From 573a5abd9d6b1668b49376b489b187780c7125c7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Tue, 27 Mar 2018 13:14:40 +0200
Subject: erts: Rewrite memory instrumentation

This commit replaces the old memory instrumentation with a new
implementation that scans carriers instead of wrapping
erts_alloc/erts_free. The old implementation could not extract
information without halting the emulator, had considerable runtime
overhead, and the memory maps it produced were noisy and lacked
critical information.

Since the new implementation walks through existing data structures
there's no longer a need to start the emulator with special flags to
get information about carrier utilization/fragmentation. Memory
fragmentation is also easier to diagnose as it's presented on a
per-carrier basis which eliminates the need to account for "holes"
between mmap segments.

To help track allocations, each allocation can now be tagged with
what it is and who allocated it at the cost of one extra word per
allocation. This is controlled on a per-allocator basis with the
+M<S>atags option, and is enabled by default for binary_alloc and
driver_alloc (which is also used by NIFs).
---
 erts/doc/src/erts_alloc.xml | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

(limited to 'erts/doc/src')

diff --git a/erts/doc/src/erts_alloc.xml b/erts/doc/src/erts_alloc.xml
index 53e136d76c..0893eb291c 100644
--- a/erts/doc/src/erts_alloc.xml
+++ b/erts/doc/src/erts_alloc.xml
@@ -559,6 +559,20 @@
             than this threshold, otherwise the carrier is shrunk.
             See also <seealso marker="#M_rsbcst"><c>rsbcst</c></seealso>.</p>
         </item>
+        <tag><marker id="M_atags"/><c><![CDATA[+M<S>atags true|false]]></c></tag>
+        <item>
+          <p>Adds a small tag to each allocated block that contains basic
+            information about what it is and who allocated it. Use the
+            <seealso marker="tools:instrument"><c>instrument</c></seealso>
+            module to inspect this information.</p>
+
+          <p>The runtime overhead is one word per allocation when enabled. This
+            may change at any time in the future.</p>
+
+          <p>The default is <c>true</c> for <c>binary_alloc</c> and
+            <c>driver_alloc</c>, and <c>false</c> for the other allocator
+            types.</p>
+        </item>
         <tag><marker id="M_e"/><c><![CDATA[+M<S>e true|false]]></c></tag>
         <item>
           <p>Enables allocator <c><![CDATA[<S>]]></c>.</p>
@@ -724,22 +738,12 @@
     <section>
       <title>Instrumentation Flags</title>
       <taglist>
-        <tag><marker id="Mim"/><c>+Mim true|false</c></tag>
-        <item>
-          <p>A map over current allocations is kept by the emulator.
-            The allocation map can be retrieved through module
-            <seealso marker="tools:instrument">
-            <c>instrument(3)</c></seealso>. <c>+Mim true</c>
-            implies <c>+Mis true</c>. <c>+Mim true</c> is the same as flag
-            <seealso marker="erl#instr"><c>-instr</c></seealso> in 
-            <c>erl(1)</c>.</p>
-        </item>
-        <tag><marker id="Mis"/><c>+Mis true|false</c></tag>
-        <item>
-          <p>Status over allocated memory is kept by the emulator.
-            The allocation status can be retrieved through module
-            <seealso marker="tools:instrument">
-            <c>instrument(3)</c></seealso>.</p>
+        <tag><c>+M&lt;S&gt;atags</c></tag>
+        <item>
+          <p>Adds a small tag to each allocated block that contains basic
+            information about what it is and who allocated it. See
+            <seealso marker="#M_atags"><c>+M&lt;S&gt;atags</c></seealso> for a
+            more complete description.</p>
         </item>
         <tag><marker id="Mit"/><c>+Mit X</c></tag>
         <item>
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