From ae75a7933f38def7ce559f0e197105d9eeff732a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Tue, 9 Jun 2020 20:55:41 +0200
Subject: [PATCH] doc: benchmark: describe _why_ Alacritty is so fast in
 'unicode-random'

---
 doc/benchmark.md | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/doc/benchmark.md b/doc/benchmark.md
index ce3f98e9..e92afe17 100644
--- a/doc/benchmark.md
+++ b/doc/benchmark.md
@@ -40,13 +40,13 @@ Scrollback: 10000 lines
 
 ### Results
 
-| Benchmark              | Foot (GCC+PGO) 1.3.0.r59 | Alacritty 0.4.3 |     URxvt 9.22 |      XTerm 356 |
-|------------------------|-------------------------:|----------------:|---------------:|---------------:|
-| alt-random             |            0.450s ±0.009 |   0.905s ±0.003 |  1.151s ±0.004 | 12.906s ±0.078 |
-| alt-random-colors      |            0.451s ±0.013 |   0.931s ±0.005 |  1.192s ±0.005 | 11.858s ±0.155 |
-| scrolling              |            1.181s ±0.036 |   1.096s ±0.022 |  1.080s ±0.007 | 38.244s ±0.118 |
-| scrolling-filled-lines |            0.878s ±0.015 |   1.320s ±0.040 |  1.251s ±0.006 |  6.812s ±0.031 |
-| unicode-random         |            0.926s ±1.179 |   0.089s ±0.001 | 24.039s ±3.559 | 26.558s ±3.841 |
+| Benchmark              | Foot (GCC+PGO) 1.3.0.r59 |      Alacritty 0.4.3 |     URxvt 9.22 |      XTerm 356 |
+|------------------------|-------------------------:|---------------------:|---------------:|---------------:|
+| alt-random             |            0.450s ±0.009 |   0.905s ±0.003      |  1.151s ±0.004 | 12.906s ±0.078 |
+| alt-random-colors      |            0.451s ±0.013 |   0.931s ±0.005      |  1.192s ±0.005 | 11.858s ±0.155 |
+| scrolling              |            1.181s ±0.036 |   1.096s ±0.022      |  1.080s ±0.007 | 38.244s ±0.118 |
+| scrolling-filled-lines |            0.878s ±0.015 |   1.320s ±0.040      |  1.251s ±0.006 |  6.812s ±0.031 |
+| unicode-random         |            0.926s ±1.179 |   0.089s ±0.001 [^1] | 24.039s ±3.559 | 26.558s ±3.841 |
 
 
 
@@ -73,10 +73,25 @@ Scrollback=10000 lines
 ### Results
 
 
-| Benchmark              | Foot (GCC+PGO) 1.3.0.r59 | Alacritty 0.4.2 |     URxvt 9.22 |      St 0.8.3 |      XTerm 356 |
-|------------------------|-------------------------:|----------------:|---------------:|--------------:|---------------:|
-| alt-random             |            0.791s ±0.080 |   1.558s ±0.038 |  1.746s ±0.065 | 2.628s ±0.085 |  1.706s ±0.064 |
-| alt-random-colors      |            0.830s ±0.076 |   1.587s ±0.041 |  2.049s ±0.118 | 3.033s ±0.129 |  2.109s ±0.131 |
-| scrolling              |            1.603s ±0.070 |   1.464s ±0.098 |  1.439s ±0.035 | 3.760s ±0.113 |  1.459s ±0.036 |
-| scrolling-filled-lines |            1.888s ±0.021 |   2.334s ±0.078 |  2.145s ±0.074 | 3.372s ±0.078 |  2.144s ±0.091 |
-| unicode-random         |            1.545s ±0.229 |   0.164s ±0.012 | 11.180s ±0.342 |       crashed | 11.389s ±0.269 |
+| Benchmark              | Foot (GCC+PGO) 1.3.0.r59 | Alacritty 0.4.2      |     URxvt 9.22 |      St 0.8.3 |      XTerm 356 |
+|------------------------|-------------------------:|---------------------:|---------------:|--------------:|---------------:|
+| alt-random             |            0.791s ±0.080 |   1.558s ±0.038      |  1.746s ±0.065 | 2.628s ±0.085 |  1.706s ±0.064 |
+| alt-random-colors      |            0.830s ±0.076 |   1.587s ±0.041      |  2.049s ±0.118 | 3.033s ±0.129 |  2.109s ±0.131 |
+| scrolling              |            1.603s ±0.070 |   1.464s ±0.098      |  1.439s ±0.035 | 3.760s ±0.113 |  1.459s ±0.036 |
+| scrolling-filled-lines |            1.888s ±0.021 |   2.334s ±0.078      |  2.145s ±0.074 | 3.372s ±0.078 |  2.144s ±0.091 |
+| unicode-random         |            1.545s ±0.229 |   0.164s ±0.012 [^1] | 11.180s ±0.342 |       crashed | 11.389s ±0.269 |
+
+
+[^1]: Alacritty is actually **really** slow at rendering this (whether
+    it is fallback fonts in general, emojis, or something else, I
+    don't know). I believe the reason it finishes the benchmark so
+    quickly is because it reads from the PTY in a separate thread,
+    into a larger receive buffer which is then consumed by the main
+    thread. This allows the client program to write its output much
+    faster since it is no longer stalling on a blocked PTY. This means
+    Alacritty only needs to render a couple of frames since it can
+    reach the final VT state almost immediately. On the other hand,
+    `cat`:ing the `unicode-random` test file in an endless loop, or
+    just manually scrolling up after the benchmark is done is
+    **slow**, which besides being felt (input lag), can be seen by
+    setting `debug.render_timer = true` in `alacritty.yml`.