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---
layout: default
---
<?xml version="1.0" encoding="utf8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!--http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd-->
<html xmlns="http://www.w3.org/1999/xhtml"
>
<head><title>16 MMTk</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf8" />
<meta name="generator" content="TeX4ht (https://tug.org/tex4ht/)" />
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</head><body
>
<!--l. 2--><div class="crosslinks"><p class="noindent"></p></div>
<h2 class="chapterHead"><span class="titlemark">Chapter 16</span><br /><a
id="x19-22000016"></a>MMTk</h2>
<!--l. 5--><p class="noindent" >The garbage collectors for Jikes RVM are provided by MMTk. The document
<a
href="http://cs.anu.edu.au/~Robin.Garner/mmtk-guide.pdf" >MMTk: The Memory Manager Toolkit</a> describes MMTk and gives a tutorial on how
to use and edit it and is the best place to start. An updated version of the tutorial is
available in this <a
href="/UserGuide/indexpa3/index.html#x21-256000III">guide</a>. A detailed description of the call chain from the compilers
through to MMTk <a
href="#x19-23000016.2">here</a> is another good place to start understanding how MMTk
integrates with Jikes RVM. <a
href="#x19-22100016.1">Anatomy of a Garbage Collector</a> describes the major
building blocks of an MMTk collector and <a
href="#x19-23500016.3">Scanning Objects in Jikes RVM</a> describes
how objects are scanned for their pointer fields during GC. MMTk also has a pure
Java <a
href="/UserGuide/TheMMTkTestHarness/index.html#x13-13800011">test harness</a> that allows development of garbage collectors in an IDE like
eclipse.
</p><!--l. 7--><p class="noindent" >Jikes RVM can be configured to employ various different allocation managers taken
from the MMTk memory management toolkit. Managers divide the available space
up as they see fit. However, they normally subdivide the available address range to
provide: </p>
<ul class="itemize1">
<li class="itemize">a metadata area which enables the manager to track the status of allocated
and unallocated storage in the rest of the heap.
</li>
<li class="itemize">an immortal data area used to service allocations of objects which are
expected to persist across the whole lifetime of the Jikes RVM runtime
(e.g. the boot image)
</li>
<li class="itemize">a large object space used to service allocations of objects which are larger
than some specified size (e.g. a virtual memory page) - the large object
space may employ a different allocation and reclamation strategy to that
used for other objects.
</li>
<li class="itemize">a small object allocation area which may be divided into e.g.two semi
spaces, a nursery space and a mature space, a set of generations, a
non-relocatable buddy hierarchy etc depending upon the allocation and
reclamation strategy employed by the memory manager.
</li>
<li class="itemize">separate spaces for code. These are designed to exclude performance
problems that can occur on some architectures when code and data are
mixed. See <a
href="http://dl.acm.org/citation.cfm?doid=1133956.1133980" >this paper</a> for the original motivation and experiments for a
separate code space.</li></ul>
<!--l. 16--><p class="noindent" >Virtual memory pages are lazily mapped into Jikes RVM’s memory image as they are
needed.
</p><!--l. 18--><p class="noindent" >The main class which is used to interface to the memory manager is called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Plan</span></span></span>.
Each flavor of the manager is implemented by substituting a different implementation
of this class. Most plans inherit from class <span
class="cmtt-10">StopTheWorld </span>which ensures that all
active mutator threads (i.e. ones which do not perform the job of reclaiming storage)
are suspended before reclamation is commenced. The argument passed to
<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">-X:gc:threads</span></span></span> determines the number of parallel collector threads that will be used
for collection.
</p><!--l. 20--><p class="noindent" >Generational collectors employ a plan which inherits from class <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Generational</span></span></span>. Inter
alia, this class ensures that a write barrier is employed so that updates from old to
new spaces are detected.
</p><!--l. 22--><p class="noindent" >Jikes RVM may also use the <a
href="#x19-23900016.4">GCSpy</a> visualization framework. GCSpy allows
developers to observe the behavior of the heap and related data structures.
</p>
<h3 class="sectionHead"><span class="titlemark">16.1 </span> <a
id="x19-22100016.1"></a>Anatomy of a Garbage Collector</h3>
<!--l. 5--><p class="noindent" ><span
class="cmbx-10">** Work in progress, contributions appreciated ** </span><br
class="newline" />
</p><!--l. 8--><p class="noindent" >This page gives a brief outline of the major control flows in the execution of a
garbage collector in MMTk. For simplicity, we focus on the MarkSweep collector,
although much of the discussion will be relevant to other collectors.
</p><!--l. 10--><p class="noindent" >This page assumes you have a basic knowledge of garbage collection. For those that
don’t, please see one of the standard texts such as <a
href="http://gchandbook.org/" >The Garbage Collection
Handbook</a>.
</p><!--l. 12--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">16.1.1 </span> <a
id="x19-22200016.1.1"></a>Structure of a Plan</h4>
<!--l. 14--><p class="noindent" >An MMTk Plan is required to provide 5 classes. They are required to have consistent
names which start with the same name and have a suffix that indicates which
class it inherits from. in the case of the MarkSweep plan, the name is ”MS”.
</p>
<ul class="itemize1">
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MS</span></span></span> - this is a singleton class that is a subclass of <span
class="cmtt-10">org.mmtk.plan.Plan</span>.
This class encapsulates data structures that are shared among multiple
threads.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MSMutator</span></span></span> - subclass of <span
class="cmtt-10">org.mmtk.plan.MutatorContext</span>. This class
encapsulates data structures that are local to a single mutator thread. In
the case of Jikes RVM, a Thread is actually a subclass of this class for
efficiency reasons.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MSCollector</span></span></span> - subclass of <span
class="cmtt-10">org.mmtk.plan.CollectorContext</span>. This
provides thread-local data structures specific to a garbage collector thread.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MSConstraints</span></span></span> - subclass of <span
class="cmtt-10">org.mmtk.plan.PlanConstraints</span>. This
provides configuration information that the host virtual machine might
need. It is separated out from the Plan class in order to prevent circular
class loading dependencies.
</li>
<li class="itemize"><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MSTraceLocal</span></span></span> - subclass of <span
class="cmtt-10">org.mmtk.plan.TraceLocal</span>. This provides
thread-local data structures specific to a particular way of traversing the
heap. In a simple collector like MarkSweep, there is only one of these
classes, but in more complex collectors there may be several. For example,
in a generational collector, there will be one <span
class="cmtt-10">TraceLocal </span>class for a nursery
collection, and another for a full-heap collection.</li></ul>
<!--l. 23--><p class="noindent" >The basic architecture of MMTk is that virtual address space is divided into chunks
(of 4MB in a 32-bit memory model) that are managed according to a specific <span
class="cmti-10">policy</span>.
A policy is implemented by an instance of the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space</span></span></span> class, and it is in the policy
class that the mechanics of a particular mechanism (like mark-sweep) is implemented.
The task of a <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Plan</span></span></span> is to create the policy (<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space</span></span></span>) objects that manage the
heap, and to integrate them into the MMTk framework. MMTk exposes
some of this memory management policy to the host VM, by allowing the
VM to specify an allocator (represented by a small integer) when allocating
space. The interface exposed to the VM allows it to choose whether an object
will move during collection or not, whether the object is large enough to
require special handling etc. The MMTk plan is free (within the semantic
guarantees exposed to the VM) to direct each of these allocators to a particular
policy.
</p><!--l. 28--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">16.1.2 </span> <a
id="x19-22300016.1.2"></a>Policies</h4>
<!--l. 29--><p class="noindent" >A policy describes how a range of virtual address space is managed. The base class of
all policies is <span
class="cmtt-10">org.mmtk.policy.Space</span>, and a particular instance of a policy is known
generically as a space. The static initializer of a <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Plan</span></span></span> and its subclasses define the
spaces that make up an MMTk plan.
</p>
<!--l. 31-->
<br />
<div class="caption"
><span class="id">23cAp0x19-22300016.1.2:
</span><span
class="content">MS.java</span></div><!--tex4ht:label?: x19-22300016.1 --><div class="lstlisting" id="listing-124"><span class="label"><a
id="x19-223001r1"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">static</span><span
class="cmtt-10"> </span><span
class="cmtt-10">final</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MarkSweepSpace</span><span
class="cmtt-10"> </span><span
class="cmtt-10">msSpace</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">new</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MarkSweepSpace</span><span
class="cmtt-10">(</span><span
class="cmtt-10">"</span><span
class="cmtt-10">ms</span><span
class="cmtt-10">"</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">VMRequest</span><span
class="cmtt-10">.</span><span
class="cmtt-10">discontiguous</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-223002r2"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">static</span><span
class="cmtt-10"> </span><span
class="cmtt-10">final</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MARK_SWEEP</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">msSpace</span><span
class="cmtt-10">.</span><span
class="cmtt-10">getDescriptor</span><span
class="cmtt-10">()</span><span
class="cmtt-10">;</span>
</div>
<!--l. 36--><p class="noindent" >In this code fragment, we see the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MS</span></span></span> plan defined. Note that we generally also define
a <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">static final</span></span></span> space descriptor. This is an optimization that allows some rapid
operations on spaces.
</p><!--l. 38--><p class="noindent" >A <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space</span></span></span> is a global object, shared among multiple mutator threads. Each policy will
also have one or more thread-local classes which provide unsynchronized allocation.
These classes are subclasses of <span
class="cmtt-10">org.mmtk.utility.alloc.Allocator</span>, and in the
case of MarkSweep, it is called <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MarkSweepLocal</span></span></span>. Instances of <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">MarkSweepLocal</span></span></span> are
created as part of a mutator context, like this
</p>
<!--l. 40-->
<br />
<div class="caption"
><span class="id">23cAp1x19-22300016.1.2:
</span><span
class="content">MSMutator.java</span></div><!--tex4ht:label?: x19-22300016.1 --><div class="lstlisting" id="listing-125"><span class="label"><a
id="x19-223003r1"></a></span><span
class="cmtt-10">protected</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MarkSweepLocal</span><span
class="cmtt-10"> </span><span
class="cmtt-10">ms</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">new</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MarkSweepLocal</span><span
class="cmtt-10">(</span><span
class="cmtt-10">MS</span><span
class="cmtt-10">.</span><span
class="cmtt-10">msSpace</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span>
</div>
<!--l. 44--><p class="noindent" >The design pattern is that the local Allocator will allocate space from a
thread-local buffer, and when that is exhausted it will allocate a new buffer from
the global <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space</span></span></span>, performing appropriate locking. The constructor of the
<span
class="cmtt-10">MarkSweepLocal </span>specifies the space from which the allocator will allocate global
memory.
</p><!--l. 48--><p class="noindent" >
</p>
<h4 class="subsectionHead"><span class="titlemark">16.1.3 </span> <a
id="x19-22400016.1.3"></a>Allocation</h4>
<!--l. 50--><p class="noindent" >MMTk provides two methods for allocating an object. These are provided by the
<span
class="cmtt-10">MSMutator </span>class, to give each plan the opportunity to use fast, unsynchronized
thread-local allocation before falling back to a slower synchronized slow-path.
</p><!--l. 52--><p class="noindent" >The version implemented in MarkSweep looks like this:
</p>
<!--l. 54-->
<br />
<div class="caption"
><span class="id">23cAp2x19-22400016.1.3:
</span><span
class="content">MSMutator.java</span></div><!--tex4ht:label?: x19-22400016.1 --><div class="lstlisting" id="listing-126"><span class="label"><a
id="x19-224001r1"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">alloc</span><span
class="cmtt-10">(</span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">align</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">allocator</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">site</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224002r2"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(</span><span
class="cmtt-10">allocator</span><span
class="cmtt-10"> </span><span
class="cmtt-10">==</span><span
class="cmtt-10"> </span><span
class="cmtt-10">MS</span><span
class="cmtt-10">.</span><span
class="cmtt-10">ALLOC_DEFAULT</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224003r3"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">ms</span><span
class="cmtt-10">.</span><span
class="cmtt-10">alloc</span><span
class="cmtt-10">(</span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">align</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224004r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224005r5"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">super</span><span
class="cmtt-10">.</span><span
class="cmtt-10">alloc</span><span
class="cmtt-10">(</span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">align</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">allocator</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">site</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224006r6"></a></span><span
class="cmtt-10">}</span>
</div>
<!--l. 63--><p class="noindent" >The basic structure of this method is common to all MMTk plans. First
they decide whether the operation applies to this level of abstraction (<span class="obeylines-h"><span class="verb"><span
class="cmtt-10">if</span>
<span
class="cmtt-10">(allocator == MS.ALLOC_DEFAULT)</span></span></span>), and if so, delegate to the appropriate place,
otherwise pass it up the chain to the super-class. In the case of MarkSweep,
<span
class="cmtt-10">MSMutator </span>delegates the allocation to its thread-local <span
class="cmtt-10">MarkSweepLocal </span>object
ms.
</p><!--l. 65--><p class="noindent" >The alloc method of <span
class="cmtt-10">MarkSweepLocal </span>is inherited from <span
class="cmtt-10">SegregatedFreeListLocal</span>
(mark-sweep is not the only way of managing free-list allocation), and looks like
this
</p>
<!--l. 67-->
<br />
<div class="caption"
><span class="id">23cAp3x19-22400016.1.3:
</span><span
class="content">SegregatedFreeListLocal.java
(simplified)</span></div><!--tex4ht:label?: x19-22400016.1 --><div class="lstlisting" id="listing-127"><span class="label"><a
id="x19-224007r1"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">final</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">alloc</span><span
class="cmtt-10">(</span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">align</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224008r2"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">sizeClass</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">getSizeClass</span><span
class="cmtt-10">(</span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224009r3"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">cell</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">freeList</span><span
class="cmtt-10">.</span><span
class="cmtt-10">get</span><span
class="cmtt-10">(</span><span
class="cmtt-10">sizeClass</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224010r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(!</span><span
class="cmtt-10">cell</span><span
class="cmtt-10">.</span><span
class="cmtt-10">isZero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224011r5"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">freeList</span><span
class="cmtt-10">.</span><span
class="cmtt-10">set</span><span
class="cmtt-10">(</span><span
class="cmtt-10">sizeClass</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">cell</span><span
class="cmtt-10">.</span><span
class="cmtt-10">loadAddress</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224012r6"></a></span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10">/*</span><span
class="cmitt-10"> </span><span
class="cmitt-10">Clear</span><span
class="cmitt-10"> </span><span
class="cmitt-10">the</span><span
class="cmitt-10"> </span><span
class="cmitt-10">free</span><span
class="cmitt-10"> </span><span
class="cmitt-10">list</span><span
class="cmitt-10"> </span><span
class="cmitt-10">link</span><span
class="cmitt-10"> </span><span
class="cmitt-10">*/</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224013r7"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">cell</span><span
class="cmtt-10">.</span><span
class="cmtt-10">store</span><span
class="cmtt-10">(</span><span
class="cmtt-10">Address</span><span
class="cmtt-10">.</span><span
class="cmtt-10">zero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224014r8"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">cell</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224015r9"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224016r10"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">allocSlow</span><span
class="cmtt-10">(</span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">align</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224017r11"></a></span><span
class="cmtt-10">}</span>
</div>
<!--l. 81--><p class="noindent" >This is a standard pattern for thread-local allocation: first we look in the thread-local
space (line 3), and if successful return the result (lines 4-8). If unsuccessful, we
request space from the global policy via the method <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Allocator.allocSlow</span></span></span>. This is
the common interface that all Allocators use to request space from the global policy.
This will eventually call the allocator-specific <span
class="cmtt-10">allocSlowOnce method</span>. The workings
of the <span
class="cmtt-10">allocSlowOnce </span>method are very policy-specific, so not appropriate to look at
at this stage, but eventually all policies will attempt to acquire fresh virtual memory
via the <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space.acquire</span></span></span> method.
</p><!--l. 83--><p class="noindent" ><span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Space.acquire</span></span></span> is the only correct way for a policy to allocate new virtual memory
for its own use.
</p>
<!--l. 86-->
<br />
<div class="caption"
><span class="id">23cAp4x19-22400016.1.3:
</span><span
class="content">Space.java
(simplified)</span></div><!--tex4ht:label?: x19-22400016.1 --><div class="lstlisting" id="listing-128"><span class="label"><a
id="x19-224018r1"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">final</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">acquire</span><span
class="cmtt-10">(</span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">pages</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224019r2"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">pr</span><span
class="cmtt-10">.</span><span
class="cmtt-10">reservePages</span><span
class="cmtt-10">(</span><span
class="cmtt-10">pages</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224020r3"></a></span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10">//</span><span
class="cmitt-10"> </span><span
class="cmitt-10">Poll</span><span
class="cmitt-10">,</span><span
class="cmitt-10"> </span><span
class="cmitt-10">either</span><span
class="cmitt-10"> </span><span
class="cmitt-10">fixing</span><span
class="cmitt-10"> </span><span
class="cmitt-10">budget</span><span
class="cmitt-10"> </span><span
class="cmitt-10">or</span><span
class="cmitt-10"> </span><span
class="cmitt-10">requiring</span><span
class="cmitt-10"> </span><span
class="cmitt-10">GC</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224021r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(</span><span
class="cmtt-10">VM</span><span
class="cmtt-10">.</span><span
class="cmtt-10">activePlan</span><span
class="cmtt-10">.</span><span
class="cmtt-10">global</span><span
class="cmtt-10">()</span><span
class="cmtt-10">.</span><span
class="cmtt-10">poll</span><span
class="cmtt-10">(</span><span
class="cmtt-10">false</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">this</span><span
class="cmtt-10">)</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224022r5"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">VM</span><span
class="cmtt-10">.</span><span
class="cmtt-10">collection</span><span
class="cmtt-10">.</span><span
class="cmtt-10">blockForGC</span><span
class="cmtt-10">()</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224023r6"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10">.</span><span
class="cmtt-10">zero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><span
class="cmitt-10">//</span><span
class="cmitt-10"> </span><span
class="cmitt-10">GC</span><span
class="cmitt-10"> </span><span
class="cmitt-10">required</span><span
class="cmitt-10">,</span><span
class="cmitt-10"> </span><span
class="cmitt-10">return</span><span
class="cmitt-10"> </span><span
class="cmitt-10">failure</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224024r7"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224025r8"></a></span><span
class="cmitt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10">//</span><span
class="cmitt-10"> </span><span
class="cmitt-10">Page</span><span
class="cmitt-10"> </span><span
class="cmitt-10">budget</span><span
class="cmitt-10"> </span><span
class="cmitt-10">is</span><span
class="cmitt-10"> </span><span
class="cmitt-10">ok</span><span
class="cmitt-10">,</span><span
class="cmitt-10"> </span><span
class="cmitt-10">try</span><span
class="cmitt-10"> </span><span
class="cmitt-10">to</span><span
class="cmitt-10"> </span><span
class="cmitt-10">acquire</span><span
class="cmitt-10"> </span><span
class="cmitt-10">virtual</span><span
class="cmitt-10"> </span><span
class="cmitt-10">memory</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224026r9"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">rtn</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">pr</span><span
class="cmtt-10">.</span><span
class="cmtt-10">getNewPages</span><span
class="cmtt-10">(</span><span
class="cmtt-10">pagesReserved</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">pages</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">zeroed</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224027r10"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(</span><span
class="cmtt-10">rtn</span><span
class="cmtt-10">.</span><span
class="cmtt-10">isZero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><span
class="cmitt-10"> </span><span
class="cmitt-10">//</span><span
class="cmitt-10"> </span><span
class="cmitt-10">Failed</span><span
class="cmitt-10">,</span><span
class="cmitt-10"> </span><span
class="cmitt-10">so</span><span
class="cmitt-10"> </span><span
class="cmitt-10">force</span><span
class="cmitt-10"> </span><span
class="cmitt-10">a</span><span
class="cmitt-10"> </span><span
class="cmitt-10">GC</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224028r11"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">boolean</span><span
class="cmtt-10"> </span><span
class="cmtt-10">gcPerformed</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">VM</span><span
class="cmtt-10">.</span><span
class="cmtt-10">activePlan</span><span
class="cmtt-10">.</span><span
class="cmtt-10">global</span><span
class="cmtt-10">()</span><span
class="cmtt-10">.</span><span
class="cmtt-10">poll</span><span
class="cmtt-10">(</span><span
class="cmtt-10">true</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">this</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224029r12"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">VM</span><span
class="cmtt-10">.</span><span
class="cmtt-10">collection</span><span
class="cmtt-10">.</span><span
class="cmtt-10">blockForGC</span><span
class="cmtt-10">()</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224030r13"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10">.</span><span
class="cmtt-10">zero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224031r14"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224032r15"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">rtn</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224033r16"></a></span><span
class="cmtt-10">}</span>
</div>
<!--l. 105--><p class="noindent" >The logic of <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">space.acquire</span></span></span> is: </p>
<ul class="itemize1">
<li class="itemize">First, poll the plan to find out whether the heap is full. This logic is
performed by the plan, because it has knowledge of copy reserves etc.
</li>
<li class="itemize">The <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">poll</span></span></span> method will request a GC if required, and return <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">true</span></span></span> if it has
done so.
</li>
<li class="itemize">Then we wait for GC if required. <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">poll</span></span></span> can’t wait, because it is called in
circumstances that aren’t GC safe.
</li>
<li class="itemize">If <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Plan.poll(...)</span></span></span> returns <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">false</span></span></span> (we are within the allowed heap size),
we call <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">pr.getNewPages</span></span></span> to allocate virtual memory. At this stage we can
find that we have run out of virtual memory, and if so, we force a GC
</li>
<li class="itemize">If a GC is performed, we return <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Address.zero()</span></span></span>, rather than retrying
locally. In many plans, the next allocation request will be satisfied by
re-using space in a page that already belongs to a policy, so the post-GC
allocation must be performed further up in the call stack. The retry logic
is handled in <span class="obeylines-h"><span class="verb"><span
class="cmtt-10">Allocator.allocSlowInline</span></span></span>.</li></ul>
<!--l. 114-->
<br />
<div class="caption"
><span class="id">23cAp5x19-22400016.1.3:
</span><span
class="content">Allocator.java
(simplified)</span></div><!--tex4ht:label?: x19-22400016.1 --><div class="lstlisting" id="listing-129"><span class="label"><a
id="x19-224034r1"></a></span><span
class="cmtt-10">public</span><span
class="cmtt-10"> </span><span
class="cmtt-10">final</span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">allocSlowInline</span><span
class="cmtt-10">(</span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">alignment</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">int</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224035r2"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">boolean</span><span
class="cmtt-10"> </span><span
class="cmtt-10">emergencyCollection</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">false</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224036r3"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">while</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(</span><span
class="cmtt-10">true</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224037r4"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">Address</span><span
class="cmtt-10"> </span><span
class="cmtt-10">result</span><span
class="cmtt-10"> </span><span
class="cmtt-10">=</span><span
class="cmtt-10"> </span><span
class="cmtt-10">allocSlowOnce</span><span
class="cmtt-10">(</span><span
class="cmtt-10">bytes</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">alignment</span><span
class="cmtt-10">,</span><span
class="cmtt-10"> </span><span
class="cmtt-10">offset</span><span
class="cmtt-10">)</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224038r5"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span
class="cmtt-10">(!</span><span
class="cmtt-10">result</span><span
class="cmtt-10">.</span><span
class="cmtt-10">isZero</span><span
class="cmtt-10">()</span><span
class="cmtt-10">)</span><span
class="cmtt-10"> </span><span
class="cmtt-10">{</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224039r6"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">return</span><span
class="cmtt-10"> </span><span
class="cmtt-10">result</span><span
class="cmtt-10">;</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224040r7"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">}</span><span
class="cmtt-10"> </span><br /><span class="label"><a
id="x19-224041r8"></a></span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10"> </span><span
class="cmtt-10">if</span><span
class="cmtt-10"> </span><span