From: means@sage.cc.purdue.edu (Neill Means) Subject: [SUMMERY] of Should I use the -m486 flag when recompiling kernel on 386 Date: Tue, 20 Apr 1993 08:35:15 GMT
Here is the summery of letters that I have received to my question:
When recompiling the kernel on .99pl5, should the -m486
be used for use in a 386 w/ FPU linux box?
Here are the responses that I received from various people. Thanks for all
the help guys! I received excellent, informative responses from linux users.
I thank all who responded to my question, You'all are great.
Neill Means
means@sage.cc.purdue.edu
P.S. When recomiling the kerel, what does profiling support for the kernel do?
Is it good/bad? Pros/Cons?
[SUMMERY]:
================================
You probably do not want to compile with the -m486 if you want to get the
most performance out of your kernel. The -m486 flag does some optimizations
for the 486 (FPU or not doesn't matter; optimizations help on a 486sx as well).
These optimizations are due to the instruction timings and pipeline handling
on the 486. The 386 will still execute these instructions, but with reduced
performance.
I have a 386/40 w/387/40 myself. I compared the latest X distribution, which
is compiled with -m486, to the previous one, which is not, and there is
a noticeable decrease in performance. (this is especially visible when
scroling in an xterm)
--
| Anatoly Ivasyuk @ Rochester Institute of Technology |
|-----------------------------------------------------|
| anatoly@nick.csh.rit.edu | ani0349@cs.rit.edu |
| Computer Science House | Computer Science Dept. |
|-----------------------------------------------------|
================
Theoretically the 486 is capable of going super-scalar (executing more
than one instruction in a clock cycle) for certain combinations of certain
instructions. These combinations are not neccesarily the obviously fastest
way of generating code to do something. So, the -m486 switch causes the
compiler to generate code that will run a little faster on a 486 and a little
slower on a 386.
You would probably do a bit better to compile with out the switch if you don't
have a 486.
>
>Sorry about the ignorance of my question.
Not a bad question at all. :)
jem.