The previous implementation maintained three exponential moving averages
(1/5/15-minute) of CPU busy-fraction, sampled every 5 seconds. This had
two problems:
1. Wrong semantics: the metric was labelled "load_avg" (a Unix concept
measuring run-queue depth), but the tracker actually measures CPU
busy-time as a fraction of FreeRTOS ticks — i.e. utilization, not
load. The name was misleading.
2. Poor responsiveness: the 1-minute EMA (DECAY = exp(-5/60) ≈ 0.920)
has a ~60-second time constant. On a live web panel, this made the
metric appear frozen, especially during short bursts of activity
(e.g. an HTTP request from the panel itself).
This commit replaces the EMA with a 64-sample simple moving average
(SMA) over a compact uint8_t circular buffer. The sample interval is
60 s / 64 = 937,500 µs, so the window covers exactly 60 seconds:
Algorithm:
- Every 937,500 µs (= 60 s / 64), the FreeRTOS tick delta (busy vs
idle ticks on core 0) is computed and stored as a uint8_t
(0–255 = 0–100% utilization).
- The circular buffer holds 64 samples, spanning exactly 60 seconds
(64 × 937,500 µs = 60,000,000 µs). The timer interval is derived
directly from the window size: 60 000 000 / SMA_WINDOW µs.
- The index wraps with a bitwise AND (& 63) instead of modulo, since
64 is a power of 2.
- The running sum is a uint16_t (max 64 × 255 = 16 320, fits easily).
- The result _sma_avg is a volatile float written atomically by the
esp_timer task (core 0) and read from the main loop (core 1).
No spinlock is needed: on Xtensa LX7, a 32-bit aligned float store
is a single instruction.
The 60-second window smooths out short bursts (e.g. WiFi/HTTP spikes)
while reacting to sustained load changes within ~10–15 seconds.
Naming:
- The public API is now getCore0Util() returning a float in [0.0, 1.0].
The name explicitly identifies which core is measured (core 0, which
runs WiFi, MQTT, HTTP, and LwIP — see task_pinning.c).
- JSON keys: "load_avg" (array) → "core0_util" (scalar float, percent)
- HistorySample field: load_avg1_pct → core0_util_pct
- StatsHistory series key: "cpu_load" → "core0_util"
- Web panel label: "Load Avg" → "Core0 Util"
The history snapshot (once per minute) reads the same _sma_avg value,
which at that point represents the rolling average of the last 60
seconds — exactly one history interval.
Valentin V. Bartenev
a9273f02bb