SyncPoint::Process thrashes heap ... fix it (#9023)

Summary:
The first parameter of SyncPoint::Process is "const std::string&".  The majority, maybe all, of the actual calls to this function use a "const char *".  The conversion before entering the function requires a construction of a std::string object on the heap.  This std::object is then typically not needed because first use of the string is a rocksdb::Slice which has a less costly conversion of char * to slice.

Example:

We have a load and iterate test.  The test loads 10m keys and iterates most via 10 rocksdb::Iterator objects.  We used TCMALLOC to gather information about allocation and space usage during iterators.

- Before this PR:  test took 32 min 17 sec
- After this PR:  test took 1 min 14 sec

The TCMALLOC top object list before this PR:

<pre>
Total: 5105999 objects
 5003717  98.0%  98.0%  5009471  98.1% rocksdb::DBIter::MergeValuesNewToOld (inline)
   20260   0.4%  98.4%    20260   0.4% std::__cxx11::basic_string::_M_mutate
   15214   0.3%  98.7%    15214   0.3% rocksdb::UncompressBlockContentsForCompressionType (inline)
   13408   0.3%  99.0%    13408   0.3% std::_Rb_tree::_M_emplace_hint_unique [clone .constprop.416] (inline)
   12957   0.3%  99.2%    12957   0.3% std::_Rb_tree::_M_emplace_hint_unique [clone .constprop.405] (inline)
    9327   0.2%  99.4%     9327   0.2% std::_Rb_tree::_M_copy (inline)
    7691   0.2%  99.5%     9919   0.2% JVM_FindSignal
    2859   0.1%  99.6%     2859   0.1% rocksdb::Cleanable::RegisterCleanup
    2844   0.1%  99.7%     2844   0.1% std::map::operator[] (inline)
</pre>

The "MergeValuesNewToOld (inline)" objects are the #define wrappers to SyncPoint::Process.  We discovered this in a 5.18 rocksdb release.  There TCMALLOC was more specific that std::basic_string was being constructed.  I believe that was before SyncPoint::Process was declared inline in subsequent releases.

The TCMALLOC top object list after this PR:

<pre>
Total: 104911 objects
   45090  43.0%  43.0%    45090  43.0% rocksdb::Cleanable::RegisterCleanup
   29995  28.6%  71.6%    29995  28.6% rocksdb::LRUCacheShard::Insert
   15229  14.5%  86.1%    15229  14.5% rocksdb::UncompressBlockContentsForCompressionType (inline)
    4373   4.2%  90.3%     4551   4.3% JVM_FindSignal
    2881   2.7%  93.0%     2881   2.7% rocksdb::::ReadBlockFromFile (inline)
    1162   1.1%  94.1%     1176   1.1% rocksdb::BlockFetcher::ReadBlockContents (inline)
    1036   1.0%  95.1%     1036   1.0% std::__cxx11::basic_string::_M_mutate
     869   0.8%  95.9%      869   0.8% std::vector::_M_realloc_insert (inline)
     806   0.8%  96.7%      806   0.8% SnmpAgent::GetVariables (inline)
</pre>

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9023

Reviewed By: pdillinger

Differential Revision: D31610907

Pulled By: mrambacher

fbshipit-source-id: 574ff51b639dd46ad253a8e664a575f06b7cc85d

test_util/sync_point.cc

@@ -60,7 +60,7 @@ void SyncPoint::ClearTrace() {
   impl_->ClearTrace();
 }
 
-void SyncPoint::Process(const std::string& point, void* cb_arg) {
+void SyncPoint::Process(const Slice& point, void* cb_arg) {
   impl_->Process(point, cb_arg);
 }
 

test_util/sync_point.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <assert.h>
+
 #include <functional>
 #include <mutex>
 #include <string>
@@ -12,6 +13,7 @@
 #include <vector>
 
 #include "rocksdb/rocksdb_namespace.h"
+#include "rocksdb/slice.h"
 
 #ifdef NDEBUG
 // empty in release build
@@ -117,7 +119,16 @@ class SyncPoint {
   // triggered by TEST_SYNC_POINT, blocking execution until all predecessors
   // are executed.
   // And/or call registered callback function, with argument `cb_arg`
-  void Process(const std::string& point, void* cb_arg = nullptr);
+  void Process(const Slice& point, void* cb_arg = nullptr);
+
+  // template gets length of const string at compile time,
+  //  avoiding strlen() at runtime
+  template <size_t kLen>
+  void Process(const char (&point)[kLen], void* cb_arg = nullptr) {
+    static_assert(kLen > 0, "Must not be empty");
+    assert(point[kLen - 1] == '\0');
+    Process(Slice(point, kLen - 1), cb_arg);
+  }
 
   // TODO: it might be useful to provide a function that blocks until all
   // sync points are cleared.

test_util/sync_point_impl.cc

@@ -101,19 +101,23 @@ void SyncPoint::Data::ClearAllCallBacks() {
   callbacks_.clear();
 }
 
-void SyncPoint::Data::Process(const std::string& point, void* cb_arg) {
+void SyncPoint::Data::Process(const Slice& point, void* cb_arg) {
   if (!enabled_) {
     return;
   }
+
   // Use a filter to prevent mutex lock if possible.
   if (!point_filter_.MayContain(point)) {
     return;
   }
 
+  // Must convert to std::string for remaining work.  Take
+  //  heap hit.
+  std::string point_string(point.ToString());
   std::unique_lock<std::mutex> lock(mutex_);
   auto thread_id = std::this_thread::get_id();
 
-  auto marker_iter = markers_.find(point);
+  auto marker_iter = markers_.find(point_string);
   if (marker_iter != markers_.end()) {
     for (auto& marked_point : marker_iter->second) {
       marked_thread_id_.emplace(marked_point, thread_id);
@@ -121,18 +125,18 @@ void SyncPoint::Data::Process(const std::string& point, void* cb_arg) {
     }
   }
 
-  if (DisabledByMarker(point, thread_id)) {
+  if (DisabledByMarker(point_string, thread_id)) {
     return;
   }
 
-  while (!PredecessorsAllCleared(point)) {
+  while (!PredecessorsAllCleared(point_string)) {
     cv_.wait(lock);
-    if (DisabledByMarker(point, thread_id)) {
+    if (DisabledByMarker(point_string, thread_id)) {
       return;
     }
   }
 
-  auto callback_pair = callbacks_.find(point);
+  auto callback_pair = callbacks_.find(point_string);
   if (callback_pair != callbacks_.end()) {
     num_callbacks_running_++;
     mutex_.unlock();
@@ -140,7 +144,7 @@ void SyncPoint::Data::Process(const std::string& point, void* cb_arg) {
     mutex_.lock();
     num_callbacks_running_--;
   }
-  cleared_points_.insert(point);
+  cleared_points_.insert(point_string);
   cv_.notify_all();
 }
 }  // namespace ROCKSDB_NAMESPACE

test_util/sync_point_impl.h

@@ -95,7 +95,7 @@ struct SyncPoint::Data {
     return marked_point_iter != marked_thread_id_.end() &&
            thread_id != marked_point_iter->second;
   }
-  void Process(const std::string& point, void* cb_arg);
+  void Process(const Slice& point, void* cb_arg);
 };
 }  // namespace ROCKSDB_NAMESPACE
 #endif // NDEBUG