With the default setting pipe is not used.

Pipe is only used for enable jvm thread pool mode only when jvm_workers > 0 (jvm_workers default = 0).

Further more pipe is never used to transfer the while request or response message.
When under jvm thread pool mode, pipe is only used to transfer a event flag (only one pointer size)。

ONLY IF you cann't resolve your performance problems by increasing worker_processes or reducing single request-response time, you can consider the way of setting jvm_workers > 0 which is not encouraged.

Thread switch cost means Thread context switch cost.