Optimized kernel builds: the straight dope

[John Richard Moser]

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Matt Zimmerman wrote:
> == Background ==
> 
> You've all seen our kernel lineup before.  In the beginning, on i386 we had
> linux-386, linux-686, linux-k7, linux-686-smp, linux-k7-smp.  Later, we
> added specialized server kernels, and folded SMP support into linux-686 and
> linux-k7.
> 

[...]

> == Conclusion ==
> 
> Having read over it, I think the numbers are fairly compelling.  The
> difference in performance between -386 and -686 is insigificant; the
> measurements are all within a reasonable error range, and within that range,
> -686 was slower as often as it was faster.
> 

Been there done that.  See benchmarks at the end of:

https://wiki.ubuntu.com/ubuntu-i686

Honestly there are places where 686 code  can become a bit faster; but
they're far and few and in practice we don't hit them much

> My recommendation is that we phase out these additional kernel builds, which
> I expect will save us a great deal of developer time, buildd time, archive
> storage and bandwidth.
> 

I personally like my i686 kernel, just because I don't like the name
'i386'  :)

Why don't you look into glibc as well, since we have i486 vs i686 glibc.

> I'm interested to hear (objective, reasoned) feedback on this data and my
> conclusions from the members of this list.

Matt, can we call the i386 kernel "i486" instead?  I see the config is
for i486 (CONFIG_M486=y)...

Aside from properly relabeling the i386 kernel, I don't have a problem.
 Himem is on in the i386 kernel; and ArchLinux and Gentoo are only
faster because users compile out a lot of the generic stuff we MUST
offer for a complete desktop system (ipv6 for example...) and don't have
as many start-up services/background programs running all the time.
(not to mention Gentoo users tend to try to get --as-needed wherever
they can to speed up load times).

I'd honestly recommend (shameless plug) looking into some new linker
optimizations for Edgy+1; processes sit mostly idle once loaded and the
kernel accounts for about 18% (oprofile) of total system CPU time
anyway[***], so our big issues are dynamic linker time (read:  process
load speed) and memory usage.  I put up an LWN.net article on the topic
at the below link:

http://lwn.net/Articles/192624/

Pretty much -Wl,-O1,--hash-style=both everywhere should be good for
Edgy+1.  -Bdirect would be difficult to maintain and the section it adds
may eventually change.  Drepper doesn't want to include support in
mainline glibc[*][**], although there's been good reaction by several
people on the Binutils mailing list and by Gentoo users and developers.
 Such a shame, too, since it's coming along so well and will probably be
killed by one man's opinions.  :(

On the other hand, every little thing adds up; --hash-style is a
combination of two minor optimizations that gives a substantial increase
overall, for example.  There are gains in some minor places using i686
instructions; and if analysis shows i486 is so insignificant that we
really don't care, I would not oppose moving up to i586 (I have one of
these!) or i686 for the whole system.  I just won't say it's significant
in its own right, especially not for just the kernel.


[*] http://sourceware.org/ml/binutils/2005-10/msg00437.html "Forget it,
we have prelinking, which is much more efficient."
[**] I more bank on this part of the argument:
http://sourceware.org/ml/binutils/2005-10/msg00438.html
[***] I oprofile'd the whole system to figure out how intensive position
independent executables would be (X spends 5% in its main exec; other
tasks spend <=0.02%; system wide total 8%).  At a side glance I noticed
vmlinuz wanted 18% total, which includes whatever the kernel is doing
AND any syscalls that userspace makes.



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