By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), August 9, 2014 1:36 pm
Room: Moderated Discussions
Brett (ggtgp.delete@this.yahoo.com) on August 9, 2014 11:51 am wrote:
>
> For those that are designing 40 watt ARM64 chips there are no legacy 32 bit apps to support
> in the laptop/desktop/server space.
Christ, you people.
You can't have it both ways. You try to argue that "ISA matters a lot for decoding", but then at the same time you try to argue that "ISA doesn't matter at all for users".
The fact that there are no legacy apps is a problem, not a feature. It means that the platform has no testing, that applications are few and hard to find, and that applications and libraries are raw, untested and likely have more bugs. In this case is also means that there are no actual performance numbers to back up your ridiculous and unlikely claims.
Who do you think buys the resulting shit?
Seriously?
Do you seriously believe that the alleged performance and power advantage (and yes, it very much is alleged - I would even go as far as call it "drug-induced hallucinations" - so far nobody has come even close to Intel in the space you claim is so ripe for ARM64 in either performance or price) are so massive that users should/would ignore the fact that the break in ISA also causes a lot of real and inevitable problems?
Guys, get a f*cking grip. It's quite clear that ISA matters at the low end, because when you are counting transistors, decode complexity really shows up. But only crazy and deluded people think that it all that noticeable in the server space. Which is not to say that such crazy and deluded people don't exist - the failed companies that were build up around that concept certainly show that such people do exist - but you need to spend a few seconds asking yourself whether you want to be counted in that group.
Even if ISA complexity is noticeable in that space (and as mentioned, Intel has so far a pretty damn good record of showing that the x86 ISA isn't a problem, since it has successfully killed off every single competing RISC/EPIC/insert-crazy-idea-here architecture), why the hell do you then think that nothing else matters?
ARM64 isn't that magical. MIPS and alpha were there before it with pretty similar "simple decoding". You're repeating arguments that didn't make sense the last time around, and that have been soundly disproven in that thing we call "the real world" (tm) or "the market".
I think ARM has a chance, but the arguments for it in this thread have been absolutely moronic. The performance claims about how much better ARM64 is over ARM32 seem to be based on Geekbench, for chrissake. Some of the other arguments have been about how relatively quickly ARM has improved, which is largely based on the fact that Cortex-A9 was a complete and utter disaster particularly from an uncore standpoint, so when people bandy about "40% improvement" and compare that to how Haswell didn't make as much of a difference, they are clearly not understanding how the ARM baseline was crap.
Even now, people seem to think that Apple A7 is somehow a high-performance chip. It's not really all that impressive, and again, the whole "look at how great it is" seems to be based almost entirely on pure crap (geekbench). And absolutely none of that is relevant to the server market.
No, if ARM has a chance, it's not because it will outperform intel server chips (I can pretty much guarantee it won't), it is because of other market forces.
For example, there are a lot of customers who want to make sure that they have alternatives, and are worried about the fact that Intel is so crushingly dominant. Those customers don't necessarily care about ARM at all, just go back a few months to look at the POWER8 motherboard news etc, but that "we really worry about a monoculture" is very much a real issue.
Similarly, there are a lot of chip companies that want to get a part of the market, and if you don't have the resources of Intel, it's really hard to compete in the x86 space. Because that complexity may not be a huge performance issue in the end, but it does mean that there is a fairly high bar of entry.
So there are reasons for ARM to be successful. We saw it in the mobile space: the licensing was a big boon there, and resulted in the proliferation of infrastructure around ARM. But it really isn't about performance or even power. It's about other market issues.
And if you really think that the decoder is the only part that is complicated, boy have I got a bridge to sell you. A high-performance IO subsystem isn't simple either. Yes, it's all PCIe these days, but go talk to hardware people about how easy it's to do high-performance PCIe, and I suspect they'll laugh at you. Or the memory subsystem. Or a good high-performance SMP fabric.
And no, those aren't exactly "small details" in a server environment. That whole "uncore" that makes sure that you can efficiently receive network packets that get DMA'd to memory at not-cacheline-aligned boundaries? Not simple either. Or just getting multi-socket interrupt controllers that work well, and can both spread things out and steer things properly? Yeah, there's a lot of work there, both in hardware and in all that system code.
And that's all just outside the core itself. Even inside the core, the instruction decoding is just one detail in the end. And a detail that you can tweak - like spending a lot of effort on the branch predictor, so your front end runs ahead better, and so that an extra cycle of decoding latency isn't so noticeable. Or perhaps have a separate decoded uop cache etc.
And then when you make such an absolutely ridiculously huge deal about instruction decoding, you at the same time entirely ignore why the customer, who is the one paying for it all in the end, might care about backwards compatibility.
Really?
So I'm dead serious when I say that anybody who talks about the alleged huge advantages of simple decoding, but then blithely ignores the advantages of backwards compatibility, is a f*cking moron.
And there's a lot of those f*cking morons in this thread. Some of them double down on their stupidity by claiming that it's a big advantage that you can jettison all that legacy baggage.
Linus "rant over" Torvalds
>
> For those that are designing 40 watt ARM64 chips there are no legacy 32 bit apps to support
> in the laptop/desktop/server space.
Christ, you people.
You can't have it both ways. You try to argue that "ISA matters a lot for decoding", but then at the same time you try to argue that "ISA doesn't matter at all for users".
The fact that there are no legacy apps is a problem, not a feature. It means that the platform has no testing, that applications are few and hard to find, and that applications and libraries are raw, untested and likely have more bugs. In this case is also means that there are no actual performance numbers to back up your ridiculous and unlikely claims.
Who do you think buys the resulting shit?
Seriously?
Do you seriously believe that the alleged performance and power advantage (and yes, it very much is alleged - I would even go as far as call it "drug-induced hallucinations" - so far nobody has come even close to Intel in the space you claim is so ripe for ARM64 in either performance or price) are so massive that users should/would ignore the fact that the break in ISA also causes a lot of real and inevitable problems?
Guys, get a f*cking grip. It's quite clear that ISA matters at the low end, because when you are counting transistors, decode complexity really shows up. But only crazy and deluded people think that it all that noticeable in the server space. Which is not to say that such crazy and deluded people don't exist - the failed companies that were build up around that concept certainly show that such people do exist - but you need to spend a few seconds asking yourself whether you want to be counted in that group.
Even if ISA complexity is noticeable in that space (and as mentioned, Intel has so far a pretty damn good record of showing that the x86 ISA isn't a problem, since it has successfully killed off every single competing RISC/EPIC/insert-crazy-idea-here architecture), why the hell do you then think that nothing else matters?
ARM64 isn't that magical. MIPS and alpha were there before it with pretty similar "simple decoding". You're repeating arguments that didn't make sense the last time around, and that have been soundly disproven in that thing we call "the real world" (tm) or "the market".
I think ARM has a chance, but the arguments for it in this thread have been absolutely moronic. The performance claims about how much better ARM64 is over ARM32 seem to be based on Geekbench, for chrissake. Some of the other arguments have been about how relatively quickly ARM has improved, which is largely based on the fact that Cortex-A9 was a complete and utter disaster particularly from an uncore standpoint, so when people bandy about "40% improvement" and compare that to how Haswell didn't make as much of a difference, they are clearly not understanding how the ARM baseline was crap.
Even now, people seem to think that Apple A7 is somehow a high-performance chip. It's not really all that impressive, and again, the whole "look at how great it is" seems to be based almost entirely on pure crap (geekbench). And absolutely none of that is relevant to the server market.
No, if ARM has a chance, it's not because it will outperform intel server chips (I can pretty much guarantee it won't), it is because of other market forces.
For example, there are a lot of customers who want to make sure that they have alternatives, and are worried about the fact that Intel is so crushingly dominant. Those customers don't necessarily care about ARM at all, just go back a few months to look at the POWER8 motherboard news etc, but that "we really worry about a monoculture" is very much a real issue.
Similarly, there are a lot of chip companies that want to get a part of the market, and if you don't have the resources of Intel, it's really hard to compete in the x86 space. Because that complexity may not be a huge performance issue in the end, but it does mean that there is a fairly high bar of entry.
So there are reasons for ARM to be successful. We saw it in the mobile space: the licensing was a big boon there, and resulted in the proliferation of infrastructure around ARM. But it really isn't about performance or even power. It's about other market issues.
And if you really think that the decoder is the only part that is complicated, boy have I got a bridge to sell you. A high-performance IO subsystem isn't simple either. Yes, it's all PCIe these days, but go talk to hardware people about how easy it's to do high-performance PCIe, and I suspect they'll laugh at you. Or the memory subsystem. Or a good high-performance SMP fabric.
And no, those aren't exactly "small details" in a server environment. That whole "uncore" that makes sure that you can efficiently receive network packets that get DMA'd to memory at not-cacheline-aligned boundaries? Not simple either. Or just getting multi-socket interrupt controllers that work well, and can both spread things out and steer things properly? Yeah, there's a lot of work there, both in hardware and in all that system code.
And that's all just outside the core itself. Even inside the core, the instruction decoding is just one detail in the end. And a detail that you can tweak - like spending a lot of effort on the branch predictor, so your front end runs ahead better, and so that an extra cycle of decoding latency isn't so noticeable. Or perhaps have a separate decoded uop cache etc.
And then when you make such an absolutely ridiculously huge deal about instruction decoding, you at the same time entirely ignore why the customer, who is the one paying for it all in the end, might care about backwards compatibility.
Really?
So I'm dead serious when I say that anybody who talks about the alleged huge advantages of simple decoding, but then blithely ignores the advantages of backwards compatibility, is a f*cking moron.
And there's a lot of those f*cking morons in this thread. Some of them double down on their stupidity by claiming that it's a big advantage that you can jettison all that legacy baggage.
Linus "rant over" Torvalds
