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endSnapshotAsync implementation (fails tests w/ multithreading)

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Former-commit-id: b401e9fcae40cf1d4e4c1584f57e760d9adf36a9
This commit is contained in:
John Sully 2020-02-01 22:28:24 -05:00
parent a223fb7bed
commit d8dcb94e37
3 changed files with 74 additions and 16 deletions
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8
src/rdb.cpp
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@ -1464,10 +1464,8 @@ void *rdbSaveThread(void *vargs)
sendChildCOWInfo(CHILD_INFO_TYPE_RDB, "RDB");
// If we were told to cancel the requesting thread holds the lock for us
aeAcquireLock();
for (int idb = 0; idb < cserver.dbnum; ++idb)
g_pserver->db[idb]->endSnapshot(args->rgpdb[idb]);
aeReleaseLock();
g_pserver->db[idb]->endSnapshotAsync(args->rgpdb[idb]);
zfree(args);
g_pserver->garbageCollector.endEpoch(vars.gcEpoch);
@ -2674,10 +2672,8 @@ void *rdbSaveToSlavesSocketsThread(void *vargs)
close(g_pserver->rdb_pipe_write); /* wake up the reader, tell it we're done. */
// If we were told to cancel the requesting thread is holding the lock for us
aeAcquireLock();
for (int idb = 0; idb < cserver.dbnum; ++idb)
g_pserver->db[idb]->endSnapshot(args->rgpdb[idb]);
aeReleaseLock();
g_pserver->db[idb]->endSnapshotAsync(args->rgpdb[idb]);
zfree(args);
return (retval == C_OK) ? (void*)0 : (void*)1;

7
src/server.h
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@ -1224,6 +1224,7 @@ class redisDbPersistentData
{
friend void dictDbKeyDestructor(void *privdata, void *key);
friend class redisDbPersistentDataSnapshot;
public:
~redisDbPersistentData();
@ -1311,6 +1312,7 @@ public:
const redisDbPersistentDataSnapshot *createSnapshot(uint64_t mvccCheckpoint, bool fOptional);
void endSnapshot(const redisDbPersistentDataSnapshot *psnapshot);
void endSnapshotAsync(const redisDbPersistentDataSnapshot *psnapshot);
void restoreSnapshot(const redisDbPersistentDataSnapshot *psnapshot);
void consolidate_snapshot();
@ -1354,6 +1356,7 @@ private:
// in a snapshot
const redisDbPersistentDataSnapshot *m_pdbSnapshot = nullptr;
std::unique_ptr<redisDbPersistentDataSnapshot> m_spdbSnapshotHOLDER;
const redisDbPersistentDataSnapshot *m_pdbSnapshotASYNC = nullptr;
int m_refCount = 0;
fastlock m_lockStorage { "storage" };
};
@ -1365,7 +1368,7 @@ protected:
bool m_fConsolidated = false;
static void gcDisposeSnapshot(redisDbPersistentDataSnapshot *psnapshot);
int snapshot_depth() const;
void consolidate_children(redisDbPersistentData *pdbPrimary);
void consolidate_children(redisDbPersistentData *pdbPrimary, bool fForce);
public:
bool FWillFreeChildDebug() const { return m_spdbSnapshotHOLDER != nullptr; }
@ -1373,6 +1376,7 @@ public:
bool iterate_threadsafe(std::function<bool(const char*, robj_roptr o)> fn, bool fKeyOnly = false) const;
using redisDbPersistentData::createSnapshot;
using redisDbPersistentData::endSnapshot;
using redisDbPersistentData::endSnapshotAsync;
using redisDbPersistentData::end;
dict_iter random_cache_threadsafe() const;
@ -2443,6 +2447,7 @@ extern dictType zsetDictType;
extern dictType clusterNodesDictType;
extern dictType clusterNodesBlackListDictType;
extern dictType dbDictType;
extern dictType dbDictTypeTombstone;
extern dictType dbSnapshotDictType;
extern dictType shaScriptObjectDictType;
extern double R_Zero, R_PosInf, R_NegInf, R_Nan;

73
src/snapshot.cpp
View File

@ -54,7 +54,7 @@ const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint6
}
m_pdict = dictCreate(&dbDictType,this);
m_pdictTombstone = dictCreate(&dbDictType, this);
m_pdictTombstone = dictCreate(&dbDictTypeTombstone, this);
serverAssert(spdb->m_pdict->iterators == 1);
@ -69,6 +69,13 @@ const redisDbPersistentDataSnapshot *redisDbPersistentData::createSnapshot(uint6
pdbSnapshotNext = pdbSnapshotNext->m_spdbSnapshotHOLDER.get();
}
if (m_pdbSnapshotASYNC != nullptr)
{
// free the async snapshot, it's done its job
endSnapshot(m_pdbSnapshotASYNC); // should be just a dec ref (FAST)
m_pdbSnapshotASYNC = nullptr;
}
return m_pdbSnapshot;
}
@ -127,10 +134,54 @@ void redisDbPersistentData::restoreSnapshot(const redisDbPersistentDataSnapshot
serverAssert(size() == expectedSize);
}
// This function is all about minimizing the amount of work done under global lock
// when there has been lots of changes since snapshot creation a naive endSnapshot()
// will block for a very long time and will cause latency spikes.
//
// Note that this function uses a lot more CPU time than a simple endSnapshot(), we
// have some internal heuristics to do a synchronous endSnapshot if it makes sense
void redisDbPersistentData::endSnapshotAsync(const redisDbPersistentDataSnapshot *psnapshot)
{
aeAcquireLock();
if (m_pdbSnapshotASYNC && m_pdbSnapshotASYNC->m_mvccCheckpoint <= psnapshot->m_mvccCheckpoint)
{
// Free a stale async snapshot so consolidate_children can clean it up later
endSnapshot(m_pdbSnapshotASYNC); // FAST: just a ref decrement
m_pdbSnapshotASYNC = nullptr;
}
size_t elements = dictSize(m_pdictTombstone);
// if neither dict is rehashing then the merge is O(1) so don't count the size
if (dictIsRehashing(psnapshot->m_pdict) || dictIsRehashing(m_pdict))
elements += dictSize(m_pdict);
if (elements < 1000000 || psnapshot != m_spdbSnapshotHOLDER.get()) // heuristic
{
// For small snapshots it makes more sense just to merge it directly
endSnapshot(psnapshot);
aeReleaseLock();
return;
}
// OK this is a big snapshot so lets do the merge work outside the lock
auto psnapshotT = createSnapshot(LLONG_MAX, false);
endSnapshot(psnapshot); // this will just dec the ref count since our new snapshot has a ref
aeReleaseLock();
// do the expensive work of merging snapshots outside the ref
reinterpret_cast<redisDbPersistentDataSnapshot*>(this)->consolidate_children(this, true /* fForce */);
// Final Cleanup
aeAcquireLock();
if (m_pdbSnapshotASYNC == nullptr)
m_pdbSnapshotASYNC = psnapshotT;
else
endSnapshot(psnapshotT); // finally clena up our temp snapshot
aeReleaseLock();
}
void redisDbPersistentData::endSnapshot(const redisDbPersistentDataSnapshot *psnapshot)
{
// Note: This function is dependent on GlobalLocksAcquried(), but rdb background saving has a weird case where
// a seperate thread holds the lock for it. Yes that's pretty crazy and should be fixed somehow...
serverAssert(GlobalLocksAcquired());
if (m_spdbSnapshotHOLDER.get() != psnapshot)
{
@ -215,7 +266,12 @@ void redisDbPersistentData::endSnapshot(const redisDbPersistentDataSnapshot *psn
m_spdbSnapshotHOLDER->m_pdict->iterators--;
}
m_spdbSnapshotHOLDER = std::move(m_spdbSnapshotHOLDER->m_spdbSnapshotHOLDER);
auto spsnapshotFree = std::move(m_spdbSnapshotHOLDER);
m_spdbSnapshotHOLDER = std::move(spsnapshotFree->m_spdbSnapshotHOLDER);
if (serverTL != nullptr)
g_pserver->garbageCollector.enqueue(serverTL->gcEpoch, std::move(spsnapshotFree));
// Sanity Checks
serverAssert(m_spdbSnapshotHOLDER != nullptr || m_pdbSnapshot == nullptr);
serverAssert(m_pdbSnapshot == m_spdbSnapshotHOLDER.get() || m_pdbSnapshot == nullptr);
serverAssert((m_refCount == 0 && m_pdict->iterators == 0) || (m_refCount != 0 && m_pdict->iterators == 1));
@ -340,14 +396,14 @@ void redisDbPersistentData::consolidate_snapshot()
}
psnapshot->m_refCount++; // ensure it's not free'd
aeReleaseLock();
psnapshot->consolidate_children(this);
psnapshot->consolidate_children(this, false /* fForce */);
aeAcquireLock();
endSnapshot(psnapshot);
aeReleaseLock();
}
// only call this on the "real" database to consolidate the first child
void redisDbPersistentDataSnapshot::consolidate_children(redisDbPersistentData *pdbPrimary)
void redisDbPersistentDataSnapshot::consolidate_children(redisDbPersistentData *pdbPrimary, bool fForce)
{
static fastlock s_lock {"consolidate_children"}; // this lock ensures only one thread is consolidating at a time
@ -355,14 +411,15 @@ void redisDbPersistentDataSnapshot::consolidate_children(redisDbPersistentData *
if (!lock.try_lock())
return; // this is a best effort function
if (snapshot_depth() < 4)
if (!fForce && snapshot_depth() < 4)
return;
serverAssert(snapshot_depth() > 1);
auto spdb = std::unique_ptr<redisDbPersistentDataSnapshot>(new (MALLOC_LOCAL) redisDbPersistentDataSnapshot());
spdb->initialize();
dictExpand(spdb->m_pdict, m_pdbSnapshot->size());
m_pdbSnapshot->iterate_threadsafe([&](const char *key, robj_roptr o){
m_pdbSnapshot->iterate_threadsafe([&](const char *key, robj_roptr o) {
if (o != nullptr) {
dictAdd(spdb->m_pdict, sdsdupshared(key), o.unsafe_robjcast());
incrRefCount(o);