60 lines
1.9 KiB
Markdown
60 lines
1.9 KiB
Markdown
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# Spinqueueing: A new locking method that only blocks requests and not threads.
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The idea is that instead of using sleeplocks like mutexes, we instead only spin
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particular request objects by re-posting them to the queue.
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Particular requests may need a given shared resource. Instead of sleeping a
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whole thread while that particular request waits for the resource, we instead
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sleep the request itself by re-posting it into the thread's queue. This
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basically implements a kind of spinlock without busy-waiting. The underlying
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thread is never blocked unless it has no requests that can make forward
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progress.
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Forward progress through requests is only halted when an external resource is
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actually being waited on. Generally this will be an actual hardware event that
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is being waited on. No software bottlenecks will be slept on.
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All locks in the program are simple spinlocks, but the algorithm to spin on them
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is:
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## Each async call has a "locker and invoker":
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int funcThatCallsAnAsyncFunc(...)
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{
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// Do preparatory stuff ...
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// Post the lockvoker to the target thread.
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targetThread.io_service.post(
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[targetThread, /* args to asyncOperationReq captured here */]()
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{
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int nAcquired;
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for (nAcquired=0; nAcquired<nLocksRequired; nAcquired++)
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{
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if (!requiredLocks->tryAcquire()) {
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break;
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}
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}
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if (nAcquired < nLocksRequired)
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{
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for (int i=0; i<nAcquired; i++) {
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requiredLocks->release();
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}
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/* Unsure how to recapture the lambda object and re-enqueue it.
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* Dunno if that's even possible. But this is the essence of the
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* queue-spin system. We re-enqueue the lockvoker until it
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* gets all locks required. Then it will invoke the async
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* frontend.
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*/
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targetThead.io_service.post(this?);
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}
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managerObject.asyncOperationReq(
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/* args to asyncOperationReq passed here */);
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}
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);
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}
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## Idk how to encapsulate lockvokers into a terse, reusable idiom.
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