Re: [PLUG] The mysterious case of the Linux Page Table Isolation patches

[Ronald Guilmet <ronpguilmet@gmail.com>]

My first thought was that hardware is cheap today. I think CISC has 
shorting instructions, but uses more memory. When the hardware (RAM) was 
the cost of a car, they pushed the software to take some of the burden, 
right?


Ron


On 1/3/2018 12:46 PM, Rich Freeman wrote:
> On Wed, Jan 3, 2018 at 12:23 PM, George Zipperlen
> <george.zipperlen@mail.com> wrote:
> > How is it that CISC beat out RISC for high performance computing?
> > I haven't payed attention to hardware for 20 years,  but I still remember
> > the Pentium FDIV bug...
> I wouldn't be surprised if some around here know more about this topic
> than I do, but here are some of the factors I'm aware of:
>
> Compatibility is a huge one.  If you have software already written for
> x86 then running it on x86 is usually going to be easier than running
> it on anything else.  I think this is less of an issue for HPC, but I
> bet it is still a factor.
>
> Commodity hardware costs probably are also a factor.  If Intel is
> making a bazillion x86 chips due to desktop demand, buying a few
> thousand for your cluster is going to be really easy and cheap
> compared to something that performs better but which requires more
> exotic hardware.  Google is pretty famous for doing all its data
> crunching on commodity hardware, not even investing in server-grade
> x86 hardware.
>
> Then you get R&D investment.  Due to the size of that desktop market
> Intel spends a fortune on making its x86 processors as capable as
> possible.  Even if the architecture handicaps them the sheer scale of
> their investment probably pays off.
>
> Those are all basically economic factors.  As far as the technology
> itself goes I think just about all new CPU designs are RISC, which
> suggests that RISC is inherently better.  Itanium went the VLIW
> direction, which is even more compiler-dependent than RISC, and that
> died due to economics before the model was really proven either way
> (IMO).
>
> I can't really think of any technical reasons why CISC would beat RISC
> other than legacy code that can't be recompiled using a decent
> compiler.  To the degree that CISC can give hints to the processor to
> better perform speculative execution I'd argue that a RISC instruction
> set could be created that does the same (have the compiler always
> output code that makes a particular branch the most common one).

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