From 7fc5bfd787794431cc386c6f1be09b42fef51d0e Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Mon, 23 Jun 2025 04:09:34 +0200
Subject: [PATCH] Reduce RAM usage for scheduled tasks (#9180)

---
 esphome/core/scheduler.cpp | 4 ++++
 esphome/core/scheduler.h   | 8 ++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/esphome/core/scheduler.cpp b/esphome/core/scheduler.cpp
index eed222c974..8144435163 100644
--- a/esphome/core/scheduler.cpp
+++ b/esphome/core/scheduler.cpp
@@ -319,13 +319,17 @@ bool HOT Scheduler::cancel_item_(Component *component, const std::string &name,
   return ret;
 }
 uint64_t Scheduler::millis_() {
+  // Get the current 32-bit millis value
   const uint32_t now = millis();
+  // Check for rollover by comparing with last value
   if (now < this->last_millis_) {
+    // Detected rollover (happens every ~49.7 days)
     this->millis_major_++;
     ESP_LOGD(TAG, "Incrementing scheduler major at %" PRIu64 "ms",
              now + (static_cast<uint64_t>(this->millis_major_) << 32));
   }
   this->last_millis_ = now;
+  // Combine major (high 32 bits) and now (low 32 bits) into 64-bit time
   return now + (static_cast<uint64_t>(this->millis_major_) << 32);
 }
 
diff --git a/esphome/core/scheduler.h b/esphome/core/scheduler.h
index 872a8bd6f6..1284bcd4a7 100644
--- a/esphome/core/scheduler.h
+++ b/esphome/core/scheduler.h
@@ -29,12 +29,16 @@ class Scheduler {
 
  protected:
   struct SchedulerItem {
+    // Ordered by size to minimize padding
     Component *component;
-    std::string name;
-    enum Type { TIMEOUT, INTERVAL } type;
     uint32_t interval;
+    // 64-bit time to handle millis() rollover. The scheduler combines the 32-bit millis()
+    // with a 16-bit rollover counter to create a 64-bit time that won't roll over for
+    // billions of years. This ensures correct scheduling even when devices run for months.
     uint64_t next_execution_;
+    std::string name;
     std::function<void()> callback;
+    enum Type : uint8_t { TIMEOUT, INTERVAL } type;
     bool remove;
 
     static bool cmp(const std::unique_ptr<SchedulerItem> &a, const std::unique_ptr<SchedulerItem> &b);