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perf: overlap wait_for_output_drain with render work #105

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Merged
RtlZeroMemory merged 1 commit into from
Feb 27, 2026
Merged

perf: overlap wait_for_output_drain with render work #105

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22 changes: 14 additions & 8 deletions src/core/zr_engine_present.inc
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Original file line number Diff line number Diff line change
Expand Up @@ -464,14 +464,6 @@ zr_result_t engine_present(zr_engine_t* e) {
/* Enforced contract: the per-frame arena is reset exactly once per present. */
zr_arena_reset(&e->arena_frame);

if (e->cfg_runtime.wait_for_output_drain != 0u) {
const int32_t timeout_ms = zr_engine_output_wait_timeout_ms(&e->cfg_runtime);
zr_result_t rc = plat_wait_output_writable(e->plat, timeout_ms);
if (rc != ZR_OK) {
return rc;
}
}

size_t out_len = 0u;
zr_term_state_t final_ts;
zr_diff_stats_t stats;
Expand Down Expand Up @@ -502,6 +494,20 @@ zr_result_t engine_present(zr_engine_t* e) {
}
}

if (out_len != 0u && e->cfg_runtime.wait_for_output_drain != 0u) {
/*
Wait right before flush so CPU diff/render work overlaps terminal drain
from the prior frame.
*/
const int32_t timeout_ms = zr_engine_output_wait_timeout_ms(&e->cfg_runtime);
rc = plat_wait_output_writable(e->plat, timeout_ms);
if (rc != ZR_OK) {
/* Keep reuse conservative when present fails before prev/next commit. */
e->diff_prev_hashes_valid = 0u;
return rc;
}
}

const uint64_t write_start_us = zr_engine_now_us();
rc = zr_engine_present_write(e, out_len);
if (rc != ZR_OK) {
Expand Down
31 changes: 31 additions & 0 deletions tests/unit/test_engine_present_backpressure.c
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,37 @@ ZR_TEST_UNIT(engine_present_wait_for_output_drain_succeeds_when_writable) {
engine_destroy(e);
}

ZR_TEST_UNIT(engine_present_wait_for_output_drain_skips_wait_when_no_flush_needed) {
mock_plat_reset();
mock_plat_set_size(10u, 4u);
mock_plat_set_output_writable(1u);

zr_engine_config_t cfg = zr_engine_config_default();
cfg.limits.out_max_bytes_per_frame = 4096u;
cfg.wait_for_output_drain = 1u;

zr_engine_t* e = NULL;
ZR_ASSERT_EQ_U32(engine_create(&e, &cfg), ZR_OK);
ZR_ASSERT_TRUE(e != NULL);

ZR_ASSERT_EQ_U32(engine_submit_drawlist(e, zr_test_dl_fixture1, (int)zr_test_dl_fixture1_len), ZR_OK);
ZR_ASSERT_EQ_U32(engine_present(e), ZR_OK);
ZR_ASSERT_EQ_U32(mock_plat_wait_output_call_count(), 1u);

/*
No drawlist/event changes for the second present => no terminal flush bytes.
wait_for_output_drain should not gate this no-op frame.
*/
mock_plat_clear_writes();
mock_plat_set_output_writable(0u);

ZR_ASSERT_EQ_U32(engine_present(e), ZR_OK);
ZR_ASSERT_EQ_U32(mock_plat_wait_output_call_count(), 1u);
ZR_ASSERT_EQ_U32((uint32_t)mock_plat_bytes_written_total(), 0u);

engine_destroy(e);
}

ZR_TEST_UNIT(engine_create_wait_for_output_drain_unsupported_fails_early) {
mock_plat_reset();
mock_plat_set_size(10u, 4u);
Expand Down
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