From 34937b0ad5b72807aa53b09a342c3d40cbd6b6bc Mon Sep 17 00:00:00 2001
From: John Sully <john@csquare.ca>
Date: Sat, 15 Aug 2020 23:05:56 +0000
Subject: [PATCH] Rehash efficiency

Former-commit-id: fab383156626ec683881101c22eb2f6c2cea4c5d
---
 src/db.cpp       |  4 ++--
 src/dict.cpp     |  6 ++++--
 src/server.cpp   | 11 ++++++-----
 src/server.h     |  2 +-
 src/snapshot.cpp |  7 ++++++-
 5 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/src/db.cpp b/src/db.cpp
index e2a50164c..1cc7e66f7 100644
--- a/src/db.cpp
+++ b/src/db.cpp
@@ -430,7 +430,7 @@ bool redisDbPersistentData::syncDelete(robj *key)
             auto itr = m_pdbSnapshot->find_cached_threadsafe(szFromObj(key));
             if (itr != nullptr)
             {
-                sds keyTombstone = sdsdup(szFromObj(key));
+                sds keyTombstone = sdsdupshared(itr.key());
                 uint64_t hash = dictGetHash(m_pdict, keyTombstone);
                 if (dictAdd(m_pdictTombstone, keyTombstone, (void*)hash) != DICT_OK)
                     sdsfree(keyTombstone);
@@ -2300,7 +2300,7 @@ void redisDbPersistentData::initialize()
 {
     m_pdbSnapshot = nullptr;
     m_pdict = dictCreate(&dbDictType,this);
-    m_pdictTombstone = dictCreate(&dbDictTypeTombstone,this);
+    m_pdictTombstone = dictCreate(&dbTombstoneDictType,this);
     m_setexpire = new(MALLOC_LOCAL) expireset();
     m_fAllChanged = 0;
     m_fTrackingChanges = 0;
diff --git a/src/dict.cpp b/src/dict.cpp
index c69c663a4..fda797363 100644
--- a/src/dict.cpp
+++ b/src/dict.cpp
@@ -326,7 +326,7 @@ int dictRehashMilliseconds(dict *d, int ms) {
 static void _dictRehashStep(dict *d) {
     unsigned long iterators;
     __atomic_load(&d->iterators, &iterators, __ATOMIC_RELAXED);
-    if (iterators == 0) dictRehash(d,10);
+    if (iterators == 0) dictRehash(d,2);
 }
 
 /* Add an element to the target hash table */
@@ -1220,7 +1220,9 @@ void dictGetStats(char *buf, size_t bufsize, dict *d) {
 
 void dictForceRehash(dict *d)
 {
-    while (dictIsRehashing(d)) _dictRehashStep(d);
+    unsigned long iterators;
+    __atomic_load(&d->iterators, &iterators, __ATOMIC_RELAXED);
+    while (iterators == 0 && dictIsRehashing(d)) _dictRehashStep(d);
 }
 
 /* ------------------------------- Benchmark ---------------------------------*/
diff --git a/src/server.cpp b/src/server.cpp
index 52648e7bd..f72047c31 100644
--- a/src/server.cpp
+++ b/src/server.cpp
@@ -1387,14 +1387,14 @@ dictType dbDictType = {
     dictObjectDestructor   /* val destructor */
 };
 
-/* db->pdict, keys are sds strings, vals uints. */
-dictType dbDictTypeTombstone = {
+/* db->pdict, keys are sds strings, vals are Redis objects. */
+dictType dbTombstoneDictType = {
     dictSdsHash,                /* hash function */
     NULL,                       /* key dup */
     NULL,                       /* val dup */
     dictSdsKeyCompare,          /* key compare */
-    dictDbKeyDestructor,          /* key destructor */
-    NULL   /* val destructor */
+    dictDbKeyDestructor,        /* key destructor */
+    NULL                        /* val destructor */
 };
 
 dictType dbSnapshotDictType = {
@@ -1539,8 +1539,9 @@ void tryResizeHashTables(int dbid) {
  * is returned. */
 int redisDbPersistentData::incrementallyRehash() {
     /* Keys dictionary */
-    if (dictIsRehashing(m_pdict)) {
+    if (dictIsRehashing(m_pdict) || dictIsRehashing(m_pdictTombstone)) {
         dictRehashMilliseconds(m_pdict,1);
+        dictRehashMilliseconds(m_pdictTombstone,1);
         return 1; /* already used our millisecond for this loop... */
     }
     return 0;
diff --git a/src/server.h b/src/server.h
index 4f9375834..ce521b2b6 100644
--- a/src/server.h
+++ b/src/server.h
@@ -2564,7 +2564,7 @@ extern dictType zsetDictType;
 extern dictType clusterNodesDictType;
 extern dictType clusterNodesBlackListDictType;
 extern dictType dbDictType;
-extern dictType dbDictTypeTombstone;
+extern dictType dbTombstoneDictType;
 extern dictType dbSnapshotDictType;
 extern dictType shaScriptObjectDictType;
 extern double R_Zero, R_PosInf, R_NegInf, R_Nan;
diff --git a/src/snapshot.cpp b/src/snapshot.cpp
index 8dd4db367..bd476be26 100644
--- a/src/snapshot.cpp
+++ b/src/snapshot.cpp
@@ -91,8 +91,13 @@ const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint6
         spdb->m_setexpire->pause_rehash();  // needs to be const
     }
 
+    if (dictIsRehashing(spdb->m_pdict) || dictIsRehashing(spdb->m_pdictTombstone)) {
+        serverLog(LL_NOTICE, "NOTICE: Suboptimal snapshot");
+    }
+
     m_pdict = dictCreate(&dbDictType,this);
-    m_pdictTombstone = dictCreate(&dbDictTypeTombstone, this);
+    dictExpand(m_pdict, 1024);   // minimize rehash overhead
+    m_pdictTombstone = dictCreate(&dbTombstoneDictType, this);
 
     serverAssert(spdb->m_pdict->iterators == 1);