MobileGL/docs/archive/notes/perf-p14/README.md

P14: the post-P13 performance pass

P14 measured the record (push) arm against the dev pull monolith and cut its CPU cost. It ran on feat/disaggregated from f8a9466b (start) to 781a7446 (close-out), on a Lenovo Y700 (Adreno 750) with pinned clocks.

• ESTIMATE.md: the code-based estimate, written before the first benchmark.
• BENCH.md: the final trace matrix, the layer split, CPU vs GPU binding, and what the remaining gap is made of.
• HANDOFF.md: harness commands, device state, open items.
• NEXT-draw-packet-cache.md: the design sketch for the next phase.

Headline numbers

Minecraft in FCL (the representative app check)

Setup:

• FCL Minecraft, in world.
• The library is swapped into FCL, which runs it as a plugin build without MOBILEGL_PIPE_POISON.
• Readings come from MobileGL's own fps log.
• Two launches × three steady windows of about 2 s each. Dev Espryt is three windows: its second launch died at start-up, twice.
• Clocks are pinned. Runs: .trace-work/perf-p14/fcl-final/.

backend	dev (pull)	feat monolith	feat inproc	feat monolith, presenting-thread CPU
Espryt	228.6	198.3 (−13 %)	241.5 (+6 %)	2.43 ms/frame (dev 1.85)
Magma	231.8	171.4 (−26 %)	194.8 (−16 %)	2.80 ms/frame (dev 1.64)

Earlier in the pass, at 94705adc, the same measurement read:

• Espryt: monolith about 192, inproc about 233;
• Magma: monolith about 169, inproc about 179;
• dev: 209-245.

FCL Magma monolith did not follow the trace's improvement. Its presenting thread is busy only about 2.8 of the roughly 5.8 ms frame, so the rest is a wait. That is the first item for the next phase: an off-CPU profile of FCL Magma monolith, the same method that found the upload stall in the trace. Two attempts at it during close-out caught no game frames (FCL start-up).

Trace replay, rd12 (Minecraft 1.21.4 RD12, 1471 draws/frame), best of 3, fps

backend	dev (pull)	monolith	inproc	shm	tcp
Espryt	115.2	91.1 (start 69.8)	111.9 (71.1)	87.8 (62.8)	68.7 (50.0)
Magma	87.0	83.8 (start 45.4)	110.0 (63.2)	95.6 (58.6)	69.9 (45.6)

openra is in BENCH.md. On openra, Magma monolith now runs 2.5x faster than dev (747 vs 293), and inproc 3.6x (1052 vs 293).

Every arm is CPU-bound: GPU busy is 19-39 %, and the top thread sits at 94-99 % of a core (70-80 % on tcp).

What landed

The commits run from f8a9466b to 781a7446. Per-commit numbers are in the commit subjects.

• Magma wire draw:
  • no per-draw SPIR-V rehash or allocation;
  • unit sampler memo; unchanged pipeline, vertex and index binds skipped;
  • direct-mapped texture and buffer fronts;
  • descriptor-set rebind with fresh offsets;
  • pass-continuation memo;
  • vertex-input layout kept on its record;
  • texture level uploads no longer wait for the GPU to drain (553eb86d, the largest single gain of the pass: rd12 70 -> 83, openra 367 -> 745);
  • slices stamped from the owner's submit counter.
• Espryt:
  • VAO twin re-points only the moved buffers;
  • clean attachments skip resync;
  • unchanged buffer sets are adopted;
  • twin lookups without SharedPtr copies;
  • one thread-local block;
  • one UBO-ring lookup per draw.
• MG_Pipe client half:
  • lifetime-id slot fronts;
  • per-VAO memo;
  • fill pins;
  • kept VAO bindings on the server;
  • uniform spans;
  • context-values gate;
  • inline process applier;
  • precomputed residual walk, with split copy and stamp loops;
  • memo-answered draw bindings.
• MG_Remote: fault-verdict polling restructured.
  • It first went from every record to every 256 records (549a34bb).
  • The drain now polls before any record the last poll could not have seen (c24b3428). This fixes the regression where a guilty context ran one more record.
  • The apply loop hands its poll to the drain (e058a450).
• Magma hang watch: a marker slot is reused only once its submission's fence completes (ec973fba).
• Tests and CI:
  • TextureUploadBetweenDrawsScenario on every arm, plus inproc sync validation (67f4f311, 86ab2835);
  • submit-time synchronization validation in the run-ahead and cache gates (781a7446);
  • ServerLoopEglLatchTest now holds the race window open.
• Tools:
  • pinned-clock matrix with a per-thread CPU and GPU-busy sampler;
  • layer-split and off-CPU profile helpers;
  • FCL harness;
  • opt-in fps log and transport override properties.

Measured and not landed

• Record-publish batching: publishing a validate's state records with the draw record. It came in within ±2 % on every arm, and it opens a deadlock class (a staging-retirement wait on a written but unpublished record).
• Native TLS (minSdk / ANDROID_PLATFORM 29): +1.7 % on Espryt, noise on Magma (rd12 monolith). This is the user's call.
• Tracker / residual mutation-epoch fast path: estimated at most 0.07 ms from the tracker alone, 0.15-0.25 ms with the residual copies. Each step is below the 2 % bar, and a missed epoch bump renders stale. It is folded into the draw-packet design.