[lldb-dap] Implement a MemoryMonitor #129332

JDevlieghere · 2025-02-28T22:18:54Z

This implements a memory monitor for macOS & Linux, and registers a callback that invokes SBDebugger::MemoryPressureDetected when a low memory event is detected. This is motivated by the new server mode, where the lldb-dap process will live across multiple debug sessions and will use more memory due to caching.

This implements a memory monitor for macOS & Linux, and registers a callback that invokes SBDebugger::MemoryPressureDetected() when a low memory event is detected.

llvmbot · 2025-02-28T22:19:31Z

@llvm/pr-subscribers-lldb

Author: Jonas Devlieghere (JDevlieghere)

Changes

This implements a memory monitor for macOS & Linux, and registers a callback that invokes SBDebugger::MemoryPressureDetected() when a low memory event is detected.

Full diff: https://github.com/llvm/llvm-project/pull/129332.diff

4 Files Affected:

(modified) lldb/tools/lldb-dap/CMakeLists.txt (+1)
(added) lldb/tools/lldb-dap/MemoryMonitor.cpp (+114)
(added) lldb/tools/lldb-dap/MemoryMonitor.h (+41)
(modified) lldb/tools/lldb-dap/lldb-dap.cpp (+15-2)

diff --git a/lldb/tools/lldb-dap/CMakeLists.txt b/lldb/tools/lldb-dap/CMakeLists.txt
index 8b3c520ec4360..8db377e31c3c6 100644
--- a/lldb/tools/lldb-dap/CMakeLists.txt
+++ b/lldb/tools/lldb-dap/CMakeLists.txt
@@ -36,6 +36,7 @@ add_lldb_tool(lldb-dap
   RunInTerminal.cpp
   SourceBreakpoint.cpp
   Watchpoint.cpp
+  MemoryMonitor.cpp
 
   Handler/ResponseHandler.cpp
   Handler/AttachRequestHandler.cpp
diff --git a/lldb/tools/lldb-dap/MemoryMonitor.cpp b/lldb/tools/lldb-dap/MemoryMonitor.cpp
new file mode 100644
index 0000000000000..da3da42fe9b0f
--- /dev/null
+++ b/lldb/tools/lldb-dap/MemoryMonitor.cpp
@@ -0,0 +1,114 @@
+//===-- MemoryMonitor.cpp -------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "MemoryMonitor.h"
+#include "llvm/ADT/ScopeExit.h"
+#include <atomic>
+#include <cstdio>
+#include <cstring>
+#include <thread>
+
+#if defined(__APPLE__)
+#include <dispatch/dispatch.h>
+#endif
+
+#if defined(__linux__)
+#include <fcntl.h>
+#include <poll.h>
+#include <unistd.h>
+#endif
+
+using namespace lldb_dap;
+
+#if defined(__APPLE__)
+class MemoryMonitorDarwin : public MemoryMonitor {
+  using MemoryMonitor::MemoryMonitor;
+  void Start() override {
+    m_memory_pressure_source = dispatch_source_create(
+        DISPATCH_SOURCE_TYPE_MEMORYPRESSURE,
+        0, // system-wide monitoring
+        DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL,
+        dispatch_get_main_queue());
+
+    dispatch_source_set_event_handler(m_memory_pressure_source, ^{
+      dispatch_source_memorypressure_flags_t pressureLevel =
+          dispatch_source_get_data(m_memory_pressure_source);
+      if (pressureLevel &
+          (DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL)) {
+        m_callback();
+      }
+    });
+  }
+
+  void Stop() override { dispatch_source_cancel(m_memory_pressure_source); }
+
+private:
+  dispatch_source_t m_memory_pressure_source;
+};
+#endif
+
+#if defined(__linux__)
+static void MonitorThread(std::atomic<bool> &done,
+                          MemoryMonitor::Callback callback) {
+  struct pollfd fds;
+  fds.fd = open("/proc/pressure/memory", O_RDWR | O_NONBLOCK);
+  if (fds.fd < 0)
+    return;
+  fds.events = POLLPRI;
+
+  auto cleanup = llvm::make_scope_exit([&]() { close(fds.fd); });
+
+  // Detect a 50ms stall in a 2 second time window.
+  const char trig[] = "some 50000 2000000";
+  if (write(fds.fd, trig, strlen(trig) + 1) < 0)
+    return;
+
+  while (!done) {
+    int n = poll(&fds, 1, 1000);
+    if (n > 0) {
+      if (fds.revents & POLLERR)
+        return;
+      if (fds.revents & POLLPRI)
+        callback();
+    }
+  }
+}
+
+class MemoryMonitorLinux : public MemoryMonitor {
+public:
+  using MemoryMonitor::MemoryMonitor;
+
+  void Start() override {
+    m_memory_pressure_thread =
+        std::thread(MonitorThread, std::ref(m_done), m_callback);
+  }
+
+  void Stop() override {
+    if (m_memory_pressure_thread.joinable()) {
+      m_done = true;
+      m_memory_pressure_thread.join();
+    }
+  }
+
+private:
+  std::atomic<bool> m_done = false;
+  std::thread m_memory_pressure_thread;
+};
+#endif
+
+std::unique_ptr<MemoryMonitor> MemoryMonitor::Create(Callback callback) {
+#if defined(__APPLE__)
+  return std::make_unique<MemoryMonitorDarwin>(callback);
+#endif
+
+#if defined(__linux__)
+  return std::make_unique<MemoryMonitorLinux>(callback);
+#endif
+
+  return nullptr;
+}
diff --git a/lldb/tools/lldb-dap/MemoryMonitor.h b/lldb/tools/lldb-dap/MemoryMonitor.h
new file mode 100644
index 0000000000000..e07c3bde9e85c
--- /dev/null
+++ b/lldb/tools/lldb-dap/MemoryMonitor.h
@@ -0,0 +1,41 @@
+//===-- MemoryMonitor.h ---------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLDB_TOOLS_LLDB_DAP_WATCHPOINT_H
+#define LLDB_TOOLS_LLDB_DAP_WATCHPOINT_H
+
+#include <functional>
+#include <memory>
+
+namespace lldb_dap {
+
+class MemoryMonitor {
+public:
+  using Callback = std::function<void()>;
+
+  MemoryMonitor(Callback callback) : m_callback(callback) {}
+  virtual ~MemoryMonitor() = default;
+
+  /// MemoryMonitor is not copyable.
+  /// @{
+  MemoryMonitor(const MemoryMonitor &) = delete;
+  MemoryMonitor &operator=(const MemoryMonitor &) = delete;
+  /// @}
+
+  static std::unique_ptr<MemoryMonitor> Create(Callback callback);
+
+  virtual void Start() = 0;
+  virtual void Stop() = 0;
+
+protected:
+  Callback m_callback;
+};
+
+} // namespace lldb_dap
+
+#endif
diff --git a/lldb/tools/lldb-dap/lldb-dap.cpp b/lldb/tools/lldb-dap/lldb-dap.cpp
index d005eccfae903..41405df548da2 100644
--- a/lldb/tools/lldb-dap/lldb-dap.cpp
+++ b/lldb/tools/lldb-dap/lldb-dap.cpp
@@ -9,7 +9,9 @@
 #include "DAP.h"
 #include "EventHelper.h"
 #include "Handler/RequestHandler.h"
+#include "MemoryMonitor.h"
 #include "RunInTerminal.h"
+#include "lldb/API/SBDebugger.h"
 #include "lldb/API/SBStream.h"
 #include "lldb/Host/Config.h"
 #include "lldb/Host/File.h"
@@ -504,9 +506,20 @@ int main(int argc, char *argv[]) {
     return EXIT_FAILURE;
   }
 
+  // Create a memory monitor. This can return nullptr if the host platform is
+  // not supported.
+  std::unique_ptr<MemoryMonitor> memory_monitor = MemoryMonitor::Create(
+      []() { lldb::SBDebugger::MemoryPressureDetected(); });
+
+  if (memory_monitor)
+    memory_monitor->Start();
+
   // Terminate the debugger before the C++ destructor chain kicks in.
-  auto terminate_debugger =
-      llvm::make_scope_exit([] { lldb::SBDebugger::Terminate(); });
+  auto terminate_debugger = llvm::make_scope_exit([&] {
+    if (memory_monitor)
+      memory_monitor->Stop();
+    lldb::SBDebugger::Terminate();
+  });
 
   std::vector<std::string> pre_init_commands;
   for (const std::string &arg :

lldb/tools/lldb-dap/MemoryMonitor.cpp

ashgti

Looks great!

vogelsgesang

Code looks good :)

Out of curiosity: What motivated you to make this change?
Maybe you might also want to add that reasoning to the commit message, might be useful to future contributors who want to understand your original motivations in the future

JDevlieghere · 2025-03-04T01:24:48Z

I moved the MemoryMonitor into Host. As I was testing this on macOS with the memory_pressure utility, I noticed that the libdispatch call would fail consistently. I was afraid this was going to happen as I'm pretty sure you're not supposed to call this from anything but Objective-C(++). Anyway, I didn't want to mimic the complexity of the Host library for the lldb-dap tool so I decided to just move it in there, as we're already relying on a bunch of other host classes.

macOS now has it's own implementation based on libdispatch and Linux uses a polling approach. I made an attempt to make this work on Windows too, but I don't have a Windows setup handy to test this on, so I'm not even sure it compiles. If someone does, I would appreciate it if you could confirm this builds and maybe even try it out with testlimit.

ashgti

LGTM!

walter-erquinigo · 2025-03-04T23:37:19Z

This looks very nice. Just out of curiosity, is something printed on the terminal/console whenever one of these low memory events happen?

JDevlieghere · 2025-03-05T06:59:00Z

This looks very nice. Just out of curiosity, is something printed on the terminal/console whenever one of these low memory events happen?

No, this should be transparent to the user. Removing the orphaned modules might not be enough to get out of this situation, so the callback might be called repeatedly. I've added a log line so we can see when this happens that way.

walter-erquinigo · 2025-03-04T23:33:58Z

lldb/source/Host/common/MemoryMonitor.cpp

+
+  lldb::thread_result_t MonitorThread() {
+#if defined(__linux__)
+    struct pollfd fds;


do you need to zero out this struct?

walter-erquinigo · 2025-03-04T23:35:50Z

lldb/source/Host/common/MemoryMonitor.cpp

+    while (!m_done) {
+      int n = poll(&fds, 1, g_timeout);
+      if (n > 0) {
+        if (fds.revents & POLLERR)


if there's an error, is there a way to restart the loop?

Motivated by #129332, which added a new .mm file that didn't get autosynced.

This implements a memory monitor for macOS, Linux and Windows. It registers a callback that invokes `SBDebugger::MemoryPressureDetected` when a low memory event is detected. This is motivated by the new server mode, where the lldb-dap process will live across multiple debug sessions and will use more memory due to caching.

Motivated by llvm#129332, which added a new .mm file that didn't get autosynced.

[lldb-dap] Implement a MemoryMonitor for macOS & Linux

c63350d

This implements a memory monitor for macOS & Linux, and registers a callback that invokes SBDebugger::MemoryPressureDetected() when a low memory event is detected.

JDevlieghere requested review from ashgti, labath, vogelsgesang and ZequanWu February 28, 2025 22:18

llvmbot added lldb lldb-dap labels Feb 28, 2025

ashgti reviewed Feb 28, 2025

View reviewed changes

lldb/tools/lldb-dap/MemoryMonitor.cpp Outdated Show resolved Hide resolved

lldb/tools/lldb-dap/MemoryMonitor.cpp Outdated Show resolved Hide resolved

Use the dispatch_get_global_queue and call dispatch_release

5959487

ashgti approved these changes Feb 28, 2025

View reviewed changes

ashgti mentioned this pull request Feb 28, 2025

[lldb-dap] lldb-dap server mode management improvements #129283

Open

vogelsgesang approved these changes Mar 1, 2025

View reviewed changes

JDevlieghere added 2 commits March 3, 2025 17:02

Move MemoryMonitor into Host

0563235

Use NativeThread and attempt a Windows implementation

9209e10

ashgti approved these changes Mar 4, 2025

View reviewed changes

JDevlieghere added 2 commits March 4, 2025 22:43

Include windows.h instead of memoryapi.h and synchapi.h

f427808

Log when memory pressure is detected

ad1e211

JDevlieghere changed the title ~~[lldb-dap] Implement a MemoryMonitor for macOS & Linux~~ [lldb-dap] Implement a MemoryMonitor Mar 5, 2025

JDevlieghere merged commit 6c4febe into llvm:main Mar 5, 2025
10 checks passed

JDevlieghere deleted the memory-monitor branch March 5, 2025 16:49

walter-erquinigo reviewed Mar 6, 2025

View reviewed changes

nico added a commit that referenced this pull request Mar 7, 2025

[gn] Make sync script sync .mm files too

e5c5e62

Motivated by #129332, which added a new .mm file that didn't get autosynced.

jph-13 pushed a commit to jph-13/llvm-project that referenced this pull request Mar 21, 2025

[gn] Make sync script sync .mm files too

2c3d031

Motivated by llvm#129332, which added a new .mm file that didn't get autosynced.

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