Peformance issues with r278325

[Stanislav Sedov]

> On Mar 18, 2016, at 11:16 AM, Stanislav Sedov <stas at freebsd.org> wrote:
> 
> FWIW we are currently testing the delay '1' change.  Unfortunately, the test is
> not easy to repeat (we didn't find a synthetic one yet that results in the same
> outcome), so it does take more time that I would like.  Will follow up with the 
> results.
> 
> We did try HEAD as well a while ago, and although it exhibited the same pattern.
> However it did not utilize the x2apic unfortunately, as it does seem to be disables
> in the BIOS (FreeBSD reports it being disables in the DMAR table).
> 
> Thanks for looking into it!
> 

Replying to my own message.

The `delay 1` test was a success, we were not able to reproduce the same issues
as we had with a 5us delay in that loop.  So perhaps we should commit the delay
change into stable/10 instead of reverting the code altogether (I assume the original
change was done to solve a real issue?).

--
Stanislav Sedov
ST4096-RIPE