Is it better to poll or wait?

问题:

I have seen a question on why "polling is bad". In terms of minimizing the amount of processor time used by one thread, would it be better to do a spin wait (i.e. poll for a required change in a while loop) or wait on a kernel object (e.g. a kernel event object in windows)?

For context, assume that the code would be required to run on any type of processor, single core, hyperthreaded, multicore, etc. Also assume that a thread that would poll or wait can't continue until the polling result is satisfactory if it polled instead of waiting. Finally, the time between when a thread starts waiting (or polling) and when the condition is satisfied can potentially vary from a very short time to a long time.

Since the OS is likely to more efficiently "poll" in the case of "waiting", I don't want to see the "waiting just means someone else does the polling" argument, that's old news, and is not necessarily 100% accurate.

回答1:

Provided the OS has reasonable implementations of these type of concurrency primitives, it's definitely better to wait on a kernel object.

Among other reasons, this lets the OS know not to schedule the thread in question for additional timeslices until the object being waited-for is in the appropriate state. Otherwise, you have a thread which is constantly getting rescheduled, context-switched-to, and then running for a time.

You specifically asked about minimizing the processor time for a thread: in this example the thread blocking on a kernel object would use ZERO time; the polling thread would use all sorts of time.

Furthermore, the "someone else is polling" argument needn't be true. When a kernel object enters the appropriate state, the kernel can look to see at that instant which threads are waiting for that object...and then schedule one or more of them for execution. There's no need for the kernel (or anybody else) to poll anything in this case.

回答2:

Waiting is the "nicer" way to behave. When you are waiting on a kernel object your thread won't be granted any CPU time as it is known by the scheduler that there is no work ready. Your thread is only going to be given CPU time when it's wait condition is satisfied. Which means you won't be hogging CPU resources needlessly.

回答3:

I think a point that hasn't been raised yet is that if your OS has a lot of work to do, blocking yeilds your thread to another process. If all processes use the blocking primitives where they should (such as kernel waits, file/network IO etc.) you're giving the kernel more information to choose which threads should run. As such, it will do more work in the same amount of time. If your application could be doing something useful while waiting for that file to open or the packet to arrive then yeilding will even help you're own app.

回答4:

Waiting does involve more resources and means an additional context switch. Indeed, some synchronization primitives like CLR Monitors and Win32 critical sections use a two-phase locking protocol - some spin waiting is done fore actually doing a true wait.

I imagine doing the two-phase thing would be very difficult, and would involve lots of testing and research. So, unless you have the time and resources, stick to the windows primitives...they already did the research for you.

回答5:

There are only few places, usually within the OS low-level things (interrupt handlers/device drivers) where spin-waiting makes sense/is required. General purpose applications are always better off waiting on some synchronization primitives like mutexes/conditional variables/semaphores.

回答6:

I agree with Darksquid, if your OS has decent concurrency primitives then you shouldn't need to poll. polling usually comes into it's own on realtime systems or restricted hardware that doesn't have an OS, then you need to poll, because you might not have the option to wait(), but also because it gives you finegrain control over exactly how long you want to wait in a particular state, as opposed to being at the mercy of the scheduler.

回答7:

Waiting (blocking) is almost always the best choice ("best" in the sense of making efficient use of processing resources and minimizing the impact to other code running on the same system). The main exceptions are:

1. When the expected polling duration is small (similar in magnitude to the cost of the blocking syscall).
2. Mostly in embedded systems, when the CPU is dedicated to performing a specific task and there is no benefit to having the CPU idle (e.g. some software routers built in the late '90s used this approach.)

Polling is generally not used within OS kernels to implement blocking system calls - instead, events (interrupts, timers, actions on mutexes) result in a blocked process or thread being made runnable.

回答8:

There are four basic approaches one might take:

1. Use some OS waiting primitive to wait until the event occurs
2. Use some OS timer primitive to check at some defined rate whether the event has occurred yet
3. Repeatedly check whether the event has occurred, but use an OS primitive to yield a time slice for an arbitrary and unknown duration any time it hasn't.
4. Repeatedly check whether the event has occurred, without yielding the CPU if it hasn't.

When #1 is practical, it is often the best approach unless delaying one's response to the event might be beneficial. For example, if one is expecting to receive a large amount of serial port data over the course of several seconds, and if processing data 100ms after it is sent will be just as good as processing it instantly, periodic polling using one of the latter two approaches might be better than setting up a "data received" event.

Approach #3 is rather crude, but may in many cases be a good one. It will often waste more CPU time and resources than would approach #1, but it will in many cases be simpler to implement and the resource waste will in many cases be small enough not to matter.

Approach #2 is often more complicated than #3, but has the advantage of being able to handle many resources with a single timer and no dedicated thread.

Approach #4 is sometimes necessary in embedded systems, but is generally very bad unless one is directly polling hardware and the won't have anything useful to do until the event in question occurs. In many circumstances, it won't be possible for the condition being waited upon to occur until the thread waiting for it yields the CPU. Yielding the CPU as in approach #3 will in fact allow the waiting thread to see the event sooner than would hogging it.