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SubjectRe: a joint letter on low latency and Linux

Larry McVoy writes:
> [somebody]

>> - Use Linus' trick of a multi threaded UP kernel using
>> SMP locks (2.5 material), also hated by Larry/Victor.
>
> I don't know where you got that idea. Sun does the same thing and I've
> always held it up as a good thing, ``eat what you cook'' type of deal.
> The nice thing about doing that is that because of interrupts even a UP
> is really an MP, and you don't have to have two different models when
> doing SMP.
>
> So I don't hate it. I agree with Ted's comment that it would be nice if
> it were possible to have it both ways so as to be able to quantify the
> effects, though I don't know how realistic that is.

Digital UNIX (now Tru64, was OSF/1) uses self-modifying code to
create a generic kernel that can do, if I remember right:

1. plain
2. real-time
3. SMP
4. real-time SMP
5. lock debugging

So, if you don't want these features, you only suffer some NOPs and
perhaps sub-optimal register allocation. I suppose the x86 version
of this could be a debug breakpoint byte followed by a byte to indicate
the required operation -- lock, unlock, or whatever. One might also
be able to make the assembler spit out addresses, like the exception
table code does.

Other uses for this technique include: Generic kernels could run
on a real i386 without being slow on a 486. Modules could work for
SMP, huge memory, and normal kernels. Exact process accounting could
be part of every kernel, but NOPed out at runtime if not enabled.
Perhaps even RT-Linux could use this ability.




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