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perf: prefetch dispatch buffers and inline timestamp reads #1243

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733cef9
perf: prefetch dispatch buffers before scheduler writes
TaoZQY Jul 1, 2026
860c15e
perf: inline dispatch timestamp counter reads
TaoZQY Jul 1, 2026
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4 changes: 4 additions & 0 deletions src/a2a3/runtime/tensormap_and_ringbuffer/runtime/scheduler/scheduler_dispatch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -161,6 +161,10 @@ SchedulerContext::PublishHandle SchedulerContext::prepare_subtask_to_core(
uint32_t buf_idx = reg_task_id & 1u;
PTO2DispatchPayload &payload = payload_per_core_[core_id][buf_idx];
DeferredCompletionSlab *deferred_slab = &deferred_slab_per_core_[core_id][buf_idx];
// Pull ownership of the per-core dispatch buffers before the hot stores below.
__builtin_prefetch(reinterpret_cast<const char *>(&payload), 1, 3);
__builtin_prefetch(reinterpret_cast<const char *>(&payload) + 64, 1, 3);
__builtin_prefetch(reinterpret_cast<const char *>(deferred_slab), 1, 3);
deferred_slab->count = 0;
deferred_slab->error_code = PTO2_ERROR_NONE;
AsyncCtx async_ctx = AsyncCtx::make(slot_state.task->task_id, deferred_slab);
Comment on lines +165 to 171

@gemini-code-assist gemini-code-assist Bot Jul 1, 2026

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medium

The prefetch for payload only covers the first two cache lines (offsets 0 and 64). However, build_payload unconditionally writes to local_context (offset 384+) and not_ready (offset 504), which reside in cache lines 6 and 7. Prefetching these tail cache lines prevents RFO cache misses on these hot stores.

Additionally, deferred_slab is written to immediately after its prefetch instruction, leaving no execution cycles to hide memory latency. Moving the construction of async_ctx before the writes provides a small execution window to overlap the prefetch.

    // Pull ownership of the per-core dispatch buffers before the hot stores below.
    __builtin_prefetch(reinterpret_cast<const char *>(&payload), 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(&payload) + 64, 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(&payload) + 384, 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(&payload) + 448, 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(deferred_slab), 1, 3);

    // Construct AsyncCtx first to give prefetch instructions some cycles to run
    AsyncCtx async_ctx = AsyncCtx::make(slot_state.task->task_id, deferred_slab);
    deferred_slab->count = 0;
    deferred_slab->error_code = PTO2_ERROR_NONE;

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4 changes: 4 additions & 0 deletions src/a5/runtime/tensormap_and_ringbuffer/runtime/scheduler/scheduler_dispatch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -145,6 +145,10 @@ void SchedulerContext::dispatch_subtask_to_core(
uint32_t buf_idx = reg_task_id & 1u;
PTO2DispatchPayload &payload = payload_per_core_[core_id][buf_idx];
DeferredCompletionSlab *deferred_slab = &deferred_slab_per_core_[core_id][buf_idx];
// Pull ownership of the per-core dispatch buffers before the hot stores below.
__builtin_prefetch(reinterpret_cast<const char *>(&payload), 1, 3);
__builtin_prefetch(reinterpret_cast<const char *>(&payload) + 64, 1, 3);
__builtin_prefetch(reinterpret_cast<const char *>(deferred_slab), 1, 3);
deferred_slab->count = 0;
deferred_slab->error_code = PTO2_ERROR_NONE;
AsyncCtx async_ctx = AsyncCtx::make(slot_state.task->task_id, deferred_slab);
Comment on lines +149 to 155

@gemini-code-assist gemini-code-assist Bot Jul 1, 2026

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medium

The prefetch for payload only covers the first two cache lines (offsets 0 and 64). However, build_payload unconditionally writes to local_context (offset 384+), which resides in cache line 6. Prefetching this tail cache line prevents RFO cache misses on these hot stores.

Additionally, deferred_slab is written to immediately after its prefetch instruction, leaving no execution cycles to hide memory latency. Moving the construction of async_ctx before the writes provides a small execution window to overlap the prefetch.

    // Pull ownership of the per-core dispatch buffers before the hot stores below.
    __builtin_prefetch(reinterpret_cast<const char *>(&payload), 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(&payload) + 64, 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(&payload) + 384, 1, 3);
    __builtin_prefetch(reinterpret_cast<const char *>(deferred_slab), 1, 3);

    // Construct AsyncCtx first to give prefetch instructions some cycles to run
    AsyncCtx async_ctx = AsyncCtx::make(slot_state.task->task_id, deferred_slab);
    deferred_slab->count = 0;
    deferred_slab->error_code = PTO2_ERROR_NONE;

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