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22 | <a href="./Home">MLton 20180207</a>\r | |
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25 | <div id="header">\r | |
26 | <h1>ConcurrentMLImplementation</h1>\r | |
27 | </div>\r | |
28 | <div id="content">\r | |
29 | <div id="preamble">\r | |
30 | <div class="sectionbody">\r | |
31 | <div class="paragraph"><p>Here are some notes on MLton’s implementation of <a href="ConcurrentML">ConcurrentML</a>.</p></div>\r | |
32 | <div class="paragraph"><p>Concurrent ML was originally implemented for SML/NJ. It was ported to\r | |
33 | MLton in the summer of 2004. The main difference between the\r | |
34 | implementations is that SML/NJ uses continuations to implement CML\r | |
35 | threads, while MLton uses its underlying <a href="MLtonThread">thread</a>\r | |
36 | package. Presently, MLton’s threads are a little more heavyweight\r | |
37 | than SML/NJ’s continuations, but it’s pretty clear that there is some\r | |
38 | fat there that could be trimmed.</p></div>\r | |
39 | <div class="paragraph"><p>The implementation of CML in SML/NJ is built upon the first-class\r | |
40 | continuations of the <span class="monospaced">SMLofNJ.Cont</span> module.</p></div>\r | |
41 | <div class="listingblock">\r | |
42 | <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">cont</span><span class="w"></span>\r | |
43 | <span class="k">val</span><span class="w"> </span><span class="n">callcc</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">cont</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
44 | <span class="k">val</span><span class="w"> </span><span class="n">isolate</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">cont</span><span class="w"></span>\r | |
45 | <span class="k">val</span><span class="w"> </span><span class="n">throw</span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">cont</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'b</span><span class="w"></span>\r | |
46 | </pre></div></div></div>\r | |
47 | <div class="paragraph"><p>The implementation of CML in MLton is built upon the first-class\r | |
48 | threads of the <a href="MLtonThread">MLtonThread</a> module.</p></div>\r | |
49 | <div class="listingblock">\r | |
50 | <div class="content"><div class="highlight"><pre><span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
51 | <span class="k">val</span><span class="w"> </span><span class="n">new</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"></span>\r | |
52 | <span class="k">val</span><span class="w"> </span><span class="n">prepare</span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="n">*</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">Runnable</span><span class="p">.</span><span class="n">t</span><span class="w"></span>\r | |
53 | <span class="k">val</span><span class="w"> </span><span class="n">switch</span><span class="p">:</span><span class="w"> </span><span class="p">(</span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">Runnable</span><span class="p">.</span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
54 | </pre></div></div></div>\r | |
55 | <div class="paragraph"><p>The port is relatively straightforward, because CML always throws to a\r | |
56 | continuation at most once. Hence, an "abstract" implementation of\r | |
57 | CML could be built upon first-class one-shot continuations, which map\r | |
58 | equally well to SML/NJ’s continuations and MLton’s threads.</p></div>\r | |
59 | <div class="paragraph"><p>The "essence" of the port is to transform:</p></div>\r | |
60 | <div class="listingblock">\r | |
61 | <div class="content monospaced">\r | |
62 | <pre>callcc (fn k => ... throw k' v')</pre>\r | |
63 | </div></div>\r | |
64 | <div class="paragraph"><p>to</p></div>\r | |
65 | <div class="listingblock">\r | |
66 | <div class="content monospaced">\r | |
67 | <pre>switch (fn t => ... prepare (t', v'))</pre>\r | |
68 | </div></div>\r | |
69 | <div class="paragraph"><p>which suffices for the vast majority of the CML implementation.</p></div>\r | |
70 | <div class="paragraph"><p>There was only one complicated transformation: blocking multiple base\r | |
71 | events. In SML/NJ CML, the representation of base events is given by:</p></div>\r | |
72 | <div class="listingblock">\r | |
73 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">event_status</span><span class="w"></span>\r | |
74 | <span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">ENABLED</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">prio</span><span class="p">:</span><span class="w"> </span><span class="n">int</span><span class="p">,</span><span class="w"> </span><span class="n">doFn</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">}</span><span class="w"></span>\r | |
75 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">BLOCKED</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="w"></span>\r | |
76 | <span class="w"> </span><span class="n">transId</span><span class="p">:</span><span class="w"> </span><span class="n">trans_id</span><span class="w"> </span><span class="n">ref</span><span class="p">,</span><span class="w"></span>\r | |
77 | <span class="w"> </span><span class="n">cleanUp</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"></span>\r | |
78 | <span class="w"> </span><span class="n">next</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="w"></span>\r | |
79 | <span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
80 | <span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">base_evt</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">event_status</span><span class="w"></span>\r | |
81 | </pre></div></div></div>\r | |
82 | <div class="paragraph"><p>When synchronizing on a set of base events, which are all blocked, we\r | |
83 | must invoke each <span class="monospaced">BLOCKED</span> function with the same <span class="monospaced">transId</span> and\r | |
84 | <span class="monospaced">cleanUp</span> (the <span class="monospaced">transId</span> is (checked and) set to <span class="monospaced">CANCEL</span> by the\r | |
85 | <span class="monospaced">cleanUp</span> function, which is invoked by the first enabled event; this\r | |
86 | "fizzles" every other event in the synchronization group that later\r | |
87 | becomes enabled). However, each <span class="monospaced">BLOCKED</span> function is implemented by\r | |
88 | a callcc, so that when the event is enabled, it throws back to the\r | |
89 | point of synchronization. Hence, the next function (which doesn’t\r | |
90 | return) is invoked by the <span class="monospaced">BLOCKED</span> function to escape the callcc and\r | |
91 | continue in the thread performing the synchronization. In SML/NJ this\r | |
92 | is implemented as follows:</p></div>\r | |
93 | <div class="listingblock">\r | |
94 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">ext</span><span class="w"> </span><span class="p">([],</span><span class="w"> </span><span class="n">blockFns</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">callcc</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">k</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
95 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">throw</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">throw</span><span class="w"> </span><span class="n">k</span><span class="w"></span>\r | |
96 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">transId</span><span class="p">,</span><span class="w"> </span><span class="n">setFlg</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">mkFlg</span><span class="p">()</span><span class="w"></span>\r | |
97 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">atomicDispatch</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
98 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="p">(</span><span class="n">blockFn::</span><span class="w"> </span><span class="n">r</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
99 | <span class="w"> </span><span class="n">throw</span><span class="w"> </span><span class="p">(</span><span class="n">blockFn</span><span class="w"> </span><span class="p">{</span><span class="w"></span>\r | |
100 | <span class="w"> </span><span class="n">transId</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">transId</span><span class="p">,</span><span class="w"></span>\r | |
101 | <span class="w"> </span><span class="n">cleanUp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">setFlg</span><span class="p">,</span><span class="w"></span>\r | |
102 | <span class="w"> </span><span class="n">next</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="n">r</span><span class="w"></span>\r | |
103 | <span class="w"> </span><span class="p">})</span><span class="w"></span>\r | |
104 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
105 | <span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="n">blockFns</span><span class="p">;</span><span class="w"> </span><span class="n">error</span><span class="w"> </span><span class="s">"[log]"</span><span class="w"></span>\r | |
106 | <span class="w"> </span><span class="k">end</span><span class="p">)</span><span class="w"></span>\r | |
107 | </pre></div></div></div>\r | |
108 | <div class="paragraph"><p>(Note that <span class="monospaced">S.atomicDispatch</span> invokes the continuation of the next\r | |
109 | continuation on the ready queue.) This doesn’t map well to the MLton\r | |
110 | thread model. Although it follows the</p></div>\r | |
111 | <div class="listingblock">\r | |
112 | <div class="content monospaced">\r | |
113 | <pre>callcc (fn k => ... throw k v)</pre>\r | |
114 | </div></div>\r | |
115 | <div class="paragraph"><p>model, the fact that <span class="monospaced">blockFn</span> will also attempt to do</p></div>\r | |
116 | <div class="listingblock">\r | |
117 | <div class="content monospaced">\r | |
118 | <pre>callcc (fn k' => ... next ())</pre>\r | |
119 | </div></div>\r | |
120 | <div class="paragraph"><p>means that the naive transformation will result in nested <span class="monospaced">switch</span>-es.</p></div>\r | |
121 | <div class="paragraph"><p>We need to think a little more about what this code is trying to do.\r | |
122 | Essentially, each <span class="monospaced">blockFn</span> wants to capture this continuation, hold\r | |
123 | on to it until the event is enabled, and continue with next; when the\r | |
124 | event is enabled, before invoking the continuation and returning to\r | |
125 | the synchronization point, the <span class="monospaced">cleanUp</span> and other event specific\r | |
126 | operations are performed.</p></div>\r | |
127 | <div class="paragraph"><p>To accomplish the same effect in the MLton thread implementation, we\r | |
128 | have the following:</p></div>\r | |
129 | <div class="listingblock">\r | |
130 | <div class="content"><div class="highlight"><pre><span class="k">datatype</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">status</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
131 | <span class="w"> </span><span class="n">ENABLED</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">prio</span><span class="p">:</span><span class="w"> </span><span class="n">int</span><span class="p">,</span><span class="w"> </span><span class="n">doitFn</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">}</span><span class="w"></span>\r | |
132 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">BLOCKED</span><span class="w"> </span><span class="k">of</span><span class="w"> </span><span class="p">{</span><span class="n">transId</span><span class="p">:</span><span class="w"> </span><span class="n">trans_id</span><span class="p">,</span><span class="w"></span>\r | |
133 | <span class="w"> </span><span class="n">cleanUp</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">unit</span><span class="p">,</span><span class="w"></span>\r | |
134 | <span class="w"> </span><span class="n">next</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">rdy_thread</span><span class="p">}</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"></span>\r | |
135 | \r | |
136 | <span class="k">type</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">base</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">status</span><span class="w"></span>\r | |
137 | \r | |
138 | <span class="k">fun</span><span class="w"> </span><span class="n">ext</span><span class="w"> </span><span class="p">([],</span><span class="w"> </span><span class="n">blockFns</span><span class="p">):</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
139 | <span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">atomicSwitch</span><span class="w"></span>\r | |
140 | <span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">(</span><span class="n">t</span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">thread</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
141 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
142 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">(</span><span class="n">transId</span><span class="p">,</span><span class="w"> </span><span class="n">cleanUp</span><span class="p">)</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">TransID</span><span class="p">.</span><span class="n">mkFlg</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
143 | <span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="n">blockFns</span><span class="p">:</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">rdy_thread</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
144 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">blockFns</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
145 | <span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">next</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
146 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">blockFn::blockFns</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
147 | <span class="w"> </span><span class="p">(</span><span class="n">S</span><span class="p">.</span><span class="n">prep</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">new</span><span class="p">)</span><span class="w"></span>\r | |
148 | <span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"></span>\r | |
149 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
150 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">atomicBegin</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
151 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">blockFn</span><span class="w"> </span><span class="p">{</span><span class="n">transId</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">transId</span><span class="p">,</span><span class="w"></span>\r | |
152 | <span class="w"> </span><span class="n">cleanUp</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">cleanUp</span><span class="p">,</span><span class="w"></span>\r | |
153 | <span class="w"> </span><span class="n">next</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="n">blockFns</span><span class="p">}</span><span class="w"></span>\r | |
154 | <span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">switch</span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">_</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">S</span><span class="p">.</span><span class="n">prepVal</span><span class="w"> </span><span class="p">(</span><span class="n">t</span><span class="p">,</span><span class="w"> </span><span class="n">x</span><span class="p">))</span><span class="w"></span>\r | |
155 | <span class="w"> </span><span class="k">end</span><span class="p">)</span><span class="w"></span>\r | |
156 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
157 | <span class="w"> </span><span class="n">log</span><span class="w"> </span><span class="n">blockFns</span><span class="w"></span>\r | |
158 | <span class="w"> </span><span class="k">end</span><span class="p">)</span><span class="w"></span>\r | |
159 | </pre></div></div></div>\r | |
160 | <div class="paragraph"><p>To avoid the nested <span class="monospaced">switch</span>-es, I run the <span class="monospaced">blockFn</span> in it’s own\r | |
161 | thread, whose only purpose is to return to the synchronization point.\r | |
162 | This corresponds to the <span class="monospaced">throw (blockFn {...})</span> in the SML/NJ\r | |
163 | implementation. I’m worried that this implementation might be a\r | |
164 | little expensive, starting a new thread for each blocked event (when\r | |
165 | there are only multiple blocked events in a synchronization group).\r | |
166 | But, I don’t see another way of implementing this behavior in the\r | |
167 | MLton thread model.</p></div>\r | |
168 | <div class="paragraph"><p>Note that another way of thinking about what is going on is to\r | |
169 | consider each <span class="monospaced">blockFn</span> as prepending a different set of actions to\r | |
170 | the thread <span class="monospaced">t</span>. It might be possible to give a\r | |
171 | <span class="monospaced">MLton.Thread.unsafePrepend</span>.</p></div>\r | |
172 | <div class="listingblock">\r | |
173 | <div class="content"><div class="highlight"><pre><span class="k">fun</span><span class="w"> </span><span class="n">unsafePrepend</span><span class="w"> </span><span class="p">(</span><span class="n">T</span><span class="w"> </span><span class="n">r</span><span class="p">:</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">t</span><span class="p">,</span><span class="w"> </span><span class="n">f</span><span class="p">:</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">):</span><span class="w"> </span><span class="n">'b</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
174 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
175 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">t</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
176 | <span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="n">!r</span><span class="w"> </span><span class="k">of</span><span class="w"></span>\r | |
177 | <span class="w"> </span><span class="n">Dead</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="k">raise</span><span class="w"> </span><span class="n">Fail</span><span class="w"> </span><span class="s">"prepend to a Dead thread"</span><span class="w"></span>\r | |
178 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">New</span><span class="w"> </span><span class="n">g</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">New</span><span class="w"> </span><span class="p">(</span><span class="n">g</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"></span>\r | |
179 | <span class="w"> </span><span class="p">|</span><span class="w"> </span><span class="n">Paused</span><span class="w"> </span><span class="p">(</span><span class="n">g</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">Paused</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="n">h</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">g</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="w"> </span><span class="n">o</span><span class="w"> </span><span class="n">h</span><span class="p">),</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"></span>\r | |
180 | <span class="w"> </span><span class="k">in</span><span class="w"> </span><span class="cm">(* r := Dead; *)</span><span class="w"></span>\r | |
181 | <span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">(</span><span class="n">ref</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"></span>\r | |
182 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
183 | </pre></div></div></div>\r | |
184 | <div class="paragraph"><p>I have commented out the <span class="monospaced">r := Dead</span>, which would allow multiple\r | |
185 | prepends to the same thread (i.e., not destroying the original thread\r | |
186 | in the process). Of course, only one of the threads could be run: if\r | |
187 | the original thread were in the <span class="monospaced">Paused</span> state, then multiple threads\r | |
188 | would share the underlying runtime/primitive thread. Now, this\r | |
189 | matches the "one-shot" nature of CML continuations/threads, but I’m\r | |
190 | not comfortable with extending <span class="monospaced">MLton.Thread</span> with such an unsafe\r | |
191 | operation.</p></div>\r | |
192 | <div class="paragraph"><p>Other than this complication with blocking multiple base events, the\r | |
193 | port was quite routine. (As a very pleasant surprise, the CML\r | |
194 | implementation in SML/NJ doesn’t use any SML/NJ-isms.) There is a\r | |
195 | slight difference in the way in which critical sections are handled in\r | |
196 | SML/NJ and MLton; since <span class="monospaced">MLton.Thread.switch</span> <em>always</em> leaves a\r | |
197 | critical section, it is sometimes necessary to add additional\r | |
198 | <span class="monospaced">atomicBegin</span>-s/<span class="monospaced">atomicEnd</span>-s to ensure that we remain in a critical\r | |
199 | section after a thread switch.</p></div>\r | |
200 | <div class="paragraph"><p>While looking at virtually every file in the core CML implementation,\r | |
201 | I took the liberty of simplifying things where it seemed possible; in\r | |
202 | terms of style, the implementation is about half-way between Reppy’s\r | |
203 | original and MLton’s.</p></div>\r | |
204 | <div class="paragraph"><p>Some changes of note:</p></div>\r | |
205 | <div class="ulist"><ul>\r | |
206 | <li>\r | |
207 | <p>\r | |
208 | <span class="monospaced">util/</span> contains all pertinent data-structures: (functional and\r | |
209 | imperative) queues, (functional) priority queues. Hence, it should be\r | |
210 | easier to switch in more efficient or real-time implementations.\r | |
211 | </p>\r | |
212 | </li>\r | |
213 | <li>\r | |
214 | <p>\r | |
215 | <span class="monospaced">core-cml/scheduler.sml</span>: in both implementations, this is where\r | |
216 | most of the interesting action takes place. I’ve made the connection\r | |
217 | between <span class="monospaced">MLton.Thread.t</span>-s and <span class="monospaced">ThreadId.thread_id</span>-s more abstract\r | |
218 | than it is in the SML/NJ implementation, and encapsulated all of the\r | |
219 | <span class="monospaced">MLton.Thread</span> operations in this module.\r | |
220 | </p>\r | |
221 | </li>\r | |
222 | <li>\r | |
223 | <p>\r | |
224 | eliminated all of the "by hand" inlining\r | |
225 | </p>\r | |
226 | </li>\r | |
227 | </ul></div>\r | |
228 | </div>\r | |
229 | </div>\r | |
230 | <div class="sect1">\r | |
231 | <h2 id="_future_extensions">Future Extensions</h2>\r | |
232 | <div class="sectionbody">\r | |
233 | <div class="paragraph"><p>The CML documentation says the following:</p></div>\r | |
234 | <div class="quoteblock">\r | |
235 | <div class="content">\r | |
236 | <div class="listingblock">\r | |
237 | <div class="content monospaced">\r | |
238 | <pre>CML.joinEvt: thread_id -> unit event</pre>\r | |
239 | </div></div>\r | |
240 | <div class="ulist"><ul>\r | |
241 | <li>\r | |
242 | <p>\r | |
243 | <span class="monospaced">joinEvt tid</span>\r | |
244 | </p>\r | |
245 | <div class="paragraph"><p>creates an event value for synchronizing on the termination of the\r | |
246 | thread with the ID tid. There are three ways that a thread may\r | |
247 | terminate: the function that was passed to spawn (or spawnc) may\r | |
248 | return; it may call the exit function, or it may have an uncaught\r | |
249 | exception. Note that <span class="monospaced">joinEvt</span> does not distinguish between these\r | |
250 | cases; it also does not become enabled if the named thread deadlocks\r | |
251 | (even if it is garbage collected).</p></div>\r | |
252 | </li>\r | |
253 | </ul></div>\r | |
254 | </div>\r | |
255 | <div class="attribution">\r | |
256 | </div></div>\r | |
257 | <div class="paragraph"><p>I believe that the <span class="monospaced">MLton.Finalizable</span> might be able to relax that\r | |
258 | last restriction. Upon the creation of a <span class="monospaced">'a Scheduler.thread</span>, we\r | |
259 | could attach a finalizer to the underlying <span class="monospaced">'a MLton.Thread.t</span> that\r | |
260 | enables the <span class="monospaced">joinEvt</span> (in the associated <span class="monospaced">ThreadID.thread_id</span>) when\r | |
261 | the <span class="monospaced">'a MLton.Thread.t</span> becomes unreachable.</p></div>\r | |
262 | <div class="paragraph"><p>I don’t know why CML doesn’t have</p></div>\r | |
263 | <div class="listingblock">\r | |
264 | <div class="content monospaced">\r | |
265 | <pre>CML.kill: thread_id -> unit</pre>\r | |
266 | </div></div>\r | |
267 | <div class="paragraph"><p>which has a fairly simple implementation — setting a kill flag in the\r | |
268 | <span class="monospaced">thread_id</span> and adjusting the scheduler to discard any killed threads\r | |
269 | that it takes off the ready queue. The fairness of the scheduler\r | |
270 | ensures that a killed thread will eventually be discarded. The\r | |
271 | semantics are little murky for blocked threads that are killed,\r | |
272 | though. For example, consider a thread blocked on <span class="monospaced">SyncVar.mTake mv</span>\r | |
273 | and a thread blocked on <span class="monospaced">SyncVar.mGet mv</span>. If the first thread is\r | |
274 | killed while blocked, and a third thread does <span class="monospaced">SyncVar.mPut (mv, x)</span>,\r | |
275 | then we might expect that we’ll enable the second thread, and never\r | |
276 | the first. But, when only the ready queue is able to discard killed\r | |
277 | threads, then the <span class="monospaced">SyncVar.mPut</span> could enable the first thread\r | |
278 | (putting it on the ready queue, from which it will be discarded) and\r | |
279 | leave the second thread blocked. We could solve this by adjusting the\r | |
280 | <span class="monospaced">TransID.trans_id types</span> and the "cleaner" functions to look for both\r | |
281 | canceled transactions and transactions on killed threads.</p></div>\r | |
282 | <div class="paragraph"><p>John Reppy says that <a href="References#MarlowEtAl01">MarlowEtAl01</a> and <a href="References#FlattFindler04">FlattFindler04</a>\r | |
283 | explain why <span class="monospaced">CML.kill</span> would be a bad idea.</p></div>\r | |
284 | <div class="paragraph"><p>Between <span class="monospaced">CML.timeOutEvt</span> and <span class="monospaced">CML.kill</span>, one could give an efficient\r | |
285 | solution to the recent <span class="monospaced">comp.lang.ml</span> post about terminating a\r | |
286 | function that doesn’t complete in a given time.</p></div>\r | |
287 | <div class="listingblock">\r | |
288 | <div class="content"><div class="highlight"><pre><span class="w"> </span><span class="k">fun</span><span class="w"> </span><span class="n">timeOut</span><span class="w"> </span><span class="p">(</span><span class="n">f</span><span class="p">:</span><span class="w"> </span><span class="n">unit</span><span class="w"> </span><span class="p">-></span><span class="w"> </span><span class="n">'a</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">:</span><span class="w"> </span><span class="n">Time</span><span class="p">.</span><span class="n">time</span><span class="p">):</span><span class="w"> </span><span class="n">'a</span><span class="w"> </span><span class="n">option</span><span class="w"> </span><span class="p">=</span><span class="w"></span>\r | |
289 | <span class="w"> </span><span class="k">let</span><span class="w"></span>\r | |
290 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">iv</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">SyncVar</span><span class="p">.</span><span class="n">iVar</span><span class="w"> </span><span class="p">()</span><span class="w"></span>\r | |
291 | <span class="w"> </span><span class="k">val</span><span class="w"> </span><span class="n">tid</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="n">CML</span><span class="p">.</span><span class="n">spawn</span><span class="w"> </span><span class="p">(</span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">SyncVar</span><span class="p">.</span><span class="n">iPut</span><span class="w"> </span><span class="p">(</span><span class="n">iv</span><span class="p">,</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="p">()))</span><span class="w"></span>\r | |
292 | <span class="w"> </span><span class="k">in</span><span class="w"></span>\r | |
293 | <span class="w"> </span><span class="n">CML</span><span class="p">.</span><span class="n">select</span><span class="w"></span>\r | |
294 | <span class="w"> </span><span class="p">[</span><span class="n">CML</span><span class="p">.</span><span class="n">wrap</span><span class="w"> </span><span class="p">(</span><span class="n">CML</span><span class="p">.</span><span class="n">timeOutEvt</span><span class="w"> </span><span class="n">t</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="p">(</span><span class="n">CML</span><span class="p">.</span><span class="n">kill</span><span class="w"> </span><span class="n">tid</span><span class="p">;</span><span class="w"> </span><span class="n">NONE</span><span class="p">)),</span><span class="w"></span>\r | |
295 | <span class="w"> </span><span class="n">CML</span><span class="p">.</span><span class="n">wrap</span><span class="w"> </span><span class="p">(</span><span class="n">SyncVar</span><span class="p">.</span><span class="n">iGetEvt</span><span class="w"> </span><span class="n">iv</span><span class="p">,</span><span class="w"> </span><span class="k">fn</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="p">=></span><span class="w"> </span><span class="n">SOME</span><span class="w"> </span><span class="n">x</span><span class="p">)]</span><span class="w"></span>\r | |
296 | <span class="w"> </span><span class="k">end</span><span class="w"></span>\r | |
297 | </pre></div></div></div>\r | |
298 | </div>\r | |
299 | </div>\r | |
300 | <div class="sect1">\r | |
301 | <h2 id="_space_safety">Space Safety</h2>\r | |
302 | <div class="sectionbody">\r | |
303 | <div class="paragraph"><p>There are some CML related posts on the MLton mailing list:</p></div>\r | |
304 | <div class="ulist"><ul>\r | |
305 | <li>\r | |
306 | <p>\r | |
307 | <a href="http://www.mlton.org/pipermail/mlton/2004-May/">http://www.mlton.org/pipermail/mlton/2004-May/</a>\r | |
308 | </p>\r | |
309 | </li>\r | |
310 | </ul></div>\r | |
311 | <div class="paragraph"><p>that discuss concerns that SML/NJ’s implementation is not space\r | |
312 | efficient, because multi-shot continuations can be held indefinitely\r | |
313 | on event queues. MLton is better off because of the one-shot nature — when an event enables a thread, all other copies of the thread\r | |
314 | waiting in other event queues get turned into dead threads (of zero\r | |
315 | size).</p></div>\r | |
316 | </div>\r | |
317 | </div>\r | |
318 | </div>\r | |
319 | <div id="footnotes"><hr></div>\r | |
320 | <div id="footer">\r | |
321 | <div id="footer-text">\r | |
322 | </div>\r | |
323 | <div id="footer-badges">\r | |
324 | </div>\r | |
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327 | </html>\r |