[gentoo-amd64] Re: Re: amd64 and kernel configuration

[Duncan]

Brett Johnson posted <42E77E5B.4040604@××××.com>, excerpted below,  on
Wed, 27 Jul 2005 07:30:19 -0500:

> netpython wrote:
>> I have enabled SMP on my gentoo AMD64 system and my box doesn't run any
>> slower (or faster).
>>
>>
> As stated earlier by Duncan (in what I thought was a great explanation!);
>
> "If you are using only a single-core AMD64, you'll want SMP off, because
> altho the kernel will work with it on, it'll be more bloated than it needs
> to be."
>
> This just means the physical size of the kernel will be larger than it
> needs to be, and consume more memory. It will have no impact on overall
> system performance.

Exactly so, because when the kernel doesn't detect a second CPU, it'll
disable most of the SMP code and not even touch it, therefore not
affecting performance.

The only exception is the size of the kernel.  Kernel memory is locked
memory -- it cannot be swapped out.  Therefore, a kernel larger than it
has to be means less real memory available for other things, and more
swapping and/or less caching than would otherwise be necessary.  The
effect isn't normally large enough to notice, but it /might/ mean
occasionally waiting an extra few seconds for a swapped out app to load,
or a file to be read from disk that otherwise would have still been in
memory cache, were it not for that additional and entirely unused kernel
bloat.

BTW, that's also a good reason to keep drivers you don't use very often,
likely floppy drivers, perhaps CD/DVD drivers and their filesystems,
perhaps FAT filesystems, perhaps printers drivers and/or anything related
such as parport drivers, perhaps scanner drivers, etc...  keep them all
compiled as modules, and only load those modules when needed, unloading
them later.  A loaded kernel module is part of the kernel, and as such,
again, locked memory, not swappable.  If you only use your floppy drive
once a month, and only use the FAT filesystem when accessing the floppy,
it simply makes no sense to compile it built-in to the kernel, or to keep
those modules loaded when not in use.  Far better to free that memory, so
it may be used by something you are actually /using/.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman in
http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html


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