svn commit: r339349 - in head/sys/amd64: amd64 include

[Gleb Smirnoff]

  Mateusz,

On Sat, Oct 13, 2018 at 09:18:32PM +0000, Mateusz Guzik wrote:
M>   Return is almost always 0. The change replaces 3 branches with 1 in the common
M>   case.

This isn't true. For a webserver working with blocking sockets
returning EAGAIN for a very large number of syscalls is normal.

I just dtraced on a random Netflix server and in our case we
get 12% of syscalls with non zero error. But our clients are
special, they request data in small chunks. I believe a regular
web server that serves mostly open ended requests will have a
greater ratio of non-zero returns, up to 50%.

Here is script:

#!/usr/sbin/dtrace -s

fbt::cpu_set_syscall_retval:entry
{
        @[args[1]] = count();
}

I would be interested if anybody reports results on a busy
web server running nginx.

So, I doubt that using __predict_true() is an optimisation here.

-- 
Gleb Smirnoff