From 1cef9c2e4bd873ebe13f5df635899231bcc467d2 Mon Sep 17 00:00:00 2001 From: Gabriel Schneider Date: Tue, 25 Aug 2026 18:41:18 -0300 Subject: A measuring instrument, and what it says about where the latency goes "Too slow for interactive use" is a real complaint and not a number. This adds the number, and the number says the wire is innocent. ## The instrument `tools/perfproto.zig` is a small framed protocol - "P4", op, length, CRC-32 of the payload, payload - shared VERBATIM by the host tool and `examples/uartperf.zig`, so a frame one writes and the other parses cannot drift. It is imported as a module by both, not copied. The checksum is the whole point. RX overrun on this UART is undetected in hardware and uncounted in the driver, so a byte that never arrived is indistinguishable from a late one; a throughput figure that is not checksummed is a guess about how fast data was corrupted. `sink` accumulates a CRC over every payload byte the board received and `report` hands it back, so the host can prove that what arrived is what it sent. `tools/rtt.zig` is the two timing functions: `roundTrip` and `measure`. Round trip is to the FIRST response byte, deliberately. A renderer that starts drawing in 8 ms and finishes in 130 ms feels immediate; one that thinks for 130 ms and then draws in 8 ms feels broken; waiting for the wire to fall quiet cannot tell them apart. Time to the last byte is recorded separately as `settle`. Microseconds, because at 115200 one byte is 87 us and a millisecond clock quantises the answer into buckets eleven bytes wide. `tools/bench_main.zig` is `p4-bench`: `--link` for the ceiling, `--editor` for how much of it the editor uses, `--sweep` for one controlled variable at a time with `--csv` raw per-trial output. ## What it measured The link is essentially perfect: 11,496 B/s up and 11,413 B/s down, 99.8% of capacity in both directions, CRC verified over 32,768 B each way, zero corruption. Typing at 6 to 100 keys/s loses nothing and never uses more than 9% of the wire, so H5 - "typing loses input" - is refuted. Latency is compute per input event, not transmission. A 40-byte motion and a 206-byte insert-and-escape cost the SAME round trip to within 0.3 ms, across a five-fold range of output. That is why raising the baud cannot fix typing: there is almost no wire in it. And an edit costs the whole document. Round trip against characters already in the line is a straight line at 54.3 us per character per keystroke - 17.0 ms at an empty line, 25.6 ms at 160. On a ~90 MHz core that is ~5,000 cycles per character, far more than a copy alone, so the full-buffer copy the source does is accompanied by at least one more full pass. One controlled intervention: building the editor object ReleaseFast instead of ReleaseSmall cuts the fixed cost 13% and the per-character cost 36%, for 35% more flash (809,536 B of a 1,536,000 B partition). Its advantage grows with the document. Nothing else measured comes close to that ratio. ## Three bugs found while building it The responder printed garbage and looked dead: it read `.rodata` before evicting the bootloader's stale cache lines. `flushFlashCache` moved from `src/pardes/app.zig` to `soc.zig` with its measured evidence, since every application that touches `.rodata` after hand-over needs it and exactly one file knew that. Then it booted, printed its marker and went silent after ten seconds: `rst:0x10 (CHIP_LP_WDT_RESET)`. The bootloader arms the RTC watchdog and expects the application to take it over. Only the editor ever did. `serial.Port.drain()` drains INPUT, not output - so timing a transfer to it reported 202% of the wire's capacity and ate the reply. Added `flushOutput` (tcdrain), named so the two cannot be confused again. Also: Zig 0.16 emits an explicit `+` for a non-negative SIGNED integer whenever a width is given (std/Io/Writer.zig:1548-1559), which put a `+` in front of every number in the first tables. ## The report `experiments/report.typ` reads the raw CSVs and computes its own figures, so a re-run changes the document instead of contradicting it. It states five hypotheses, settles each against one experiment, and is explicit about the one that failed: the geometry sweep is confounded, because characters accumulated across conditions and the length experiment then proved that matters. It is reported as unsupported rather than dressed up as a result. --- experiments/length-ReleaseFast.csv | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 experiments/length-ReleaseFast.csv (limited to 'experiments/length-ReleaseFast.csv') diff --git a/experiments/length-ReleaseFast.csv b/experiments/length-ReleaseFast.csv new file mode 100644 index 0000000..043fb83 --- /dev/null +++ b/experiments/length-ReleaseFast.csv @@ -0,0 +1,36 @@ +label,experiment,cols,rows,length,op,rep,rtt_us,settle_us,bytes +ReleaseFast,length,0,0,0,insert,0,15072,26542,141 +ReleaseFast,length,0,0,0,insert,1,14793,20959,80 +ReleaseFast,length,0,0,0,insert,2,14767,20944,81 +ReleaseFast,length,0,0,0,insert,3,14794,20970,81 +ReleaseFast,length,0,0,0,insert,4,14808,20937,81 +ReleaseFast,length,0,0,0,insert,5,14875,20991,81 +ReleaseFast,length,0,0,0,insert,6,14870,20971,81 +ReleaseFast,length,0,0,20,insert,0,15305,21531,81 +ReleaseFast,length,0,0,20,insert,1,15317,21534,81 +ReleaseFast,length,0,0,20,insert,2,15425,21719,81 +ReleaseFast,length,0,0,20,insert,3,15408,21759,81 +ReleaseFast,length,0,0,20,insert,4,15513,21702,81 +ReleaseFast,length,0,0,20,insert,5,15821,28801,158 +ReleaseFast,length,0,0,20,insert,6,15740,21843,80 +ReleaseFast,length,0,0,40,insert,0,16198,22320,81 +ReleaseFast,length,0,0,40,insert,1,16261,22352,81 +ReleaseFast,length,0,0,40,insert,2,16229,22505,81 +ReleaseFast,length,0,0,40,insert,3,16309,22374,81 +ReleaseFast,length,0,0,40,insert,4,16312,22466,81 +ReleaseFast,length,0,0,40,insert,5,16289,22563,81 +ReleaseFast,length,0,0,40,insert,6,16341,22625,81 +ReleaseFast,length,0,0,80,insert,0,17772,23963,81 +ReleaseFast,length,0,0,80,insert,1,17803,23933,81 +ReleaseFast,length,0,0,80,insert,2,17817,24108,81 +ReleaseFast,length,0,0,80,insert,3,17818,23893,81 +ReleaseFast,length,0,0,80,insert,4,17917,24037,81 +ReleaseFast,length,0,0,80,insert,5,17919,24158,81 +ReleaseFast,length,0,0,80,insert,6,17920,24078,81 +ReleaseFast,length,0,0,160,insert,0,20271,26409,81 +ReleaseFast,length,0,0,160,insert,1,20230,26412,81 +ReleaseFast,length,0,0,160,insert,2,20249,26522,81 +ReleaseFast,length,0,0,160,insert,3,20302,26487,81 +ReleaseFast,length,0,0,160,insert,4,20648,33557,158 +ReleaseFast,length,0,0,160,insert,5,20563,26664,80 +ReleaseFast,length,0,0,160,insert,6,20699,26920,81 -- cgit v1.3