-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathjava-garbage-collection.html
More file actions
283 lines (209 loc) · 10.5 KB
/
java-garbage-collection.html
File metadata and controls
283 lines (209 loc) · 10.5 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
<!DOCTYPE html>
<html>
<head>
<!-- [[! Document Settings ]] -->
<meta charset="utf-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<!-- [[! Page Meta ]] -->
<title>Understanding Java Garbage Collection</title>
<meta name="description" content="与机器，人，神共舞 - 编程，读书，思考，旅行，与机器对话，与人交谈，对神发问，探索，体验人生美丽的风景" />
<meta name="HandheldFriendly" content="True" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<link rel="shortcut icon" href="/assets/images/favicon.ico" >
<!-- [[! Styles'n'Scripts ]] -->
<link rel="stylesheet" type="text/css" href="/assets/css/screen.css" />
<link rel="stylesheet" type="text/css"
href="//fonts.googleapis.com/css?family=Merriweather:300,700,700italic,300italic|Open+Sans:700,400" />
<link rel="stylesheet" type="text/css" href="/assets/css/syntax.css" />
<!-- [[! Ghost outputs important style and meta data with this tag ]] -->
<link rel="canonical" href="/" />
<meta name="referrer" content="origin" />
<link rel="next" href="/page2/" />
<meta property="og:site_name" content="与机器，人，神共舞" />
<meta property="og:type" content="website" />
<meta property="og:title" content="与机器，人，神共舞" />
<meta property="og:description" content="编程，读书，思考，旅行，与机器对话，与人交谈，对神发问，探索，体验人生美丽的风景" />
<meta property="og:url" content="/" />
<meta property="og:image" content="/assets/images/cover1.jpg" />
<meta name="twitter:card" content="summary_large_image" />
<meta name="twitter:title" content="与机器，人，神共舞" />
<meta name="twitter:description" content="编程，读书，思考，旅行，与机器对话，与人交谈，对神发问，探索，体验人生美丽的风景" />
<meta name="twitter:url" content="/" />
<meta name="twitter:image:src" content="/assets/images/cover1.jpg" />
<script type="application/ld+json">
{
"@context": "http://schema.org",
"@type": "Website",
"publisher": "Tao's Page",
"url": "/",
"image": "/assets/images/cover1.jpg",
"description": "编程，读书，思考，旅行，与机器对话，与人交谈，对神发问，探索，体验人生美丽的风景"
}
</script>
<meta name="generator" content="Jekyll 3.0.0" />
<link rel="alternate" type="application/rss+xml" title="与机器，人，神共舞" href="/rss.xml" />
</head>
<body class="home-template nav-closed">
<div class="nav">
<h3 class="nav-title">Menu</h3>
<a href="#" class="nav-close">
<span class="hidden">Close</span>
</a>
<ul>
<li class="nav-home " role="presentation"><a href="/">Home</a></li>
<li class="nav-about " role="presentation"><a href="/about.html">About</a></li>
<li class="nav-fables " role="presentation"><a href="/tag/machine/">Machine</a></li>
<li class="nav-speeches " role="presentation"><a href="/tag/human/">Human</a></li>
<li class="nav-fiction " role="presentation"><a href="/tag/god/">God</a></li>
<li class="nav-author " role="presentation"><a href="/author/hetao/">Author</a></li>
</ul>
<a class="subscribe-button icon-feed" href="/vocab.html">Apps</a>
</div>
<span class="nav-cover"></span>
<div class="site-wrapper">
<!-- [[! Everything else gets inserted here ]] -->
<!-- default -->
<!-- The comment above "< default" means - insert everything in this file into -->
<!-- the [body] of the default.hbs template, which contains our header/footer. -->
<!-- Everything inside the #post tags pulls data from the post -->
<!-- #post -->
<header class="main-header post-head no-cover">
<nav class="main-nav clearfix">
</nav>
</header>
<main class="content" role="main">
<article class="post tag-machine">
<header class="post-header">
<h1 class="post-title">Understanding Java Garbage Collection</h1>
<section class="post-meta">
<!-- <a href='/'>Tao He</a> -->
<time class="post-date" datetime="2016-08-04">04 Aug 2016</time>
<!-- [[tags prefix=" on "]] -->
on
<a href='/tag/machine'>Machine</a>
</section>
</header>
<section class="post-content">
<p><strong>stop-the-world</strong> Stop-the-world means that the JVM is stopping the application from running to execute a GC. When stop-the-world occurs, every thread except for the threads needed for the GC will stop their tasks. The interrupted tasks will resume only after the GC task has completed. GC tuning often means reducing this stop-the-world time.</p>
<h4 id="generational-garbage-collection">Generational Garbage Collection</h4>
<p>Java does not explicitly specify a memory and remove it in the program code.Maybe there is to way to remove the memory explicitly :</p>
<ul>
<li>
<p>Sets the relevant object to null</p>
</li>
<li>
<p>Use System.gc() method to remove the memory - will affect the system performance drastically</p>
</li>
</ul>
<p>But nobody do that. GC responsible to finds the unnecessary objects to remove them. GC created based on the following two preconditions:</p>
<ul>
<li>
<p>Most objects soon become unreachable.</p>
</li>
<li>
<p>References from old objects to young objects only exist in small numbers.</p>
</li>
</ul>
<h5 id="young-generation">Young generation</h5>
<p>Most of the newly created objects are located here. Since most objects soon become unreachable, many objects are created in the young generation, then disappear. When objects disappear from this area, we say a “minor GC” has occurred.</p>
<h5 id="old-generation">Old generation</h5>
<p>The objects that did not become unreachable and survived from the young generation are copied here. It is generally larger than the young generation. As it is bigger in size, the GC occurs less frequently than in the young generation. When objects disappear from the old generation, we say a “major GC” (or a “full GC”) has occurred.</p>
<h5 id="permanent-generation">Permanent generation</h5>
<p>It stores classes or interned character strings. So, this area is definitely not for objects that survived from the old generation to stay permanently. A GC may occur in this area. The GC that took place here is still counted as a major GC.</p>
<h4 id="composition-of-the-young-generation">Composition of the Young Generation</h4>
<p>The young generation is divided into 3 spaces. </p>
<ul>
<li>
<p>One Eden space</p>
</li>
<li>
<p>Two Survivor spaces</p>
</li>
</ul>
<p>There are 3 spaces in total, two of which are Survivor spaces. The order of execution process of each space is as below:</p>
<ul>
<li>
<p>The majority of newly created objects are located in the Eden space.</p>
</li>
<li>
<p>After one GC in the Eden space, the surviving objects are moved to one of the Survivor spaces. </p>
</li>
<li>
<p>After a GC in the Eden space, the objects are piled up into the Survivor space, where other surviving objects already exist.</p>
</li>
<li>
<p>Once a Survivor space is full, surviving objects are moved to the other Survivor space. Then, the Survivor space that is full will be changed to a state where there is no data at all.</p>
</li>
<li>
<p>The objects that survived these steps that have been repeated a number of times are moved to the old generation.</p>
</li>
</ul>
<h5 id="gc-for-the-old-generation">GC for the Old Generation</h5>
<p>The old generation basically performs a GC when the data is full. The execution procedure varies by the GC type, so it would be easier to understand if you know different types of GC.
According to JDK 7, there are 5 GC types.</p>
<ul>
<li>
<p>Serial GC</p>
</li>
<li>
<p>Parallel GC</p>
</li>
<li>
<p>Parallel Old GC (Parallel Compacting GC)</p>
</li>
<li>
<p>Concurrent Mark & Sweep GC (or “CMS”)</p>
</li>
<li>
<p>Garbage First (G1) GC</p>
</li>
</ul>
<p>This article will not introduce 5 GCs, if you interested in, please google it.</p>
</section>
<footer class="post-footer">
<!-- Everything inside the #author tags pulls data from the author -->
<!-- #author-->
<!-- Add Disqus Comments -->
</footer>
</article>
</main>
<aside class="read-next">
<!-- [[! next_post ]] -->
<a class="read-next-story no-cover" href="/thread-looper-handler-in-android">
<section class="post">
<h2>Android Thread Related Stuff</h2>
<p>It's widely known that it's illegal to update UI components directly from threads other than...</p>
</section>
</a>
<!-- [[! /next_post ]] -->
<!-- [[! prev_post ]] -->
<a class="read-next-story prev no-cover" href="/android-binder-mechanism">
<section class="post">
<h2>Android Binder Mechanism</h2>
<p>What is Binder The Binder mechanism has started from a simple idea. “Let requests and...</p>
</section>
</a>
<!-- [[! /prev_post ]] -->
</aside>
<!-- /post -->
<footer class="site-footer clearfix">
<section class="copyright"><a href="/">与机器，人，神共舞</a> © 2024</section>
</footer>
</div>
<!-- [[! Ghost outputs important scripts and data with this tag ]] -->
<script type="text/javascript" src="https://code.jquery.com/jquery-1.11.3.min.js"></script>
<!-- [[! The main JavaScript file for Casper ]] -->
<script type="text/javascript" src="/assets/js/jquery.fitvids.js"></script>
<script type="text/javascript" src="/assets/js/index.js"></script>
<!-- Add Google Analytics -->
<!-- Google Analytics Tracking code -->
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','//www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-78960009-1', 'auto');
ga('send', 'pageview');
</script>
</body>
</html>