Re: kernel cpu entries

Mark Kirkwood <markir@xxxxxxxxxxxxxxx> · Thu, 15 Dec 2005 17:10:07 +1300

Scott Long wrote:

Jonathan Noack wrote:
Kevin Oberman wrote:
Scott Long wrote:
Also, taking out CPU_I586 is usually a bad idea.  It offers no 
performance penalties (unlike CPU_I386 and maybe CPU_I486), but

enables things like optimized bcopy.

Ahh, This is the sort of thing I never realized. Is there anything in
the handbook that covers this? I had always been under the impression
that CPU_I686 enabled all things that the 686 was capable of. I will
build a new kernel to add that back in.

 From tuning(7):

**************************************************

There are a number of *_CPU options that can be commented out.  If you 
only want the kernel to run on a Pentium class CPU, you can easily 
remove I486_CPU, but only remove I586_CPU if you are sure your CPU is 
being recognized as a Pentium II or better.  Some clones may be 
recognized as a Pentium or even a 486 and not be able to boot without 
those options.  If it works, great!  The operating system will be able 
to better use higher-end CPU features for MMU, task switching, 
timebase, and even device operations...

**************************************************
 From /sys/i386/conf/NOTES:
**************************************************
# You must specify at least one CPU (the one you intend to run on);
# deleting the specification for CPUs you don't need to use may make
# parts of the system run faster.
**************************************************
 From npx(4) (also see /sys/i386/i386/support.s):

**************************************************

The NPX registers are normally used to optimize copying and zeroing 
when all of the following conditions are satisfied:

1.   cpu I586_CPU is an option

...

Then copying and zeroing using the NPX registers is normally 30-100% 
faster.

**************************************************
All is rosy until you see that I586_CPU looks like a loss for blowfish 
(if you have an i686 CPU):

/sys/crypto/blowfish/arch/i386/bf_enc.S
As I use AES, I guess I586_CPU is a win for me.  Despite this, I think 
it makes the most sense for I686_CPU to enable the optimized bcopy if 
it really is a win for i686 CPUs.
-Jonathan

I agree, but frankly I've been loath to touch it out of pure fear of the
correctness geeks.  I know that if I go near it, someone will point out
that it's not 100% correct in all cases of some buggy i686 derivative
that hasn't been sold since 1998, and therefore it's better to just
leave it alone and satify that .00001% of the problem.  Or, the
alternate scenario is that people will moan that we should be using
SSE instead, and that any change that doesn't involve SSE is wrong and
a waste of time.  Then a meta-argument will break out over SSE vs SSE2
vs 3DNow, and how again some buggy derivative chip can't use it and
can't be detected or worked around.  I make my peace by just remembering
to leave CPU_I586 enabled on all of my local systems =-)

Is a minor update to the handbook needed in order avoid confusion then? 
e.g. I have been commenting out CPU_I586 on all my PIII systems in the 
(mistaken it would seem) belief that having CPU_I686 only was better.
cheers
Mark
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Re: kernel cpu entries

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