defrag: eliminate persistent pointers

Signed-off-by: Jim Brunner <[email protected]>
valkey-io · Dec 12, 2024 · 0ed833b · 0ed833b
1 parent 0c8ad5c
commit 0ed833b
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Showing 2 changed files with 42 additions and 28 deletions.
diff --git a/src/defrag.c b/src/defrag.c
@@ -120,7 +120,7 @@ typedef doneStatus (*kvstoreHelperPreContinueFn)(monotime endtime, void *privdat
 // Private data for main dictionary keys
 typedef struct {
     kvstoreIterState kvstate;
-    serverDb *db;
+    int dbid;
 } defragKeysCtx;
 static_assert(offsetof(defragKeysCtx, kvstate) == 0, "defragStageKvstoreHelper requires this");
 
@@ -735,7 +735,7 @@ static void defragModule(serverDb *db, robj *obj) {
 /* for each key we scan in the main dict, this function will attempt to defrag
  * all the various pointers it has. */
 static void defragKey(defragKeysCtx *ctx, robj **elemref) {
-    serverDb *db = ctx->db;
+    serverDb *db = &server.db[ctx->dbid];
     int slot = ctx->kvstate.slot;
     robj *newob, *ob;
     unsigned char *newzl;
@@ -919,7 +919,7 @@ static doneStatus defragLaterStep(monotime endtime, void *privdata) {
         robj *ob = found;
 
         long long key_defragged = server.stat_active_defrag_hits;
-        bool timeout = (defragLaterItem(ob, &defrag_later_cursor, endtime, ctx->db->id) == 1);
+        bool timeout = (defragLaterItem(ob, &defrag_later_cursor, endtime, ctx->dbid) == 1);
         if (key_defragged != server.stat_active_defrag_hits) {
             server.stat_active_defrag_key_hits++;
         } else {
@@ -962,7 +962,10 @@ static doneStatus defragStageKvstoreHelper(monotime endtime,
         state.cursor = 0;
         return DEFRAG_NOT_DONE;
     }
-    serverAssert(kvs == state.kvs); // Shouldn't change during the stage
+    if (kvs != state.kvs) {
+        // There has been a change of the kvs (flushdb, swapdb, etc.).  Just complete the stage.
+        return DEFRAG_DONE;
+    }
 
     unsigned int iterations = 0;
     unsigned long long prev_defragged = server.stat_active_defrag_hits;
@@ -1012,26 +1015,30 @@ static doneStatus defragStageKvstoreHelper(monotime endtime,
 }
 
 
-// Note: target is a DB, (not a KVS like most stages)
+// Target is a DBID
 static doneStatus defragStageDbKeys(monotime endtime, void *target, void *privdata) {
     UNUSED(privdata);
-    serverDb *db = (serverDb *)target;
+    int dbid = (uintptr_t)target;
+    serverDb *db = &server.db[dbid];
 
     static defragKeysCtx ctx; // STATIC - this persists
     if (endtime == 0) {
-        ctx.db = db;
+        ctx.dbid = dbid;
         // Don't return yet.  Call the helper with endtime==0 below.
     }
-    serverAssert(ctx.db == db);
+    serverAssert(ctx.dbid == dbid);
 
     return defragStageKvstoreHelper(endtime, db->keys,
                                     dbKeysScanCallback, defragLaterStep, &ctx);
 }
 
 
+// Target is a DBID
 static doneStatus defragStageExpiresKvstore(monotime endtime, void *target, void *privdata) {
     UNUSED(privdata);
-    return defragStageKvstoreHelper(endtime, (kvstore *)target,
+    int dbid = (uintptr_t)target;
+    serverDb *db = &server.db[dbid];
+    return defragStageKvstoreHelper(endtime, db->expires,
                                     scanHashtableCallbackCountScanned, NULL, NULL);
 }
 
@@ -1222,29 +1229,38 @@ static long long activeDefragTimeProc(struct aeEventLoop *eventLoop, long long i
     }
 
     monotime starttime = getMonotonicUs();
-    monotime endtime = starttime + computeDefragCycleUs();
+    int dutyCycleUs = computeDefragCycleUs();
+    monotime endtime = starttime + dutyCycleUs;
+    bool haveMoreWork = true;
 
     mstime_t latency;
     latencyStartMonitor(latency);
 
-    if (!defrag.current_stage) {
-        defrag.current_stage = listNodeValue(listFirst(defrag.remaining_stages));
-        listDelNode(defrag.remaining_stages, listFirst(defrag.remaining_stages));
-        // Initialize the stage with endtime==0
-        doneStatus status = defrag.current_stage->stage_fn(0, defrag.current_stage->target, defrag.current_stage->privdata);
-        serverAssert(status == DEFRAG_NOT_DONE); // Initialization should always return DEFRAG_NOT_DONE
-    }
+    do {
+        if (!defrag.current_stage) {
+            defrag.current_stage = listNodeValue(listFirst(defrag.remaining_stages));
+            listDelNode(defrag.remaining_stages, listFirst(defrag.remaining_stages));
+            // Initialize the stage with endtime==0
+            doneStatus status = defrag.current_stage->stage_fn(0, defrag.current_stage->target, defrag.current_stage->privdata);
+            serverAssert(status == DEFRAG_NOT_DONE); // Initialization should always return DEFRAG_NOT_DONE
+        }
 
-    doneStatus status = defrag.current_stage->stage_fn(endtime, defrag.current_stage->target, defrag.current_stage->privdata);
-    if (status == DEFRAG_DONE) {
-        zfree(defrag.current_stage);
-        defrag.current_stage = NULL;
-    }
+        doneStatus status = defrag.current_stage->stage_fn(endtime, defrag.current_stage->target, defrag.current_stage->privdata);
+        if (status == DEFRAG_DONE) {
+            zfree(defrag.current_stage);
+            defrag.current_stage = NULL;
+        }
+
+        haveMoreWork = (defrag.current_stage || listLength(defrag.remaining_stages) > 0);
+        /* If we've completed a stage early, and still have a standard time allotment remaining,
+         * we'll start another stage.  This can happen when defrag is running infrequently, and
+         * starvation protection has increased the duty-cycle. */
+    } while (haveMoreWork && getMonotonicUs() <= endtime - server.active_defrag_cycle_us);
 
     latencyEndMonitor(latency);
     latencyAddSampleIfNeeded("active-defrag-cycle", latency);
 
-    if (defrag.current_stage || listLength(defrag.remaining_stages) > 0) {
+    if (haveMoreWork) {
         return computeDelayMs(endtime);
     } else {
         endDefragCycle(true);
@@ -1283,9 +1299,8 @@ static void beginDefragCycle(void) {
     defrag.remaining_stages = listCreate();
 
     for (int dbid = 0; dbid < server.dbnum; dbid++) {
-        serverDb *db = &server.db[dbid];
-        addDefragStage(defragStageDbKeys, db, NULL);
-        addDefragStage(defragStageExpiresKvstore, db->expires, NULL);
+        addDefragStage(defragStageDbKeys, (void*)(uintptr_t)dbid, NULL);
+        addDefragStage(defragStageExpiresKvstore, (void*)(uintptr_t)dbid, NULL);
     }
 
     static getClientChannelsFnWrapper getClientPubSubChannelsFn = {getClientPubSubChannels};

diff --git a/src/server.h b/src/server.h
@@ -1900,8 +1900,7 @@ struct valkeyServer {
     int sanitize_dump_payload;                   /* Enables deep sanitization for ziplist and listpack in RDB and RESTORE. */
     int skip_checksum_validation;                /* Disable checksum validation for RDB and RESTORE payload. */
     int jemalloc_bg_thread;                      /* Enable jemalloc background thread */
-    int active_defrag_configuration_changed;     /* defrag configuration has been changed and need to reconsider
-                                                  * active_defrag_running in computeDefragCycles. */
+    int active_defrag_configuration_changed;     /* Config changed; need to recompute active_defrag_cpu_percent. */
     size_t active_defrag_ignore_bytes;           /* minimum amount of fragmentation waste to start active defrag */
     int active_defrag_threshold_lower;           /* minimum percentage of fragmentation to start active defrag */
     int active_defrag_threshold_upper;           /* maximum percentage of fragmentation at which we use maximum effort */