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Switch from rand() to an XOR Shift to favor speed over random quality #89

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Merged
merged 1 commit into from
Sep 17, 2025
Merged

Switch from rand() to an XOR Shift to favor speed over random quality #89

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13 changes: 11 additions & 2 deletions src/AsyncTCP.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -190,6 +190,15 @@ class queue_mutex_guard {
static SimpleIntrusiveList<lwip_tcp_event_packet_t> _async_queue;
static TaskHandle_t _async_service_task_handle = NULL;

static uint32_t _xor_shift_state = 31; // any nonzero seed will do
static uint32_t _xor_shift_next() {
uint32_t x = _xor_shift_state;
x ^= x << 13;
x ^= x >> 17;
x ^= x << 5;
return _xor_shift_state = x;
}

static void _free_event(lwip_tcp_event_packet_t *evpkt) {
if ((evpkt->event == LWIP_TCP_RECV) && (evpkt->recv.pb != nullptr)) {
pbuf_free(evpkt->recv.pb);
Expand Down Expand Up @@ -246,7 +255,7 @@ static inline lwip_tcp_event_packet_t *_get_async_event() {
Let's discard poll events processing using linear-increasing probability curve when queue size grows over 3/4
Poll events are periodic and connection could get another chance next time
*/
if (_async_queue.size() > (rand() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 4 + CONFIG_ASYNC_TCP_QUEUE_SIZE * 3 / 4)) {
if (_async_queue.size() > (_xor_shift_next() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 4 + CONFIG_ASYNC_TCP_QUEUE_SIZE * 3 / 4)) {
_free_event(e);
async_tcp_log_d("discarding poll due to queue congestion");
continue;
Expand Down Expand Up @@ -422,7 +431,7 @@ int8_t AsyncTCP_detail::tcp_poll(void *arg, struct tcp_pcb *pcb) {
{
queue_mutex_guard guard;
// async_tcp_log_d("qs:%u", _async_queue.size());
if (_async_queue.size() > (rand() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 2 + CONFIG_ASYNC_TCP_QUEUE_SIZE / 4)) {
if (_async_queue.size() > (_xor_shift_next() % CONFIG_ASYNC_TCP_QUEUE_SIZE / 2 + CONFIG_ASYNC_TCP_QUEUE_SIZE / 4)) {
async_tcp_log_d("throttling");
return ERR_OK;
}
Expand Down