{"id":2049,"date":"2019-12-17T08:06:42","date_gmt":"2019-12-17T00:06:42","guid":{"rendered":"https:\/\/coderbee.net\/?p=2049"},"modified":"2019-12-16T22:56:50","modified_gmt":"2019-12-16T14:56:50","slug":"%e3%80%8a%e5%9e%83%e5%9c%be%e5%9b%9e%e6%94%b6%e7%ae%97%e6%b3%95%e6%89%8b%e5%86%8c%e3%80%8b-%e7%ac%ac2%e7%ab%a0-%e6%a0%87%e8%ae%b0-%e6%b8%85%e6%89%ab%e5%9b%9e%e6%94%b6","status":"publish","type":"post","link":"https:\/\/coderbee.net\/index.php\/jvm\/20191217\/2049","title":{"rendered":"\u300a\u5783\u573e\u56de\u6536\u7b97\u6cd5\u624b\u518c\u300b&#8211;\u7b2c2\u7ae0 \u6807\u8bb0-\u6e05\u626b\u56de\u6536"},"content":{"rendered":"<h1>1\u3001\u5783\u573e\u56de\u6536\u57fa\u7840<\/h1>\n<p>\u5783\u573e\u56de\u6536\u5668\u7684\u5b89\u5168\u6027\u662f\u6307\uff1a\u5728\u4efb\u4f55\u65f6\u5019\u90fd\u4e0d\u80fd\u56de\u6536\u5b58\u6d3b\u5bf9\u8c61\u3002<\/p>\n<p>\u6d6e\u52a8\u5783\u573e\uff08floating garbage\uff09\uff1a\u5982\u679c\u67d0\u4e2a\u5bf9\u8c61\u5728\u56de\u6536\u8fc7\u7a0b\u542f\u52a8\u4e4b\u540e\u624d\u53d8\u6210\u5783\u573e\uff0c\u90a3\u4e48\u8be5\u5bf9\u8c61\u53ea\u80fd\u5728\u4e0b\u4e00\u4e2a\u56de\u6536\u5468\u671f\u5185\u5f97\u5230\u56de\u6536\u3002<\/p>\n<p>\u56de\u6536\u5668\u5728\u6807\u8bb0\u5bf9\u8c61\u65f6\u53ef\u4ee5\u4f7f\u7528\u5bf9\u8c61\u4e2d\u7684\u57df\uff0c\u4e5f\u53ef\u4ee5\u4f7f\u7528\u989d\u5916\u7684\u6570\u636e\u7ed3\u6784\u5982\u4f4d\u56fe bitmap\uff1b\u5bf9\u4e8e\u5e76\u53d1\u56de\u6536\u5668\u6216\u5176\u4ed6\u9700\u8981\u5c06\u5806\u5206\u4e3a\u6570\u4e2a\u72ec\u7acb\u533a\u57df\u7684\u56de\u6536\u5668\uff0c\u5176\u9700\u8981\u989d\u5916\u7684\u8bb0\u5fc6\u96c6\uff08remembered set\uff09\u6765\u4fdd\u5b58\u8d4b\u503c\u5668\u6240\u4fee\u6539\u7684\u6307\u9488\u503c\u6216\u8005\u8de8\u533a\u57df\u6307\u9488\u7684\u4f4d\u7f6e\u3002<\/p>\n<p>\u8d4b\u503c\u5668\uff1a\u5e94\u7528\u7ebf\u7a0b\u3002<\/p>\n<p>\u5728\u7c7b\u578b\u5b89\u5168\u7684\u8bed\u8a00\u4e2d\uff0c\u4e00\u65e6\u67d0\u4e2a\u5bf9\u8c61\u5728\u5806\u4e2d\u4e0d\u53ef\u8fbe\uff0c\u8d4b\u503c\u5668\u5728\u6ca1\u6709\u5916\u90e8\u7cfb\u7edf\u534f\u52a9\u7684\u60c5\u51b5\u4e0b\u662f\u65e0\u6cd5\u8bbf\u95ee\u5230\u4e0d\u53ef\u8fbe\u5bf9\u8c61\u7684\u3002<\/p>\n<p>\u8d4b\u503c\u5668\u5728\u5de5\u4f5c\u8fc7\u7a0b\u4e2d\u4f1a\u6267\u884c\u4e09\u79cd\u4e0e\u56de\u6536\u5668\u76f8\u5173\u7684\u64cd\u4f5c\uff1a\u521b\u5efa\uff08New\uff09\u3001\u8bfb\uff08Read\uff09\u3001\u5199\uff08Write\uff09\u3002\u67d0\u4e9b\u7279\u6b8a\u7684\u5185\u5b58\u7ba1\u7406\u5668\u4f1a\u4e3a\u57fa\u672c\u64cd\u4f5c\u589e\u52a0\u4e00\u4e9b\u989d\u5916\u529f\u80fd\uff0c\u5373\u5c4f\u969c\uff08barrier\uff09\uff0c\u5c4f\u969c\u64cd\u4f5c\u4f1a\u540c\u6b65\u6216\u5f02\u6b65\u5730\u4e0e\u56de\u6536\u5668\u4ea7\u751f\u4ea4\u4e92\u3002<\/p>\n<p>\u5728\u65b0\u5206\u914d\u7684\u5bf9\u8c61\u53ef\u88ab\u8d4b\u503c\u5668\u64cd\u4f5c\u4e4b\u524d\uff0c\u56de\u6536\u5668\u90fd\u9700\u8981\u5148\u5bf9\u5176\u5143\u6570\u636e\u8fdb\u884c\u521d\u59cb\u5316\u3002<\/p>\n<p>\u4efb\u4f55\u81ea\u52a8\u5185\u5b58\u7ba1\u7406\u7cfb\u7edf\u90fd\u9762\u4e34\u4e09\u4e2a\u4efb\u52a1\uff1a<br \/>\n1\u3001\u4e3a\u65b0\u5bf9\u8c61\u5206\u914d\u7a7a\u95f4\u3002<br \/>\n2\u3001\u786e\u5b9a\u5b58\u6d3b\u5bf9\u8c61\u3002<br \/>\n3\u3001\u56de\u6536\u6b7b\u4ea1\u5bf9\u8c61\u6240\u5360\u7528\u7684\u7a7a\u95f4\u3002<\/p>\n<p>\u56de\u6536\u7a7a\u95f4\u7684\u65b9\u6cd5\u5f71\u54cd\u8fd9\u5206\u914d\u65b0\u7a7a\u95f4\u7684\u65b9\u6cd5\u3002<\/p>\n<p>\u57fa\u4e8e\u6307\u9488\u53ef\u8fbe\u6027\u5206\u6790\u5f97\u5230\u7684\u5b58\u6d3b\u5bf9\u8c61\u96c6\u5408\u4e2d\u53ef\u80fd\u5305\u542b\u4e00\u4e9b\u6c38\u8fdc\u4e0d\u4f1a\u518d\u88ab\u8d4b\u503c\u5668\u8bbf\u95ee\u7684\u5bf9\u8c61\uff0c\u4f46\u662f\u6b7b\u4ea1\u5bf9\u8c61\u96c6\u5408\u4e2d\u7684\u5bf9\u8c61\u5374\u5fc5\u5b9a\u662f\u6b7b\u4ea1\u7684\u3002<\/p>\n<p><!--more--><\/p>\n<h1>2\u3001\u6807\u8bb0-\u6e05\u626b\u56de\u6536<\/h1>\n<p>\u662f\u5728\u6307\u9488\u53ef\u8fbe\u6027\u9012\u5f52\u5b9a\u4e49\u6307\u5bfc\u4e0b\u6700\u76f4\u63a5\u7684\u56de\u6536\u65b9\u6848\u3002\u5b83\u7684\u56de\u6536\u8fc7\u7a0b\u5206\u4e3a\u4e24\u4e2a\u9636\u6bb5\uff1a<br \/>\n\u7b2c\u4e00\u9636\u6bb5\u8ffd\u8e2a\uff08trace\uff09\u9636\u6bb5\uff0c\u5373\u56de\u6536\u5668\u4ece\u6839\u96c6\u5408\uff08\u5bc4\u5b58\u5668\u3001\u7ebf\u7a0b\u6808\u3001\u5168\u5c40\u53d8\u91cf\u7b49\uff09\u5f00\u59cb\u904d\u5386\u5bf9\u8c61\u56fe\uff0c\u5e76\u6807\u8bb0\uff08mark\uff09\u6240\u9047\u5230\u7684\u6bcf\u4e2a\u5bf9\u8c61\uff1b<br \/>\n\u7b2c\u4e8c\u9636\u6bb5\u4e3a\u6e05\u626b\uff08sweep\uff09\u9636\u6bb5\uff0c\u5373\u56de\u6536\u5668\u68c0\u67e5\u5806\u4e2d\u7684<strong>\u6bcf<\/strong>\u4e00\u4e2a\u5bf9\u8c61\uff0c\u5e76\u5c06\u6240\u6709<strong>\u672a\u6807\u8bb0<\/strong>\u7684\u5bf9\u8c61\u5f53\u4f5c\u5783\u573e\u8fdb\u884c\u56de\u6536\u3002<\/p>\n<p>\u6807\u8bb0-\u6e05\u626b\u7b97\u6cd5\u662f\u4e00\u79cd\u95f4\u63a5\u56de\u6536\u7b97\u6cd5\uff0c\u5b83\u5e76\u975e\u76f4\u63a5\u68c0\u6d4b\u5783\u573e\u672c\u8eab\uff0c\u800c\u662f\u5148\u786e\u5b9a\u6240\u6709\u5b58\u6d3b\u5bf9\u8c61\uff0c\u7136\u540e\u53cd\u8fc7\u6765\u5224\u5b9a\u5176\u4ed6\u5bf9\u8c61\u90fd\u662f\u5783\u573e\u3002\u8be5\u7b97\u6cd5\u6bcf\u6b21\u8c03\u7528\u90fd\u9700\u8981\u91cd\u65b0\u8ba1\u7b97\u5b58\u6d3b\u5bf9\u8c61\u96c6\u5408\u3002<\/p>\n<h1>3\u3001# \u6807\u8bb0-\u6e05\u626b\u7b97\u6cd5<\/h1>\n<pre><code>New():\n    ref &lt;- allocate()\n    if ref = null           \/\/ \u5806\u4e2d\u65e0\u53ef\u7528\u7a7a\u95f4\n        collect()\n        ref &lt;- allocate()\n        if ref = null       \/\/ \u5806\u4e2d\u4ecd\u7136\u65e0\u53ef\u7528\u7a7a\u95f4\n            error \"Out of memory\"\n    return ref\n\natomic collect():\n    markFromRoots()\n    sweep(heapStart, heapEnd)\n<\/code><\/pre>\n<h2>3.1 \u6807\u8bb0 mark<\/h2>\n<p>\u56de\u6536\u5668\u5728\u904d\u5386\u5bf9\u8c61\u56fe\u4e4b\u524d\u5fc5\u987b\u5148\u6784\u5efa\u6807\u8bb0\u8fc7\u7a0b\u9700\u8981\u7528\u5230\u7684\u8d77\u59cb\u5de5\u4f5c\u5217\u8868\uff08wor klist\uff09\uff0c\u5c06\u6839\u5bf9\u8c61\u8fdb\u884c\u6807\u8bb0\u5e76\u5c06\u5176\u52a0\u5165\u5de5\u4f5c\u5217\u8868\u3002<\/p>\n<p>\u56de\u6536\u5668\u53ef\u4ee5\u901a\u8fc7\u8bbe\u7f6e\u5bf9\u8c61\u5934\u90e8\u67d0\u4e2a\u4f4d\uff08\u6216\u5b57\u8282\uff09\u7684\u65b9\u5f0f\u5bf9\u5176\u8fdb\u884c\u6807\u8bb0\uff0c\u8be5\u4f4d\u6216\u5b57\u8282\u4e5f\u53ef\u4f4d\u4e8e\u4e00\u5f20\u989d\u5916\u7684\u8868\u4e2d\u3002<\/p>\n<pre><code>markFromRoots():\n    initialise(worklist)\n    for each fld in Roots\n        ref &lt;- *fld\n        if ref != null &amp;&amp; not isMarked(ref)\n            setMarked(ref)      \/\/ \u6807\u8bb0\u5b8c\u540e\u518d\u653e\u8fdb\u53bb\n            add(worklist, ref)\n            mark()\n\ninitialise(worklist):\n    worklist &lt;- empty\n\nmark():\n    while not isEmpty(worklist)\n        ref &lt;- remove(worklist)     \/\/ ref \u5df2\u7ecf\u6807\u8bb0\u8fc7\n        for each fld in Pointers(ref)\n            child &lt;- *fld\n            if child != null &amp;&amp; not isMarked(child)\n                setMarked(child)\n                add(worklist, child)\n<\/code><\/pre>\n<p>markFromRoots \u5728\u5c06\u6839\u5bf9\u8c61\u52a0\u5165 worklist \u540e\u5c31\u9a6c\u4e0a\u8c03\u7528 mark \u662f\u4e3a\u4e86\u51cf\u5c11\u5de5\u4f5c\u5217\u8868\u7684\u5927\u5c0f\u3002\u4ece mark \u65b9\u6cd5\u4ece\u5faa\u73af\u4e2d\u63d0\u51fa\u5219\u53ef\u4ee5\u52a0\u5feb\u6839\u626b\u63cf\uff0c\u5bf9\u4e8e\u5e76\u53d1\u56de\u6536\u5668\u53ef\u80fd\u53ea\u9700\u6302\u8d77\u7ebf\u7a0b\u5e76\u626b\u63cf\u5176\u6808\uff0c\u63a5\u4e0b\u6765\u5bf9\u8c61\u56fe\u904d\u5386\u5219\u53ef\u4ee5\u4e0e\u8d4b\u503c\u5668\u5e76\u53d1\u8fdb\u884c\u3002<\/p>\n<p>\u6807\u8bb0\u9636\u6bb5\u5b8c\u6210\u7684\u6807\u5fd7\u662f\u5de5\u4f5c\u5217\u8868\uff0c\u800c\u4e0d\u662f\u5c06\u6240\u6709\u5df2\u6807\u8bb0\u5bf9\u8c61\u90fd\u6dfb\u52a0\u5230\u5de5\u4f5c\u5217\u8868\uff0c\u4efb\u4f55\u6ca1\u6709\u6253\u4e0a\u6807\u8bb0\u4f4d\u7684\u5bf9\u8c61\u90fd\u662f\u5783\u573e\u3002<\/p>\n<h2>3.2 \u6e05\u626b sweep<\/h2>\n<pre><code>sweep(heapStart, heapEnd):\n    scan &lt;- heapStart\n    while scan &lt; heapEnd\n        if isMarked(scan)\n            unsetMarked(scan)   \/\/ \u5b58\u6d3b\u5bf9\u8c61\uff0c\u6e05\u9664\u6807\u8bb0\u4f4d\uff0c\u4ee5\u4fbf\u4e0b\u4e2a\u56de\u6536\u5468\u671f\u8fdb\u884c\u6807\u8bb0\n        else\n            free(scan)          \/\/ \u672a\u6807\u8bb0\u7684\u662f\u5783\u573e\u5bf9\u8c61\n        scan &lt;- nextObject(scan)\n<\/code><\/pre>\n<p>nextObject \u4e0d\u4ec5\u8981\u80fd\u83b7\u53d6\u5bf9\u8c61\u7684\u5927\u5c0f\u4fe1\u606f\uff0c\u8fd8\u8981\u80fd\u5904\u7406\u7528\u4e8e\u5bf9\u9f50\u7684\u586b\u5145\u5b57\u8282\u3002<\/p>\n<blockquote>\n<p>\u5728 JVM \u91cc\uff0c\u6bcf\u4e2a\u5bf9\u8c61\u5b9e\u4f8b\u90fd\u6709\u4e00\u4e2a\u6307\u5411\u5176\u6240\u5c5e\u7c7b\u578b Class \u7684\u6307\u9488\u5b57\u6bb5\uff0c\u901a\u8fc7 Class \u53ef\u4ee5\u77e5\u9053\u5bf9\u8c61\u7684\u5927\u5c0f\u3002\u662f\u5426\u586b\u5145\u3001\u5982\u4f55\u586b\u5145\u5176\u5b9e\u53d6\u51b3\u4e8e\u5177\u4f53\u5185\u5b58\u7ba1\u7406\u5668\u7684\u5b9e\u73b0\u3002<br \/>\n  JVM \u4e5f\u53ef\u4ee5\u6839\u636e\u8fd9\u4e2a Class \u83b7\u53d6\u6240\u6709\u5f15\u7528\u7c7b\u578b\u7684\u5b57\u6bb5\u3002\u8fdb\u800c\u904d\u5386\u8fd9\u4e9b\u5f15\u7528\u6240\u6307\u5411\u7684\u5bf9\u8c61\u3002\u4e5f\u53ef\u80fd\u5305\u542b\u5c06\u5bf9\u8c61\u81ea\u8eab\u6807\u8bb0\u3001\u5e76\u5c06\u5176\u5b57\u7ed3\u70b9\u6dfb\u52a0\u5230 worklist \u7684\u4ee3\u7801\u3002<\/p>\n<\/blockquote>\n<h2>3.3 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src=\"https:\/\/coderbee.net\/wp-content\/uploads\/2019\/12\/tricolour-abstraction.jpg\" alt=\"\" width=\"1440\" height=\"1080\" class=\"alignnone size-full wp-image-2053\" \/><\/p>\n<p>\u4e00\u6b21\u5806\u904d\u5386\u8fc7\u7a0b\u53ef\u4ee5\u5f62\u8c61\u7b2c\u770b\u4f5c\u662f\u56de\u6536\u5668\u4ee5\u7070\u8272\u5bf9\u8c61\u4f5c\u4e3a\u201c\u6ce2\u9762\u201d\uff08wavefront\uff09\uff0c\u5c06\u9ed1\u8272\u5bf9\u8c61\u548c\u767d\u8272\u5bf9\u8c61\u5206\u79bb\uff0c\u4e0d\u65ad\u5411\u524d\u63a8\u8fdb\u6ce2\u9762\uff0c\u76f4\u5230\u6240\u6709\u53ef\u5927\u5bf9\u8c61\u90fd\u53d8\u6210\u9ed1\u8272\u7684\u8fc7\u7a0b\u3002<br \/>\n<img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/coderbee.net\/wp-content\/uploads\/2019\/12\/wavefront.png\" alt=\"\u6ce2\u9762,wavefront\" width=\"247\" height=\"169\" class=\"alignnone size-full wp-image-2050\" 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