Round out process collector

Also improve implementation comments

CMakeLists.txt

@@ -114,6 +114,7 @@ set(SOURCES
     src/arena_allocator.cpp
     src/format.cpp
     src/metric.cpp
+    src/process_collector.cpp
     ${CMAKE_BINARY_DIR}/json_tokens.cpp)
 
 add_executable(weaseldb ${SOURCES})

benchmarks/bench_format_comparison.cpp

@@ -15,7 +15,8 @@
 
 // Test data for consistent benchmarks
 constexpr int TEST_INT = 42;
-constexpr double TEST_DOUBLE = 3.14159;
+constexpr double TEST_DOUBLE =
+    3.141592653589793; // Exact IEEE 754 representation of π
 const std::string TEST_STRING = "Hello World";
 
 // Benchmark simple string concatenation: "Hello " + "World" + "!"
@@ -23,20 +24,21 @@ void benchmark_simple_concatenation() {
   std::cout << "\n=== Simple String Concatenation: 'Hello World!' ===\n";
 
   ankerl::nanobench::Bench bench;
-  bench.title("Simple Concatenation").unit("op").warmup(100).epochs(1000);
+  bench.title("Simple Concatenation").unit("op").warmup(100);
 
+  ArenaAllocator arena(64);
   // Arena-based static_format
   bench.run("static_format", [&] {
-    ArenaAllocator arena(64);
     auto result = static_format(arena, "Hello ", "World", "!");
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // Arena-based format
   bench.run("format", [&] {
-    ArenaAllocator arena(64);
     auto result = format(arena, "Hello %s!", "World");
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // std::stringstream
@@ -54,16 +56,6 @@ void benchmark_simple_concatenation() {
     ankerl::nanobench::doNotOptimizeAway(result);
   });
 #endif
-
-  // Raw snprintf with malloc
-  bench.run("snprintf + malloc", [&] {
-    char buffer[64];
-    int len = std::snprintf(buffer, sizeof(buffer), "Hello %s!", "World");
-    char *result = static_cast<char *>(std::malloc(len + 1));
-    std::memcpy(result, buffer, len + 1);
-    ankerl::nanobench::doNotOptimizeAway(result);
-    std::free(result);
-  });
 }
 
 // Benchmark mixed type formatting: "Count: 42, Rate: 3.14159"
@@ -71,21 +63,22 @@ void benchmark_mixed_types() {
   std::cout << "\n=== Mixed Type Formatting: 'Count: 42, Rate: 3.14159' ===\n";
 
   ankerl::nanobench::Bench bench;
-  bench.title("Mixed Types").unit("op").warmup(100).epochs(1000);
+  bench.title("Mixed Types").unit("op").warmup(100);
 
+  ArenaAllocator arena(128);
   // Arena-based static_format
   bench.run("static_format", [&] {
-    ArenaAllocator arena(128);
     auto result =
         static_format(arena, "Count: ", TEST_INT, ", Rate: ", TEST_DOUBLE);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // Arena-based format
   bench.run("format", [&] {
-    ArenaAllocator arena(128);
     auto result = format(arena, "Count: %d, Rate: %.5f", TEST_INT, TEST_DOUBLE);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // std::stringstream
@@ -104,17 +97,6 @@ void benchmark_mixed_types() {
     ankerl::nanobench::doNotOptimizeAway(result);
   });
 #endif
-
-  // Raw snprintf with malloc
-  bench.run("snprintf + malloc", [&] {
-    char buffer[128];
-    int len = std::snprintf(buffer, sizeof(buffer), "Count: %d, Rate: %.5f",
-                            TEST_INT, TEST_DOUBLE);
-    char *result = static_cast<char *>(std::malloc(len + 1));
-    std::memcpy(result, buffer, len + 1);
-    ankerl::nanobench::doNotOptimizeAway(result);
-    std::free(result);
-  });
 }
 
 // Benchmark complex formatting with precision and alignment
@@ -122,14 +104,15 @@ void benchmark_complex_formatting() {
   std::cout << "\n=== Complex Formatting: '%-10s %5d %8.2f' ===\n";
 
   ankerl::nanobench::Bench bench;
-  bench.title("Complex Formatting").unit("op").warmup(100).epochs(1000);
+  bench.title("Complex Formatting").unit("op").warmup(100);
 
+  ArenaAllocator arena(128);
   // Arena-based format (static_format doesn't support printf specifiers)
   bench.run("format", [&] {
-    ArenaAllocator arena(128);
     auto result = format(arena, "%-10s %5d %8.2f", TEST_STRING.c_str(),
                          TEST_INT, TEST_DOUBLE);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // std::stringstream
@@ -150,17 +133,6 @@ void benchmark_complex_formatting() {
     ankerl::nanobench::doNotOptimizeAway(result);
   });
 #endif
-
-  // Raw snprintf with malloc
-  bench.run("snprintf + malloc", [&] {
-    char buffer[128];
-    int len = std::snprintf(buffer, sizeof(buffer), "%-10s %5d %8.2f",
-                            TEST_STRING.c_str(), TEST_INT, TEST_DOUBLE);
-    char *result = static_cast<char *>(std::malloc(len + 1));
-    std::memcpy(result, buffer, len + 1);
-    ankerl::nanobench::doNotOptimizeAway(result);
-    std::free(result);
-  });
 }
 
 // Benchmark error message formatting (common use case)
@@ -169,26 +141,27 @@ void benchmark_error_messages() {
                "(line 123)' ===\n";
 
   ankerl::nanobench::Bench bench;
-  bench.title("Error Messages").unit("op").warmup(100).epochs(1000);
+  bench.title("Error Messages").unit("op").warmup(100);
 
   constexpr int error_code = 404;
   constexpr int line_number = 123;
   const std::string error_msg = "File not found";
 
+  ArenaAllocator arena(128);
   // Arena-based static_format (using string literals only)
   bench.run("static_format", [&] {
-    ArenaAllocator arena(128);
     auto result = static_format(arena, "Error ", error_code, ": ",
                                 "File not found", " (line ", line_number, ")");
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // Arena-based format
   bench.run("format", [&] {
-    ArenaAllocator arena(128);
     auto result = format(arena, "Error %d: %s (line %d)", error_code,
                          error_msg.c_str(), line_number);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
   // std::stringstream
@@ -210,37 +183,73 @@ void benchmark_error_messages() {
 #endif
 }
 
-// Benchmark memory allocation overhead by testing arena reuse
-void benchmark_memory_reuse() {
-  std::cout << "\n=== Memory Allocation Patterns ===\n";
+// Benchmark simple double formatting (common in metrics)
+void benchmark_double_formatting() {
+  std::cout << "\n=== Simple Double Formatting ===\n";
+
+  // Validate that all formatters produce identical output
+  ArenaAllocator arena(128);
+
+  auto static_result = static_format(arena, TEST_DOUBLE);
+  auto format_result = format(arena, "%.17g", TEST_DOUBLE);
+
+  std::stringstream ss;
+  ss << std::setprecision(17) << TEST_DOUBLE;
+  auto stringstream_result = ss.str();
+
+#if HAS_STD_FORMAT
+  auto std_format_result = std::format("{}", TEST_DOUBLE);
+#endif
+
+  std::cout << "Validation (note: precision algorithms may differ):\n";
+  std::cout << "  static_format:   '" << static_result
+            << "' (length: " << static_result.length() << ")\n";
+  std::cout << "  format(%.17g):   '" << format_result
+            << "' (length: " << format_result.length() << ")\n";
+  std::cout << "  std::stringstream: '" << stringstream_result
+            << "' (length: " << stringstream_result.length() << ")\n";
+#if HAS_STD_FORMAT
+  std::cout << "  std::format:     '" << std_format_result
+            << "' (length: " << std_format_result.length() << ")\n";
+#endif
+
+  std::cout
+      << "Note: Different formatters may use different precision algorithms\n";
+  std::cout << "Proceeding with performance comparison...\n";
 
   ankerl::nanobench::Bench bench;
-  bench.title("Memory Patterns").unit("op").warmup(100).epochs(100);
+  bench.title("Double Formatting").unit("op").warmup(100);
 
-  // Arena with fresh allocation each time (realistic usage)
-  bench.run("fresh arena", [&] {
-    ArenaAllocator arena(128);
-    auto result = format(arena, "Test %d: %s %.2f", TEST_INT,
-                         TEST_STRING.c_str(), TEST_DOUBLE);
+  // Arena-based static_format (double only)
+  bench.run("static_format(double)", [&] {
+    auto result = static_format(arena, TEST_DOUBLE);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
-  // Pre-allocated arena (reuse scenario)
-  ArenaAllocator shared_arena(1024);
-  bench.run("reused arena", [&] {
-    auto result = format(shared_arena, "Test %d: %s %.2f", TEST_INT,
-                         TEST_STRING.c_str(), TEST_DOUBLE);
+  // Arena-based format with equivalent precision
+  bench.run("format(%.17g)", [&] {
+    // Use %.17g to match static_format's full precision behavior
+    auto result = format(arena, "%.17g", TEST_DOUBLE);
     ankerl::nanobench::doNotOptimizeAway(result);
+    arena.reset();
   });
 
-  // Fresh std::string allocations
+  // std::stringstream (full precision)
   bench.run("std::stringstream", [&] {
     std::stringstream ss;
-    ss << "Test " << TEST_INT << ": " << TEST_STRING << " " << std::fixed
-       << std::setprecision(2) << TEST_DOUBLE;
+    ss << std::setprecision(17) << TEST_DOUBLE;
     auto result = ss.str();
     ankerl::nanobench::doNotOptimizeAway(result);
   });
+
+#if HAS_STD_FORMAT
+  // std::format (C++20) - default formatting
+  bench.run("std::format", [&] {
+    auto result = std::format("{}", TEST_DOUBLE);
+    ankerl::nanobench::doNotOptimizeAway(result);
+  });
+#endif
 }
 
 int main() {
@@ -257,7 +266,7 @@ int main() {
   benchmark_mixed_types();
   benchmark_complex_formatting();
   benchmark_error_messages();
-  benchmark_memory_reuse();
+  benchmark_double_formatting();
 
   std::cout << "\n=== Summary ===\n";
   std::cout
@@ -267,8 +276,6 @@ int main() {
   std::cout
       << "* std::stringstream: Flexible but slower due to heap allocation\n";
   std::cout << "* std::format: Modern C++20 alternative (if available)\n";
-  std::cout << "* snprintf + malloc: Low-level but requires manual memory "
-               "management\n";
 
   return 0;
 }

benchmarks/bench_metric.cpp

@@ -158,21 +158,28 @@ int main() {
 
   // Render performance scaling
   {
+    bench.unit("metric");
+    bench.title("render performance");
     // Test render performance as number of metrics increases
-    std::vector<metric::Counter> counters;
-    std::vector<metric::Gauge> gauges;
-    std::vector<metric::Histogram> histograms;
-
-    auto counter_family =
-        metric::create_counter("scale_counter", "Scale counter");
-    auto gauge_family = metric::create_gauge("scale_gauge", "Scale gauge");
-    auto histogram_family = metric::create_histogram(
-        "scale_histogram", "Scale histogram",
-        std::initializer_list<double>{0.1, 0.5, 1.0, 2.5, 5.0, 10.0, 25.0,
-                                      50.0});
 
     // Create varying numbers of metrics
     for (int scale : {10, 100, 1000}) {
+      metric::reset_metrics_for_testing();
+      std::vector<metric::Counter> counters;
+      std::vector<metric::Gauge> gauges;
+      std::vector<metric::Histogram> histograms;
+
+      auto counter_family =
+          metric::create_counter("scale_counter", "Scale counter");
+      auto gauge_family = metric::create_gauge("scale_gauge", "Scale gauge");
+      auto buckets = std::initializer_list<double>{0.1, 0.5,  1.0,  2.5,
+                                                   5.0, 10.0, 25.0, 50.0};
+      auto histogram_family = metric::create_histogram(
+          "scale_histogram", "Scale histogram", buckets);
+
+      std::atomic<double> counter_value{3.1415924654};
+      bench.batch(scale * (/*counter*/ 1 + /*gauge*/ 1 + /*callback*/ 1 +
+                           /*histogram*/ (buckets.size() * 2 + 2)));
       // Clear previous metrics by creating new families
       // (Note: In real usage, metrics persist for application lifetime)
       for (int i = 0; i < scale; ++i) {
@@ -186,6 +193,12 @@ int main() {
         counters.back().inc(static_cast<double>(i));
         gauges.back().set(static_cast<double>(i * 2));
         histograms.back().observe(static_cast<double>(i) * 0.1);
+        // Register callbacks
+        counter_family.register_callback(
+            {{"type", "callback"}, {"id", std::to_string(i)}},
+            [&counter_value]() {
+              return counter_value.load(std::memory_order_relaxed);
+            });
       }
 
       ArenaAllocator arena;
@@ -200,50 +213,5 @@ int main() {
     }
   }
 
-  // Callback metrics performance
-  {
-    auto counter_family =
-        metric::create_counter("callback_counter", "Callback counter");
-    auto gauge_family =
-        metric::create_gauge("callback_gauge", "Callback gauge");
-
-    std::atomic<double> counter_value{0};
-    std::atomic<double> gauge_value{100};
-
-    // Register callbacks
-    counter_family.register_callback(
-        {{"type", "callback"}}, [&counter_value]() {
-          return counter_value.load(std::memory_order_relaxed);
-        });
-
-    gauge_family.register_callback({{"type", "callback"}}, [&gauge_value]() {
-      return gauge_value.load(std::memory_order_relaxed);
-    });
-
-    // Background thread updating callback values
-    std::atomic<bool> stop_callback{false};
-    std::latch start_latch{2}; // Background thread + benchmark thread
-
-    std::thread callback_updater([&]() {
-      start_latch.arrive_and_wait(); // Wait for benchmark to start
-      while (!stop_callback.load()) {
-        counter_value.fetch_add(1);
-        gauge_value.store(gauge_value.load() + 1);
-      }
-    });
-
-    ArenaAllocator arena;
-
-    start_latch.arrive_and_wait(); // Wait for background thread to be ready
-    bench.run("render() - with callback metrics", [&]() {
-      auto output = metric::render(arena);
-      ankerl::nanobench::doNotOptimizeAway(output);
-      arena.reset();
-    });
-
-    stop_callback.store(true);
-    callback_updater.join();
-  }
-
   return 0;
 }

src/connection.cpp

@@ -5,8 +5,29 @@
 #include <cstdio>
 #include <cstdlib>
 
+#include "metric.hpp"
 #include "server.hpp" // Need this for release_back_to_server implementation
 
+namespace {
+// Thread-local metric instances
+thread_local auto connections_total =
+    metric::create_counter("weaseldb_connections_total",
+                           "Total number of connections accepted")
+        .create({});
+thread_local auto connections_active =
+    metric::create_gauge("weaseldb_connections_active",
+                         "Number of currently active connections")
+        .create({});
+thread_local auto bytes_read =
+    metric::create_counter("weaseldb_bytes_read_total",
+                           "Total number of bytes read from clients")
+        .create({});
+thread_local auto bytes_written =
+    metric::create_counter("weaseldb_bytes_written_total",
+                           "Total number of bytes written to clients")
+        .create({});
+} // namespace
+
 // Static thread-local storage for iovec buffer
 static thread_local std::vector<struct iovec> g_iovec_buffer{IOV_MAX};
 
@@ -16,6 +37,11 @@ Connection::Connection(struct sockaddr_storage addr, int fd, int64_t id,
     : fd_(fd), id_(id), epoll_index_(epoll_index), addr_(addr), arena_(),
       handler_(handler), server_(server.weak_from_this()) {
   server.active_connections_.fetch_add(1, std::memory_order_relaxed);
+
+  // Increment connection metrics using thread-local instances
+  connections_total.inc();
+  connections_active.inc();
+
   assert(handler_);
   handler_->on_connection_established(*this);
 }
@@ -27,6 +53,10 @@ Connection::~Connection() {
   if (auto server_ptr = server_.lock()) {
     server_ptr->active_connections_.fetch_sub(1, std::memory_order_relaxed);
   }
+
+  // Decrement active connections gauge
+  connections_active.dec();
+
   int e = close(fd_);
   if (e == -1 && errno != EINTR) {
     perror("close");
@@ -63,11 +93,18 @@ int Connection::readBytes(char *buf, size_t buffer_size) {
     if (r == 0) {
       return -1;
     }
+
+    // Increment bytes read metric
+    if (r > 0) {
+      bytes_read.inc(r);
+    }
+
     return r;
   }
 }
 
 bool Connection::writeBytes() {
+  ssize_t total_bytes_written = 0;
   while (!messages_.empty()) {
     // Build iovec array up to IOV_MAX limit using thread-local vector
     assert(g_iovec_buffer.size() == IOV_MAX);
@@ -93,6 +130,10 @@ bool Connection::writeBytes() {
           continue; // Standard practice: retry on signal interruption
         }
         if (errno == EAGAIN) {
+          // Increment bytes written metric before returning
+          if (total_bytes_written > 0) {
+            bytes_written.inc(total_bytes_written);
+          }
           return false;
         }
         perror("writev");
@@ -102,6 +143,7 @@ bool Connection::writeBytes() {
     }
 
     assert(w > 0);
+    total_bytes_written += w;
 
     // Handle partial writes by updating string_view data/size
     size_t bytes_written = static_cast<size_t>(w);
@@ -123,5 +165,10 @@ bool Connection::writeBytes() {
   }
   assert(messages_.empty());
 
+  // Increment bytes written metric
+  if (total_bytes_written > 0) {
+    bytes_written.inc(total_bytes_written);
+  }
+
   return false;
 }

src/format.cpp

@@ -948,13 +948,7 @@ char *to_chars(char *first, const char *last, double value) {
 } // namespace
 
 namespace detail {
-void DoubleTerm::write(char *&buf) const {
-  char scratch[kMaxLength];
-  char *end = to_chars(scratch, nullptr, s);
-  const auto len = end - scratch;
-  std::memcpy(buf, scratch, static_cast<std::size_t>(len));
-  buf += len;
-}
+void DoubleTerm::write(char *&buf) const { buf = to_chars(buf, nullptr, s); }
 
 } // namespace detail
 

src/format.hpp

@@ -270,7 +270,6 @@ std::string_view static_format(ArenaAllocator &arena, Ts &&...ts) {
   char *buf = result;
   (detail::term(ts).write(buf), ...);
   const int size = static_cast<int>(buf - result);
-  return std::string_view(
-      arena.realloc(result, upper_bound, upper_bound - size),
-      static_cast<std::size_t>(size));
+  return std::string_view(arena.realloc(result, upper_bound, size),
+                          static_cast<std::size_t>(size));
 }

src/main.cpp

@@ -1,10 +1,10 @@
 #include "config.hpp"
 #include "connection.hpp"
-#include "connection_handler.hpp"
 #include "http_handler.hpp"
+#include "metric.hpp"
 #include "perfetto_categories.hpp"
+#include "process_collector.hpp"
 #include "server.hpp"
-#include <atomic>
 #include <csignal>
 #include <cstring>
 #include <fcntl.h>
@@ -176,6 +176,9 @@ int main(int argc, char *argv[]) {
   perfetto::TrackEvent::Register();
 #endif
 
+  // Register the process collector for default metrics.
+  metric::register_collector(std::make_shared<ProcessCollector>());
+
   std::string config_file = "config.toml";
 
   // Parse command line arguments

src/metric.hpp

@@ -20,6 +20,14 @@
 // typical Prometheus client libraries that support multiple registries.
 // This design choice prioritizes simplicity and performance over flexibility.
 //
+// PERFORMANCE NOTE:
+// Family registration operations (create_counter/gauge/histogram), metric
+// instance creation (.create()), and render() use a global mutex for thread
+// safety. Registration operations should be performed during application
+// initialization, not in performance-critical paths. Metric update operations
+// (inc/dec/set/observe) are designed for high-frequency use and do not contend
+// on the global mutex.
+//
 // METRIC LIFECYCLE:
 // Metrics are created once and persist for the application lifetime. There is
 // no unregistration mechanism - this prevents accidental metric loss and
@@ -37,6 +45,7 @@
 
 #include <functional>
 #include <initializer_list>
+#include <memory>
 #include <span>
 #include <type_traits>
 #include <vector>
@@ -223,6 +232,36 @@ bool is_valid_label_value(std::string_view value);
 // when no metric objects are in use and no concurrent render() calls.
 void reset_metrics_for_testing();
 
+/**
+ * @brief Interface for a custom collector that can be registered with the
+ * metrics system.
+ *
+ * This is used for complex metric gathering, such as reading from /proc, where
+ * multiple metrics need to be updated from a single data source.
+ */
+struct Collector {
+  /**
+   * @brief Virtual destructor.
+   */
+  virtual ~Collector() = default;
+
+  /**
+   * @brief Called by the metrics system to update the metrics this collector is
+   * responsible for.
+   */
+  virtual void collect() = 0;
+};
+
+/**
+ * @brief Register a collector with the metrics system.
+ *
+ * The system will hold a shared_ptr to the collector and call its collect()
+ * method during each metric rendering.
+ *
+ * @param collector A shared_ptr to the collector to be registered.
+ */
+void register_collector(std::shared_ptr<Collector> collector);
+
 // Note: Histograms do not support callbacks due to their multi-value nature
 // (buckets + sum + count). Use static histogram metrics only.
 

src/process_collector.cpp

@@ -0,0 +1,239 @@
+#include "process_collector.hpp"
+
+#include <cstdio>
+#include <cstring>
+#include <dirent.h>
+#include <sys/resource.h>
+#include <unistd.h>
+#include <vector>
+
+namespace {
+// Helper function to read the system boot time from /proc/stat.
+// Returns boot time in seconds since epoch, or 0 on error.
+double get_boot_time() {
+  FILE *fp = std::fopen("/proc/stat", "r");
+  if (!fp) {
+    return 0;
+  }
+
+  char line[256];
+  double boot_time = 0;
+  while (std::fgets(line, sizeof(line), fp)) {
+    if (std::strncmp(line, "btime ", 6) == 0) {
+      if (std::sscanf(line + 6, "%lf", &boot_time) != 1) {
+        boot_time = 0;
+      }
+      break;
+    }
+  }
+
+  std::fclose(fp);
+  return boot_time;
+}
+} // namespace
+
+ProcessCollector::ProcessCollector()
+    : cpu_seconds_total_(metric::create_counter(
+                             "process_cpu_seconds_total",
+                             "Total user and system CPU time spent in seconds")
+                             .create({})),
+      resident_memory_bytes_(
+          metric::create_gauge("process_resident_memory_bytes",
+                               "Resident memory size in bytes")
+              .create({})),
+      virtual_memory_bytes_(metric::create_gauge("process_virtual_memory_bytes",
+                                                 "Virtual memory size in bytes")
+                                .create({})),
+      open_fds_(metric::create_gauge("process_open_fds",
+                                     "Number of open file descriptors")
+                    .create({})),
+      max_fds_(metric::create_gauge("process_max_fds",
+                                    "Maximum number of open file descriptors")
+                   .create({})),
+      start_time_seconds_(
+          metric::create_gauge(
+              "process_start_time_seconds",
+              "Start time of the process since unix epoch in seconds")
+              .create({})),
+      threads_(metric::create_gauge("process_threads",
+                                    "Number of OS threads in this process")
+                   .create({})),
+      context_switches_total_voluntary_(
+          metric::create_counter("process_context_switches_total",
+                                 "Total number of context switches")
+              .create({{"type", "voluntary"}})),
+      context_switches_total_nonvoluntary_(
+          metric::create_counter("process_context_switches_total",
+                                 "Total number of context switches")
+              .create({{"type", "nonvoluntary"}})),
+      page_faults_total_minor_(
+          metric::create_counter("process_page_faults_total",
+                                 "Total number of page faults")
+              .create({{"type", "minor"}})),
+      page_faults_total_major_(
+          metric::create_counter("process_page_faults_total",
+                                 "Total number of page faults")
+              .create({{"type", "major"}})) {
+  // Set the constant max_fds metric.
+  struct rlimit rlim;
+  if (getrlimit(RLIMIT_NOFILE, &rlim) == 0) {
+    max_fds_.set(rlim.rlim_cur);
+  }
+
+  // Perform an initial collection to populate the other metrics and set the
+  // initial counter values.
+  collect();
+}
+
+void ProcessCollector::collect() {
+  // --- CPU Time, Memory, and Start Time from /proc/self/stat ---
+  FILE *fp = std::fopen("/proc/self/stat", "r");
+  if (!fp) {
+    return;
+  }
+
+  char buf[2048];
+  if (std::fgets(buf, sizeof(buf), fp) == nullptr) {
+    std::fclose(fp);
+    return;
+  }
+  std::fclose(fp);
+
+  // Find the end of the command name, which is in parentheses
+  const char *stats_start = std::strrchr(buf, ')');
+  if (!stats_start) {
+    return;
+  }
+  stats_start += 2; // Skip the ')' and the space
+
+  // Tokenize the rest of the string
+  std::vector<const char *> stats;
+  char *p = const_cast<char *>(stats_start);
+  while (*p) {
+    stats.push_back(p);
+    while (*p && *p != ' ') {
+      p++;
+    }
+    if (*p) {
+      *p = '\0';
+      p++;
+    }
+  }
+
+  // We need at least 24 fields for rss, and also fields 9,11 for page faults
+  if (stats.size() < 24) {
+    return;
+  }
+
+  long clk_tck = sysconf(_SC_CLK_TCK);
+
+  // --- Page Faults ---
+  unsigned long long minor_faults = std::strtoull(stats[7], nullptr, 10);
+  unsigned long long major_faults = std::strtoull(stats[9], nullptr, 10);
+
+  if (last_minor_faults_ > 0) {
+    if (minor_faults > last_minor_faults_) {
+      page_faults_total_minor_.inc(minor_faults - last_minor_faults_);
+    }
+  } else {
+    page_faults_total_minor_.inc(minor_faults);
+  }
+  last_minor_faults_ = minor_faults;
+
+  if (last_major_faults_ > 0) {
+    if (major_faults > last_major_faults_) {
+      page_faults_total_major_.inc(major_faults - last_major_faults_);
+    }
+  } else {
+    page_faults_total_major_.inc(major_faults);
+  }
+  last_major_faults_ = major_faults;
+
+  // --- CPU Time ---
+  unsigned long long utime_ticks = std::strtoull(stats[11], nullptr, 10);
+  unsigned long long stime_ticks = std::strtoull(stats[12], nullptr, 10);
+  unsigned long long current_total_ticks = utime_ticks + stime_ticks;
+
+  if (last_total_ticks_ > 0) { // If we have a previous value
+    if (current_total_ticks > last_total_ticks_) {
+      double delta_seconds =
+          (double)(current_total_ticks - last_total_ticks_) / clk_tck;
+      cpu_seconds_total_.inc(delta_seconds);
+    }
+  } else { // First run, initialize the counter
+    cpu_seconds_total_.inc((double)current_total_ticks / clk_tck);
+  }
+  last_total_ticks_ = current_total_ticks;
+
+  // --- Memory ---
+  unsigned long long vsize = std::strtoull(stats[20], nullptr, 10);
+  long rss_pages = std::strtol(stats[21], nullptr, 10);
+  virtual_memory_bytes_.set(vsize);
+  resident_memory_bytes_.set(rss_pages * sysconf(_SC_PAGESIZE));
+
+  // --- Start Time (only needs to be set once) ---
+  if (!start_time_set_) {
+    long long start_time_ticks = std::strtoll(stats[19], nullptr, 10);
+    double boot_time = get_boot_time();
+    if (boot_time > 0) {
+      start_time_seconds_.set(boot_time + (double)start_time_ticks / clk_tck);
+      start_time_set_ = true;
+    }
+  }
+
+  // --- File Descriptors ---
+  int fd_count = 0;
+  DIR *dp = opendir("/proc/self/fd");
+  if (dp) {
+    while (readdir(dp) != nullptr) {
+      fd_count++;
+    }
+    closedir(dp);
+    // Subtract 3 for '.', '..', and the opendir handle itself
+    open_fds_.set(fd_count > 3 ? fd_count - 3 : 0);
+  }
+
+  // --- Parse /proc/self/status for additional metrics ---
+  FILE *status_fp = std::fopen("/proc/self/status", "r");
+  if (status_fp) {
+    char status_line[256];
+    while (std::fgets(status_line, sizeof(status_line), status_fp)) {
+      if (std::strncmp(status_line, "Threads:\t", 9) == 0) {
+        int thread_count;
+        if (std::sscanf(status_line + 9, "%d", &thread_count) == 1) {
+          threads_.set(thread_count);
+        }
+      } else if (std::strncmp(status_line, "voluntary_ctxt_switches:\t", 25) ==
+                 0) {
+        unsigned long long voluntary_switches;
+        if (std::sscanf(status_line + 25, "%llu", &voluntary_switches) == 1) {
+          if (last_voluntary_context_switches_ > 0) {
+            if (voluntary_switches > last_voluntary_context_switches_) {
+              context_switches_total_voluntary_.inc(
+                  voluntary_switches - last_voluntary_context_switches_);
+            }
+          } else {
+            context_switches_total_voluntary_.inc(voluntary_switches);
+          }
+          last_voluntary_context_switches_ = voluntary_switches;
+        }
+      } else if (std::strncmp(status_line, "nonvoluntary_ctxt_switches:\t",
+                              29) == 0) {
+        unsigned long long nonvoluntary_switches;
+        if (std::sscanf(status_line + 29, "%llu", &nonvoluntary_switches) ==
+            1) {
+          if (last_nonvoluntary_context_switches_ > 0) {
+            if (nonvoluntary_switches > last_nonvoluntary_context_switches_) {
+              context_switches_total_nonvoluntary_.inc(
+                  nonvoluntary_switches - last_nonvoluntary_context_switches_);
+            }
+          } else {
+            context_switches_total_nonvoluntary_.inc(nonvoluntary_switches);
+          }
+          last_nonvoluntary_context_switches_ = nonvoluntary_switches;
+        }
+      }
+    }
+    std::fclose(status_fp);
+  }
+}

src/process_collector.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include "metric.hpp"
+
+/**
+ * @brief A metric collector for standard process-level statistics.
+ *
+ * Gathers metrics like CPU usage, memory, and file descriptors by reading
+ * files from the /proc filesystem.
+ */
+struct ProcessCollector : public metric::Collector {
+  /**
+   * @brief Constructs the collector and initializes the process metrics.
+   */
+  ProcessCollector();
+
+  /**
+   * @brief Called by the metrics system to update the process metrics.
+   */
+  void collect() override;
+
+private:
+  // Metrics for process statistics
+  metric::Counter cpu_seconds_total_;
+  metric::Gauge resident_memory_bytes_;
+  metric::Gauge virtual_memory_bytes_;
+  metric::Gauge open_fds_;
+  metric::Gauge max_fds_;
+  metric::Gauge start_time_seconds_;
+
+  // Additional process metrics from /proc/self/status
+  metric::Gauge threads_;
+  metric::Counter context_switches_total_voluntary_;
+  metric::Counter context_switches_total_nonvoluntary_;
+
+  // Page fault metrics from /proc/self/stat
+  metric::Counter page_faults_total_minor_;
+  metric::Counter page_faults_total_major_;
+
+  // Last observed values for calculating counter increments
+  unsigned long long last_total_ticks_ = 0;
+  unsigned long long last_minor_faults_ = 0;
+  unsigned long long last_major_faults_ = 0;
+  unsigned long long last_voluntary_context_switches_ = 0;
+  unsigned long long last_nonvoluntary_context_switches_ = 0;
+  bool start_time_set_ = false;
+};

src/server.cpp

@@ -487,6 +487,7 @@ void Server::process_connection_writes(std::unique_ptr<Connection> &conn,
   if ((events & EPOLLOUT) || ((events & EPOLLIN) && conn->hasMessages())) {
     bool had_messages = conn->hasMessages();
     bool error = conn->writeBytes();
+
     if (error) {
       conn.reset(); // Connection should be closed
       return;