[nongnu] elpa/flx 4c67675437 032/182: add GC tuning information to README

[ELPA Syncer]

branch: elpa/flx
commit 4c67675437e9c2e338f091322bb967ba93f2d86d
Author: Le Wang <le.wang@agworld.com.au>
Commit: Le Wang <le.wang@agworld.com.au>

    add GC tuning information to README
---
 README.md | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/README.md b/README.md
index b6c1eeb3e5..f1b3e5bc26 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,61 @@ It's unlikely to crash your Emacs, but do expect to encounter 
bugs.
 
 [Screencast showing rationale and ido workflow][]
 
+# Memory Usage
+
+The flx algorithm willingly sacrifices memory usage for speed.
+
+For 10k file names, about 10 MB of memory will be used to speed up future
+matching.  This memory is never released to keep the match speed fast.
+
+So far with modern computers, this feels like a reasonable design decision.
+
+It may change in future.
+
+
+# GC Optimization
+
+Emacs garbage collector is fairly primitive stop the world type.  GC time can
+contribute significantly to the run-time of computation that allocates and
+frees a lot of memory.
+
+Consider:
+
+    (defun uuid ()
+      (format "%08x-%08x-%08x-%08x"
+              (random (expt 16 4))
+              (random (expt 16 4))
+              (random (expt 16 4))
+              (random (expt 16 4))))
+
+    (benchmark-run 1
+      (let ((cache (flx-make-filename-cache)))
+        (dolist (i (number-sequence 0 10000))
+          (flx-process-cache (uuid) cache))))
+            ;;; ⇒ (0.899678 9 0.33650300000000044)
+
+This means that roughly 30% of time is spent just doing garbage-collection.
+
+`flx` can benefit significantly from garbage collection tuning.
+
+By default Emacs will initiate GC every 0.76 MB allocated (`gc-cons-threshold`
+== 800000).  If we increase this to 20 MB (`gc-cons-threshold` == 20000000)
+
+We get:
+
+    (benchmark-run 1
+      (setq gc-cons-threshold 20000000)
+      (let ((cache (flx-make-filename-cache)))
+        (dolist (i (number-sequence 0 10000))
+          (flx-process-cache (uuid) cache))))
+        ;;; ⇒ (0.62035 1 0.05461100000000041)
+
+So if you have a modern machine, I encourage you to add
+
+    (setq gc-cons-threshold 20000000)
+
+to your init file.
+
 # ido support
 
 Add this to your init file and *flx* match will be enabled for ido.