Per the QPU-default substrate decree 2026-05-23: the per-dispatch
vkAllocateMemory in dispatch_*_qpu was the biggest single fixable
contributor to QPU dispatch overhead. This pools v3d_buffer
allocations by power-of-2 size class so the second-and-subsequent
dispatch hits a freelist instead of paying ~10-50us of Mesa-V3D7
memory-allocation cost per call.
API additions (v3d_runner.h):
- v3d_runner_acquire_buffer(): pulls from per-bucket freelist;
falls through to v3d_runner_create_buffer() on miss.
- v3d_runner_release_buffer(): pushes back onto the freelist; the
backing VkBuffer/VkDeviceMemory only get vkFreeMemory'd in
v3d_runner_destroy().
- v3d_runner_pool_total_bytes(): diagnostic watermark.
Size classes 2^8..2^23 (256 B to 8 MiB). Oversize requests fall
through to non-pooled (vkAllocateMemory) for both ends — pool stays
correct, just degenerates to old behaviour for those calls.
Migration: daedalus_core.c dispatch_*_qpu paths globally swap
create_buffer → acquire_buffer and destroy_buffer → release_buffer.
All five QPU dispatch functions (idct8 / lpf / mc_8h / cdef /
h264_deblock) now reuse buffers across calls. test_api_idct stays
bit-exact (4096/4096 bytes on CPU/QPU/AUTO substrates on hertz).
Microbench (tests/bench_pool_overhead.c) on hertz (Pi 5,
6.18.29+rpt-rpi-2712, V3D 7.1):
call 0: 434.89 us (cold — 3x vkAllocateMemory)
call 1: 100.06 us (pool hit on all 3 buffers)
steady-state:
p50: 76.44 us
p90: 90.52 us
mean: 77.95 us
first-call / steady-state ratio: 5.7x
The remaining ~76us steady-state is dominated by vkQueueWaitIdle +
shader execution + per-call descriptor-set update + command-buffer
allocation — addressed in follow-on tasks 161 (persistent cmdbuf)
and 162 (dmabuf import for dst, eliminates memcpy in/out).
Refs daedalus-fourier task #160.
claude-noether
0a042a8e95