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In some situations, especially when dealing with low latency on processors supporting a variable frequency or when running inside virtual machines, each time the process waits for an I/O using the poller, the processor goes back to sleep or is offered to another VM for a long time, and it causes excessively high latencies. A solution to this provided by this patch is to enable busy polling using a global option. When busy polling is enabled, the pollers never sleep and loop over themselves waiting for an I/O event to happen or for a timeout to occur. On multi-processor machines it can significantly overheat the processor but it usually results in much lower latencies. A typical test consisting in injecting traffic over a single connection at a time over the loopback shows a bump from 4640 to 8540 connections per second on forwarded connections, indicating a latency reduction of 98 microseconds for each connection, and a bump from 12500 to 21250 for locally terminated connections (redirects), indicating a reduction of 33 microseconds. It is only usable with epoll and kqueue because select() and poll()'s API is not convenient for such usages, and the level of performance they are used in doesn't benefit from this anyway. The option, which obviously remains disabled by default, can be turned on using "busy-polling" in the global section, and turned off later using "no busy-polling". Its status is reported in "show info" to help troubleshooting suspicious CPU spikes. |
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