A keystroke is now 8.37 ms, from 16.99; the remaining 6.4 ms is located

Progression on the die, five document lengths x seven trials at each step:

    step                fixed    per char   at 160 chars
    ReleaseSmall      16.99 ms    54.3 us      25.56 ms
    ReleaseFast       14.85 ms    34.7 us      20.30 ms  0.79x
    + ASCII grapheme  14.56 ms    12.0 us      16.46 ms  0.64x
    + ASCII print     14.27 ms     6.9 us      15.36 ms  0.60x
    + shadow grid      8.87 ms     7.3 us      10.02 ms  0.39x
    + byte compare     8.37 ms     7.1 us       9.48 ms  0.37x

Round trip is time to the FIRST response byte: ~1.95 ms of host and USB latency plus
compute. Compute is 6.42 ms and the 4 ms goal needs it under 2.05 ms, so 3.1x remains.

Every microsecond of it is now measured rather than guessed, by stage, on the die:

    our walk of the grid        979 us   this repo's own present()
    vaxis diff + emit          2455 us   vaxis walks all 480 cells, per-cell strings
    pardes Surface rebuild     2000 us   pardes rebuilds every cell every frame
    input parse + edit          250 us

The honest reading is that the two large items are not each other's alternative.
vaxis's diff is redundant work - present() already computes exactly which cells moved,
so emitting ANSI directly from the shadow grid would remove most of that 2.46 ms. But
even a perfect renderer leaves pardes rebuilding a whole Surface per keystroke, and 4 ms
at 40x12 needs that too.

Raising the baud does not move this number. At 115200 an 81-byte reply is 7.0 ms of
wire, but almost none of it lands before the first byte; 921600 takes `settle` from
23 ms to ~16 ms and leaves the round trip where it is. Worth doing, not for this.

1 files changed, 36 insertions, 0 deletions

diff --git a/experiments/length-RFast-fastcmp.csv b/experiments/length-RFast-fastcmp.csv
new file mode 100644
index 0000000..d8937f2
--- /dev/null
+++ b/experiments/length-RFast-fastcmp.csv
@@ -0,0 +1,36 @@
+label,experiment,cols,rows,length,op,rep,rtt_us,settle_us,bytes
+RFast-fastcmp,length,0,0,0,insert,0,8967,20293,141
+RFast-fastcmp,length,0,0,0,insert,1,8359,14491,80
+RFast-fastcmp,length,0,0,0,insert,2,8317,14505,81
+RFast-fastcmp,length,0,0,0,insert,3,8329,14497,81
+RFast-fastcmp,length,0,0,0,insert,4,8405,14681,81
+RFast-fastcmp,length,0,0,0,insert,5,8424,14734,81
+RFast-fastcmp,length,0,0,0,insert,6,8360,14482,81
+RFast-fastcmp,length,0,0,20,insert,0,8409,14602,81
+RFast-fastcmp,length,0,0,20,insert,1,8433,14612,81
+RFast-fastcmp,length,0,0,20,insert,2,8566,14751,81
+RFast-fastcmp,length,0,0,20,insert,3,8490,14723,81
+RFast-fastcmp,length,0,0,20,insert,4,8481,14673,81
+RFast-fastcmp,length,0,0,20,insert,5,8705,21462,158
+RFast-fastcmp,length,0,0,20,insert,6,8589,14632,80
+RFast-fastcmp,length,0,0,40,insert,0,8607,14792,81
+RFast-fastcmp,length,0,0,40,insert,1,8647,14810,81
+RFast-fastcmp,length,0,0,40,insert,2,8671,14958,81
+RFast-fastcmp,length,0,0,40,insert,3,8661,14769,81
+RFast-fastcmp,length,0,0,40,insert,4,8664,14902,81
+RFast-fastcmp,length,0,0,40,insert,5,8674,14772,81
+RFast-fastcmp,length,0,0,40,insert,6,8637,14979,81
+RFast-fastcmp,length,0,0,80,insert,0,9010,15048,81
+RFast-fastcmp,length,0,0,80,insert,1,8965,15295,81
+RFast-fastcmp,length,0,0,80,insert,2,8918,15317,81
+RFast-fastcmp,length,0,0,80,insert,3,9010,15223,81
+RFast-fastcmp,length,0,0,80,insert,4,8974,15153,81
+RFast-fastcmp,length,0,0,80,insert,5,9047,15299,81
+RFast-fastcmp,length,0,0,80,insert,6,9039,15215,81
+RFast-fastcmp,length,0,0,160,insert,0,9439,15670,81
+RFast-fastcmp,length,0,0,160,insert,1,9449,15591,81
+RFast-fastcmp,length,0,0,160,insert,2,9492,15671,81
+RFast-fastcmp,length,0,0,160,insert,3,9472,15629,81
+RFast-fastcmp,length,0,0,160,insert,4,9665,22480,158
+RFast-fastcmp,length,0,0,160,insert,5,9561,15724,80
+RFast-fastcmp,length,0,0,160,insert,6,9482,15679,81