By: Björn Ragnar Björnsson (bjorn.ragnar.delete@this.gmail.com), May 20, 2022 5:54 pm
Room: Moderated Discussions
Mark (nospamplease.delete@this.nothereorthere.com) on May 20, 2022 12:15 pm wrote:
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on May 20, 2022 11:32 am wrote:
> > Brendan (btrotter.delete@this.gmail.com) on May 20, 2022 4:06 am wrote:
> > >
> > > To me, supporting dissimilar CPUs (for both "different ISA" and "same ISA with different instruction
> > > timings") is an interesting engineering challenge [...]
> >
> > That's an odd way to spell "stupid and pointless".
> >
> > We already know what the problem is. Intel already did the "expose that cores
> > are different" thing in their desktop/laptop chips. It was an abject and fundamental
> > failure that is basically unfixable, and Intel already disabled it.
> >
> > So no, it's not an "engineering challenge". It was a broken product. It's the
> > same kind of "engineering challenge" that Itanium was. Just give it up.
> >
> > Linus
>
> Impossible or merely painful?
>
> Catch the unimplemented instruction.
> Tag the process as fat core only.
> Reschedule and restart the instruction
On the surface it looks to be solvable. But it isn't really. As someone pointed else pointed out you would end up with everything running on fat cores because of libraries ...
Couldn't you just continue with library architecture detection? Well how? From the core a process starts on? That's going to go off the rails fast. So, are you going to do statistics in the kernel to move processes around, even after the paths through the librarie's have been selected (and the exotic opcodes are gone, or live as the case may be)?
No, the complexity of supporting this is unsupportable. Not unsupportable in the "technical possibility" sense, but unsupportable in the real world. It's too much of a burden on software development/support to be possible in any meaningly gainful sense. It's never going to realize any useful gains for a measurable amount of users.
It's a total mess, that's what it is. Par for the course: Linus is right. Does that surprise anyone here :) Just forget Intel's brief interlude with mixed ISA cores ever happened.
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on May 20, 2022 11:32 am wrote:
> > Brendan (btrotter.delete@this.gmail.com) on May 20, 2022 4:06 am wrote:
> > >
> > > To me, supporting dissimilar CPUs (for both "different ISA" and "same ISA with different instruction
> > > timings") is an interesting engineering challenge [...]
> >
> > That's an odd way to spell "stupid and pointless".
> >
> > We already know what the problem is. Intel already did the "expose that cores
> > are different" thing in their desktop/laptop chips. It was an abject and fundamental
> > failure that is basically unfixable, and Intel already disabled it.
> >
> > So no, it's not an "engineering challenge". It was a broken product. It's the
> > same kind of "engineering challenge" that Itanium was. Just give it up.
> >
> > Linus
>
> Impossible or merely painful?
>
> Catch the unimplemented instruction.
> Tag the process as fat core only.
> Reschedule and restart the instruction
On the surface it looks to be solvable. But it isn't really. As someone pointed else pointed out you would end up with everything running on fat cores because of libraries ...
Couldn't you just continue with library architecture detection? Well how? From the core a process starts on? That's going to go off the rails fast. So, are you going to do statistics in the kernel to move processes around, even after the paths through the librarie's have been selected (and the exotic opcodes are gone, or live as the case may be)?
No, the complexity of supporting this is unsupportable. Not unsupportable in the "technical possibility" sense, but unsupportable in the real world. It's too much of a burden on software development/support to be possible in any meaningly gainful sense. It's never going to realize any useful gains for a measurable amount of users.
It's a total mess, that's what it is. Par for the course: Linus is right. Does that surprise anyone here :) Just forget Intel's brief interlude with mixed ISA cores ever happened.