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//===--- Trace.cpp - Performance tracing facilities -----------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "support/Trace.h"
#include "support/Context.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/Chrono.h"
#include "llvm/Support/FormatProviders.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/Threading.h"
#include <atomic>
#include <chrono>
#include <memory>
#include <mutex>
namespace clang {
namespace clangd {
namespace trace {
namespace {
// The current implementation is naive: each thread writes to Out guarded by Mu.
// Perhaps we should replace this by something that disturbs performance less.
class JSONTracer : public EventTracer {
JSONTracer(llvm::raw_ostream &OS, bool Pretty)
: Out(OS, Pretty ? 2 : 0), Start(std::chrono::system_clock::now()) {
// The displayTimeUnit must be ns to avoid low-precision overlap
// calculations!
Out.attribute("displayTimeUnit", "ns");
rawEvent("M", llvm::json::Object{
{"name", "process_name"},
{"args", llvm::json::Object{{"name", "clangd"}}},
~JSONTracer() {
// We stash a Span object in the context. It will record the start/end,
// and this also allows us to look up the parent Span's information.
Context beginSpan(
llvm::StringRef Name,
llvm::function_ref<void(llvm::json::Object *)> AttachDetails) override {
auto JS = std::make_unique<JSONSpan>(this, Name);
return Context::current().derive(SpanKey, std::move(JS));
// Trace viewer requires each thread to properly stack events.
// So we need to mark only duration that the span was active on the thread.
// (Hopefully any off-thread activity will be connected by a flow event).
// Record the end time here, but don't write the event: Args aren't ready yet.
void endSpan() override {
void instant(llvm::StringRef Name, llvm::json::Object &&Args) override {
llvm::json::Object{{"name", Name}, {"args", std::move(Args)}});
// Record an event on the current thread. ph, pid, tid, ts are set.
// Contents must be a list of the other JSON key/values.
void jsonEvent(llvm::StringRef Phase, llvm::json::Object &&Contents,
uint64_t TID = llvm::get_threadid(), double Timestamp = 0) {
Contents["ts"] = Timestamp ? Timestamp : timestamp();
Contents["tid"] = int64_t(TID);
std::lock_guard<std::mutex> Lock(Mu);
rawEvent(Phase, Contents);
class JSONSpan {
JSONSpan(JSONTracer *Tracer, llvm::StringRef Name)
: StartTime(Tracer->timestamp()), EndTime(0), Name(Name),
TID(llvm::get_threadid()), Tracer(Tracer) {
// ~JSONSpan() may run in a different thread, so we need to capture now.
// We don't record begin events here (and end events in the destructor)
// because B/E pairs have to appear in the right order, which is awkward.
// Instead we send the complete (X) event in the destructor.
// If our parent was on a different thread, add an arrow to this span.
auto *Parent = Context::current().get(SpanKey);
if (Parent && *Parent && (*Parent)->TID != TID) {
// If the parent span ended already, then show this as "following" it.
// Otherwise show us as "parallel".
double OriginTime = (*Parent)->EndTime;
if (!OriginTime)
OriginTime = (*Parent)->StartTime;
auto FlowID = nextID();
llvm::json::Object{{"id", FlowID},
{"name", "Context crosses threads"},
{"cat", "mock_cat"}},
(*Parent)->TID, (*Parent)->StartTime);
llvm::json::Object{{"id", FlowID},
{"bp", "e"},
{"name", "Context crosses threads"},
{"cat", "mock_cat"}},
~JSONSpan() {
// Finally, record the event (ending at EndTime, not timestamp())!
llvm::json::Object{{"name", std::move(Name)},
{"args", std::move(Args)},
{"dur", EndTime - StartTime}},
TID, StartTime);
// May be called by any thread.
void markEnded() { EndTime = Tracer->timestamp(); }
llvm::json::Object Args;
static int64_t nextID() {
static std::atomic<int64_t> Next = {0};
return Next++;
double StartTime;
std::atomic<double> EndTime; // Filled in by markEnded().
std::string Name;
uint64_t TID;
JSONTracer *Tracer;
static Key<std::unique_ptr<JSONSpan>> SpanKey;
// Record an event. ph and pid are set.
// Contents must be a list of the other JSON key/values.
void rawEvent(llvm::StringRef Phase,
const llvm::json::Object &Event) /*REQUIRES(Mu)*/ {
// PID 0 represents the clangd process.
Out.object([&] {
Out.attribute("pid", 0);
Out.attribute("ph", Phase);
for (const auto &KV : Event)
Out.attribute(KV.first, KV.second);
// If we haven't already, emit metadata describing this thread.
void captureThreadMetadata() {
uint64_t TID = llvm::get_threadid();
std::lock_guard<std::mutex> Lock(Mu);
if (ThreadsWithMD.insert(TID).second) {
llvm::SmallString<32> Name;
if (!Name.empty()) {
rawEvent("M", llvm::json::Object{
{"tid", int64_t(TID)},
{"name", "thread_name"},
{"args", llvm::json::Object{{"name", Name}}},
double timestamp() {
using namespace std::chrono;
return duration<double, std::micro>(system_clock::now() - Start).count();
std::mutex Mu;
llvm::json::OStream Out /*GUARDED_BY(Mu)*/;
llvm::DenseSet<uint64_t> ThreadsWithMD /*GUARDED_BY(Mu)*/;
const llvm::sys::TimePoint<> Start;
// We emit CSV as specified in RFC 4180:
// \r\n line endings are used, cells with \r\n," are quoted, quotes are doubled.
class CSVMetricTracer : public EventTracer {
CSVMetricTracer(llvm::raw_ostream &Out) : Out(Out) {
Start = std::chrono::steady_clock::now();
Out.SetUnbuffered(); // We write each line atomically.
Out << "Kind,Metric,Label,Value,Timestamp\r\n";
void record(const Metric &Metric, double Value,
llvm::StringRef Label) override {
std::string QuotedLabel;
if (needsQuote(Label))
Label = QuotedLabel = quote(Label);
uint64_t Micros = std::chrono::duration_cast<std::chrono::microseconds>(
std::chrono::steady_clock::now() - Start)
std::lock_guard<std::mutex> Lock(Mu);
Out << llvm::formatv("{0},{1},{2},{3:e},{4}.{5:6}\r\n",
typeName(Metric.Type), Metric.Name, Label, Value,
Micros / 1000000, Micros % 1000000);
llvm::StringRef typeName(Metric::MetricType T) {
switch (T) {
case Metric::Value:
return "v";
case Metric::Counter:
return "c";
case Metric::Distribution:
return "d";
llvm_unreachable("Unknown Metric::MetricType enum");
static bool needsQuote(llvm::StringRef Text) {
// section 2.6
return Text.find_first_of(",\"\r\n") != llvm::StringRef::npos;
std::string quote(llvm::StringRef Text) {
std::string Result = "\"";
for (char C : Text) {
if (C == '"')
return Result;
std::mutex Mu;
llvm::raw_ostream &Out /*GUARDED_BY(Mu)*/;
std::chrono::steady_clock::time_point Start;
Key<std::unique_ptr<JSONTracer::JSONSpan>> JSONTracer::SpanKey;
EventTracer *T = nullptr;
} // namespace
Session::Session(EventTracer &Tracer) {
assert(!T && "Resetting global tracer is not allowed.");
T = &Tracer;
Session::~Session() { T = nullptr; }
std::unique_ptr<EventTracer> createJSONTracer(llvm::raw_ostream &OS,
bool Pretty) {
return std::make_unique<JSONTracer>(OS, Pretty);
std::unique_ptr<EventTracer> createCSVMetricTracer(llvm::raw_ostream &OS) {
return std::make_unique<CSVMetricTracer>(OS);
void log(const llvm::Twine &Message) {
if (!T)
T->instant("Log", llvm::json::Object{{"Message", Message.str()}});
bool enabled() { return T != nullptr; }
// The JSON object is event args (owned by context), if the tracer wants them.
static std::pair<Context, llvm::json::Object *>
makeSpanContext(llvm::Twine Name, const Metric &LatencyMetric) {
if (!T)
return std::make_pair(Context::current().clone(), nullptr);
llvm::Optional<WithContextValue> WithLatency;
using Clock = std::chrono::high_resolution_clock;
[StartTime = Clock::now(), Name = Name.str(), &LatencyMetric] {
std::chrono::duration_cast<std::chrono::milliseconds>(Clock::now() -
llvm::json::Object *Args = nullptr;
Context Ctx = T->beginSpan(
Name.isSingleStringRef() ? Name.getSingleStringRef()
: llvm::StringRef(Name.str()),
[&](llvm::json::Object *A) {
assert(A && A->empty() && "Invalid AttachDetails() placeholder!");
Args = A;
return std::make_pair(std::move(Ctx), Args);
// Fallback metric that measures latencies for spans without an explicit latency
// metric. Labels are span names.
constexpr Metric SpanLatency("span_latency", Metric::Distribution, "span_name");
// Span keeps a non-owning pointer to the args, which is how users access them.
// The args are owned by the context though. They stick around until the
// beginSpan() context is destroyed, when the tracing engine will consume them.
Span::Span(llvm::Twine Name) : Span(Name, SpanLatency) {}
Span::Span(llvm::Twine Name, const Metric &LatencyMetric)
: Span(makeSpanContext(Name, LatencyMetric)) {}
Span::Span(std::pair<Context, llvm::json::Object *> Pair)
: Args(Pair.second), RestoreCtx(std::move(Pair.first)) {}
Span::~Span() {
if (T)
void Metric::record(double Value, llvm::StringRef Label) const {
if (!T)
assert((LabelName.empty() == Label.empty()) &&
"recording a measurement with inconsistent labeling");
T->record(*this, Value, Label);
Context EventTracer::beginSpan(
llvm::StringRef Name,
llvm::function_ref<void(llvm::json::Object *)> AttachDetails) {
return Context::current().clone();
} // namespace trace
} // namespace clangd
} // namespace clang