How does x86 pause instruction work in spinlock *and* can it be used in other scenarios?
The pause instruction is commonly used in the loop of testing spinlock, when some other thread owns the spinlock, to mitigate the tight loop. It's said that it is equivalent to some NOP instructions. Could somebody tell me how exactly it works for spinlock optimization? It seems to me that even the NOP instructions are a waste of CPU time. Will they decrease CPU usage?
Another question is that could I use pause instruction for other similar purposes. For example, I have a busy thread which keeps scanning some places (e.g. a queue) to retrieve new nodes; however, sometimes the queue is empty and the thread is just wasting cpu time. Sleep the thread and wake it up by other threads may be an option, however the thread is critical, so I don't want to make it sleep.
Could pause instruction work for my purpose to mitigate the CPU usage? Currently it uses 100% cpu of a physical core?
Answer
PAUSE notifies the CPU that this is a spinlock wait loop so memory and cache accesses may be optimized. See also pause instruction in x86 for some more details about avoiding the memory-order mis-speculation when leaving the spin-loop.
PAUSE may actually stop CPU for some time to save power. Older CPUs decode it as REP NOP, so you don't have to check if its supported. Older CPUs will simply do nothing (NOP) as fast as possible.
See also https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
Update: I don't think it's a good idea to use PAUSE in queue checking unless you are going to make your queue spinlock-like (and there is no obvious way to do it).
Spinning for a very long time is still very bad, even with PAUSE.