Author: lquack
Date: Wed Oct 21 10:16:26 2015
New Revision: 1709788
URL: http://svn.apache.org/viewvc?rev=1709788&view=rev
Log:
QPID-6802: [Documentation, Java Broker] Clarify memory usage of messages.
This addresses review comments from Keith Wall
Modified:
qpid/java/trunk/doc/book/src/java-broker/Java-Broker-Runtime-Memory.xml
Modified:
qpid/java/trunk/doc/book/src/java-broker/Java-Broker-Runtime-Memory.xml
URL:
http://svn.apache.org/viewvc/qpid/java/trunk/doc/book/src/java-broker/Java-Broker-Runtime-Memory.xml?rev=1709788&r1=1709787&r2=1709788&view=diff
==============================================================================
--- qpid/java/trunk/doc/book/src/java-broker/Java-Broker-Runtime-Memory.xml
(original)
+++ qpid/java/trunk/doc/book/src/java-broker/Java-Broker-Runtime-Memory.xml Wed
Oct 21 10:16:26 2015
@@ -93,9 +93,15 @@
<section>
<title>Messages</title>
<para>
- Messages are obviously the largest consumer of memory in Qpid.
- Except for some moderate overhead (â1 kB) which is kept in heap the
message and its headers are kept in direct memory.
+ Messages and their headers are kept in direct memory and have an
additional overhead of approximately 1 kB heap memory each.
This means that most brokers will want to have more direct memory than
heap memory.
+ When many small messages accumulate on the broker the 1 kB heap memory
overhead can become a <link
linkend="Java-Broker-Runtime-Memory-Low-Memory-Heap">limiting factor</link>.
+ </para>
+ <para>
+ When the broker is <link
linkend="Java-Broker-Runtime-Memory-Low-Memory-Direct">running low on direct
memory</link> it will evict messages from memory and flow them to disk.
+ For persistent messages this only means freeing the direct memory
representation because they always have an on-disk representation to guard
against unexpected failure (e.g., a power cut).
+ For transient messages this implies additional disk I/O.
+ After being flown to disk messages need to be re-read from disk before
delivery.
</para>
</section>
<section id="Java-Broker-Runtime-Memory-Usage-BDB">
@@ -125,13 +131,13 @@
<title>HTTP Management</title>
<para>
Qpid uses Jetty for the HTTP Management (both REST and Web Management
Console).
- When the management plugin is loaded it will allocate the memory it
needs and should not require more memeory during operation and can thus be
largely ignored.
+ When the management plugin is loaded it will allocate the memory it
needs and should not require more memory during operation and can thus be
largely ignored.
</para>
</section>
</section>
<section id="Java-Broker-Runtime-Memory-Low-Memory">
<title>Low Memory Conditions</title>
- <section>
+ <section id="Java-Broker-Runtime-Memory-Low-Memory-Heap">
<title>Low on Heap Memory</title>
<para>
When the broker runs low on heap memory performance will degrade
because the JVM will trigger full garbage collection (GC) events in a struggle
to free memory.
@@ -140,7 +146,7 @@
Should the heap memory demands rise even further the JVM will
eventually throw an OutOfMemoryError which will cause the broker to shut down.
</para>
</section>
- <section>
+ <section id="Java-Broker-Runtime-Memory-Low-Memory-Direct">
<title>Low on Direct Memory</title>
<para>
When the broker detects that it uses 40% of available direct memory it
will start flowing incoming transient messages to disk and reading them back
before delivery.
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