From 6e42b112b1cebaa486c926fda759a1144cd0fe45 Mon Sep 17 00:00:00 2001
From: Sam Thursfield <sam.thursfield@codethink.co.uk>
Date: Mon, 9 Nov 2015 12:43:40 +0000
Subject: Only run as many parallel jobs as there are available CPU cores

YBD has been doing this for a while and seems like it is faster as a
result.

Change-Id: I8f95a53195cdbc2c75c06a8abe9eb089a84b1c1b
---
 morphlib/util.py | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/morphlib/util.py b/morphlib/util.py
index e34799df..ec83df25 100644
--- a/morphlib/util.py
+++ b/morphlib/util.py
@@ -93,10 +93,12 @@ def make_concurrency(cores=None):
     '''
 
     n = cpu_count() if cores is None else cores
-    # Experimental results (ref. Kinnison) says a factor of 1.5
-    # gives about the optimal result for build times, since much of
-    # builds are I/O bound, not CPU bound.
-    return max(int(n * 1.5 + 0.5), 1)
+    # Research shows that max-jobs should be == cores by default, up to max.
+    # 20 cores.
+    #
+    # http://listmaster.pepperfish.net/pipermail/
+    #   baserock-dev-baserock.org/2015-September/013264.html
+    return min(n, 20)
 
 
 def create_cachedir(settings):  # pragma: no cover
-- 
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