From d5dc5584884488c8fcb5710997a14c2c418692f3 Mon Sep 17 00:00:00 2001
From: Lukas Larsson <lukas@erlang.org>
Date: Tue, 15 May 2018 16:32:26 +0200
Subject: erts: Make cpu_timestamp use per thread on Linux

If we don't use per thread the value becomes completely
nonsensical on systems with more than one scheduler.
We keep the old solaris behaviour in order to support the
option, but it only really works when using a single
scheduler.
---
 lib/tools/doc/src/fprof.xml | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

(limited to 'lib')

diff --git a/lib/tools/doc/src/fprof.xml b/lib/tools/doc/src/fprof.xml
index 4c9e48045e..72624bd33b 100644
--- a/lib/tools/doc/src/fprof.xml
+++ b/lib/tools/doc/src/fprof.xml
@@ -328,10 +328,16 @@
            purposes. This option is only
            allowed with the <c>start</c> option.</item>
           <tag><c>cpu_time</c>| <c>{cpu_time, bool()}</c></tag>
-          <item>The options <c>cpu_time</c> or <c>{cpu_time, true></c>
+          <item>The options <c>cpu_time</c> or <c>{cpu_time, true}</c>
            makes the timestamps in the trace be in CPU time instead
            of wallclock time which is the default. This option is
-           only allowed with the <c>start</c> option.</item>
+           only allowed with the <c>start</c> option.
+           <warning><p>Getting correct values out of cpu_time can be difficult.
+           The best way to get correct values is to run using a single
+           scheduler and bind that scheduler to a specific CPU,
+           i.e. <c>erl +S 1 +sbt db</c>.</p>
+           </warning>
+          </item>
           <tag><c>{procs, PidSpec}</c>| <c>{procs, [PidSpec]}</c></tag>
           <item>Specifies which processes that shall be traced. If
            this option is not given, the calling process is
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