[kern/sys_pipe.c] PIPE_NODIRECT and pipe throughput

[Dmitry Antipov]

Hello all,

recently I've tried a few benchmarks around pipe throughput on Linux vs. FreeBSD.
Everyone interesting can see my stuff at http://213.148.29.37/PipeBench, and
initial post to Linux kernel developer mailing list at
http://www.uwsg.iu.edu/hypermail/linux/kernel/0803.0/1837.html

It was noticed (http://www.uwsg.iu.edu/hypermail/linux/kernel/0803.0/1842.html) that
the page flipping may be a reason of FreeBSD advantage. I've looked at kern/sys_pipe.c
and found that defining PIPE_NODIRECT should disable it. Is that correct ?

Moreover, when I've tried to run the kernel (7.0-STABLE) with PIPE_NODIRECT defined,
my benchmark didn't see 30% slowdown, as promised in kern/sys_pipe.c comments.
So, what should be done with a pipe to see a difference between PIPE_NODIRECT enabled
and disabled ?

Dmitry