4db7c51b5c
Having a hundred threads doing I/O at the same time is bad on magnetic disks because of the excessive disk seeks. So allow only 4 threads to copy closures in parallel.
68 lines
1.6 KiB
C++
68 lines
1.6 KiB
C++
#pragma once
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#include <atomic>
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#include "sync.hh"
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/* This class hands out tokens. There are only ‘maxTokens’ tokens
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available. Calling get() will return a Token object, representing
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ownership of a token. If no token is available, get() will sleep
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until another thread returns a token. */
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class TokenServer
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{
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unsigned int maxTokens;
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Sync<unsigned int> curTokens{0};
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std::condition_variable_any wakeup;
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public:
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TokenServer(unsigned int maxTokens) : maxTokens(maxTokens) { }
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class Token
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{
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friend TokenServer;
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TokenServer * ts;
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bool acquired = false;
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Token(TokenServer * ts, unsigned int timeout) : ts(ts)
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{
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auto curTokens(ts->curTokens.lock());
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while (*curTokens >= ts->maxTokens)
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if (timeout) {
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if (!curTokens.wait_for(ts->wakeup, std::chrono::seconds(timeout),
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[&]() { return *curTokens < ts->maxTokens; }))
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return;
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} else
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curTokens.wait(ts->wakeup);
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(*curTokens)++;
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acquired = true;
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}
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public:
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Token(Token && t) : ts(t.ts) { t.ts = 0; }
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Token(const Token & l) = delete;
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~Token()
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{
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if (!ts || !acquired) return;
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{
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auto curTokens(ts->curTokens.lock());
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assert(*curTokens);
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(*curTokens)--;
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}
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ts->wakeup.notify_one();
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}
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bool operator ()() { return acquired; }
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};
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Token get(unsigned int timeout = 0)
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{
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return Token(this, timeout);
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}
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};
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