Remove synchronization in monitor. (#7164)

* Remove synchronization in monitor.

Calling rabit functions during destruction is flaky.

* Add xgboost prefix to nvtx marker.

src/common/timer.cc

@@ -8,7 +8,6 @@
 #include <vector>
 #include <sstream>
 #include "timer.h"
-#include "xgboost/json.h"
 
 #if defined(XGBOOST_USE_NVTX)
 #include <nvToolsExt.h>
@@ -22,7 +21,7 @@ void Monitor::Start(std::string const &name) {
     auto &stats = statistics_map_[name];
     stats.timer.Start();
 #if defined(XGBOOST_USE_NVTX)
-    std::string nvtx_name = label_ + "::" + name;
+    std::string nvtx_name = "xgboost::" + label_ + "::" + name;
     stats.nvtx_id = nvtxRangeStartA(nvtx_name.c_str());
 #endif  // defined(XGBOOST_USE_NVTX)
   }
@@ -39,60 +38,6 @@ void Monitor::Stop(const std::string &name) {
   }
 }
 
-std::vector<Monitor::StatMap> Monitor::CollectFromOtherRanks() const {
-  // Since other nodes might have started timers that this one haven't, so
-  // we can't simply call all reduce.
-  size_t const world_size = rabit::GetWorldSize();
-  size_t const rank = rabit::GetRank();
-
-  // It's much easier to work with rabit if we have a string serialization.  So we go with
-  // json.
-  Json j_statistic { Object() };
-  j_statistic["rank"] = Integer(rank);
-  j_statistic["statistic"] = Object();
-
-  auto& statistic = j_statistic["statistic"];
-  for (auto const& kv : statistics_map_) {
-    statistic[kv.first] = Object();
-    auto& j_pair = statistic[kv.first];
-    j_pair["count"] = Integer(kv.second.count);
-    j_pair["elapsed"] = Integer(static_cast<int64_t>(
-        std::chrono::duration_cast<std::chrono::microseconds>(
-        kv.second.timer.elapsed).count()));
-  }
-
-  std::string str;
-  Json::Dump(j_statistic, &str);
-
-  size_t str_size = str.size();
-  rabit::Allreduce<rabit::op::Max>(&str_size, 1);
-  std::string buffer;
-  buffer.resize(str_size);
-
-  // vector storing stat from all workers
-  std::vector<StatMap> world(world_size);
-
-  // Actually only rank 0 is printing.
-  for (size_t i = 0; i < world_size; ++i) {
-    std::copy(str.cbegin(), str.cend(), buffer.begin());
-    rabit::Broadcast(&buffer, i);
-    auto j_other = Json::Load(StringView{buffer.c_str(), buffer.size()});
-    auto& other = world[i];
-
-    auto const& j_statistic = get<Object>(j_other["statistic"]);
-
-    for (auto const& kv : j_statistic) {
-      std::string const& timer_name = kv.first;
-      auto const& pair = kv.second;
-      other[timer_name] = {get<Integer>(pair["count"]), get<Integer>(pair["elapsed"])};
-    }
-
-    // FIXME(trivialfis): How to ask rabit to block here?
-  }
-
-  return world;
-}
-
 void Monitor::PrintStatistics(StatMap const& statistics) const {
   for (auto &kv : statistics) {
     if (kv.second.first == 0) {
@@ -109,30 +54,16 @@ void Monitor::PrintStatistics(StatMap const& statistics) const {
 
 void Monitor::Print() const {
   if (!ConsoleLogger::ShouldLog(ConsoleLogger::LV::kDebug)) { return; }
-
+  auto rank = rabit::GetRank();
-  bool is_distributed = rabit::IsDistributed();
+  StatMap stat_map;
-
+  for (auto const &kv : statistics_map_) {
-  if (is_distributed) {
+    stat_map[kv.first] = std::make_pair(
-    auto world = this->CollectFromOtherRanks();
+        kv.second.count, std::chrono::duration_cast<std::chrono::microseconds>(
-    // rank zero is in charge of printing
+                             kv.second.timer.elapsed)
-    if (rabit::GetRank() == 0) {
+                             .count());
-      LOG(CONSOLE) << "======== Monitor: " << label_ << " ========";
-      for (size_t i = 0; i < world.size(); ++i) {
-        LOG(CONSOLE) << "From rank: " << i << ": " << std::endl;
-        auto const& statistic = world[i];
-        this->PrintStatistics(statistic);
-      }
-    }
-  } else {
-    StatMap stat_map;
-    for (auto const& kv : statistics_map_) {
-      stat_map[kv.first] = std::make_pair(
-          kv.second.count, std::chrono::duration_cast<std::chrono::microseconds>(
-              kv.second.timer.elapsed).count());
-    }
-    LOG(CONSOLE) << "======== Monitor: " << label_ << " ========";
-    this->PrintStatistics(stat_map);
   }
+  LOG(CONSOLE) << "======== Monitor (" << rank << "): " << label_ << " ========";
+  this->PrintStatistics(stat_map);
 }
 
 }  // namespace common

src/common/timer.h

@@ -59,8 +59,6 @@ struct Monitor {
   std::map<std::string, Statistics> statistics_map_;
   Timer self_timer_;
 
-  /*! \brief Collect time statistics across all workers. */
-  std::vector<StatMap> CollectFromOtherRanks() const;
   void PrintStatistics(StatMap const& statistics) const;
 
  public: