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RE: [mat-dev] my own HOWTO / tutorial
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Hi Victor,
this is great stuff!
Those are the little things we - knowing MAT very good - often forget.
Like the very first instruction to look out for the heap parameters when
parsing big heap dumps. Thanks.
> Also, to make MAT more "easy-to-use" I would propose to change the
> behavior of "Dominator Tree" link, shown at the "Overview" page by
> default ("List the biggest objects and what they keep alive") - if you
> click it on the large dump, you will see some strange dominator tree
> which shows ~10% of objects, and I had never seen that those objects
had
> something to do with real memory problems. And where are other 90% of
> objects? :)
> I would propose to open (by default) a dominator tree as shown in my
> HOWTO, see below - grouped by packets, include all classes excepting
> standard java.*.
One word of explanation here: The dominator tree contains all objects in
the heap (e.g. the retained size adds up to 100% of the heap). It just
does not contain all objects on the "root" level of the tree. If you
group the objects by package, then all objects from the root level of
the tree are taken, grouped by package and a tree is created. So the
tree contains only the objects of the root level. The next levels always
contain the objects kept alive by the objects on the "root" level. Those
are distinct object graphs.
Andreas.
-----Original Message-----
From: mat-dev-bounces@xxxxxxxxxxx [mailto:mat-dev-bounces@xxxxxxxxxxx]
On Behalf Of Victor
Sent: Donnerstag, 23. Juli 2009 07:18
To: mat-dev@xxxxxxxxxxx
Subject: [mat-dev] my own HOWTO / tutorial
Hello, MAT developers!
I am using MAT during some time, maybe several months - our system is
now on that (un)happy stage which requires constant memory & performance
analysis, so I tried many tools, especially YourKit, JProfiler,
OptimizeIt and of course MAT ;)
At the first look, when I started MAT I did not understand at all what
to do and how to use it :) I spent several days to find out how to do
something real with MAT. After that I understood that the tool was
really great. But every time after non-using of MAT for some time, I
forgot (again and again) those steps I did to find memory leaks or just
understand memory usage, so I decided to write a small HOWTO for myself
:)
It would be great if we include some kind of tutorial based on it, with
examples and screen-shots to MAT help system.
Also, to make MAT more "easy-to-use" I would propose to change the
behavior of "Dominator Tree" link, shown at the "Overview" page by
default ("List the biggest objects and what they keep alive") - if you
click it on the large dump, you will see some strange dominator tree
which shows ~10% of objects, and I had never seen that those objects had
something to do with real memory problems. And where are other 90% of
objects? :)
I would propose to open (by default) a dominator tree as shown in my
HOWTO, see below - grouped by packets, include all classes excepting
standard java.*.
OK, sorry for the long introduction, here is the HOWTO itself, I have
already posted it to Markus Kohler's blog:
http://kohlerm.blogspot.com/2009/07/eclipse-memory-analyzer-10-useful.ht
ml
===============================================
How to use Eclipse MAT to analyze memory dumps.
===============================================
Initial steps:
- if working with large heap dumps (> 1 Gb), it is recommended to
increase Xmx value in eclipse.ini (ex. -Xmx2400m)
- open your dump (File -> Open heap dump), wait until parsing is
finished
- for a fast preview, make a summary histogram (button "Create a
histogram from an arbitrary set of object" on the toolbar),
sort by retaind heap, look at object types which retain most of
memory
(usually, String/char[] and byte[])
1. The first way - using dominator tree grouped by packets/classes
1.1 Press button "Open query browser", in the drop-down menu select
"Java Basics -> Open in dominator tree",
type a filter for your classes in "objects" field, ex.
"com.yourcompany.yourproject.*",
in the "groupby" field, select "by package", look at the results
(retained heap), expand your packets and subpackets,
see how much memory is retained by each packet/subpacket.
This way you may understand which module(s) of your system
consume most
of memory.
1.2 Select a packet (or a class) you want to investigate (which retains
much memory),
in the drop-down menu select "Show retained set",
look at object types which consume most of memory (in the
context of
the selected package),
you should sort the results by retained heap.
So, you see which types of objects are consumed by some of your
modules/subsystems.
1.3 Click on some object type (usually, you look at char[] and byte[]),
in the context menu select "List objects -> with incoming
references",
sort the results by retained heap.
This way, you will see largest objects of the selected type. You
may
see their contents, copy them into clipboard
or a file ("Copy" in the context menu)
To see more objects, click on the "Total" line (in the bottom)
and
select "Next" in the context menu.
1.4 Select an object instance from the result, look at its contents, and
also expand incoming references,
analyze why this object is referenced and why this value is here
(this
depends on your business logic).
It makes sense to look at more objects from the result and
analyze them.
1.5 Repeat steps for different object types and also different packets
and/or classes
(you are interested in objects which retain most of heap).
2. Another way
2.1 On the summary histogram (see Initial Steps above) select a type
(class) which retains much memory (ususally char[]/String, byte[])
2.2 In the context menu select "Show object by class -> by incoming
references"
2.3 Expand the resulting tree, look - which class references to it, then
look which class references to that class, etc.
2.4 At each step you may look at the retained set of any class ("Show
retained set" in the context menu)
or at the concrete object instances ("List objects -> with
incoming
references" in the context menu).
===============================================
--
Vatel (Victor)
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