ int processCommandAndResetClient(client *c) {
     }
     if (server.current_client == NULL) deadclient = 1;
     server.current_client = NULL;
-    /* freeMemoryIfNeeded may flush slave output buffers. This may
-     * result into a slave, that may be the active client, to be
+    /* performEvictions may flush slave output buffers. This may
+     * result in a slave, that may be the active client, to be
      * freed. */
     return deadclient ? C_ERR : C_OK;
 }
@@ -2797,7 +2797,7 @@ void asyncCloseClientOnOutputBufferLimitReached(client *c) {
     }
 }
 
-/* Helper function used by freeMemoryIfNeeded() in order to flush slaves
+/* Helper function used by performEvictions() in order to flush slaves
  * output buffers without returning control to the event loop.
  * This is also called by SHUTDOWN for a best-effort attempt to send
  * slaves the latest writes. */

src/server.c

@@ -3681,9 +3681,9 @@ int processCommand(client *c) {
      * condition, to avoid mixing the propagation of scripts with the
      * propagation of DELs due to eviction. */
     if (server.maxmemory && !server.lua_timedout) {
-        int out_of_memory = freeMemoryIfNeededAndSafe() == C_ERR;
-        /* freeMemoryIfNeeded may flush slave output buffers. This may result
-         * into a slave, that may be the active client, to be freed. */
+        int out_of_memory = (performEvictions() == EVICT_FAIL);
+        /* performEvictions may flush slave output buffers. This may result
+         * in a slave, that may be the active client, to be freed. */
         if (server.current_client == NULL) return C_ERR;
 
         int reject_cmd_on_oom = is_denyoom_command;