SMT flock of chickens stupidity.

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), August 6, 2018 9:32 am
Room: Moderated Discussions
Brett (ggtgp.delete@this.yahoo.com) on August 5, 2018 10:27 pm wrote:
>
> WOW, this whole tree of SMT threads instantly died after my post. ;)

I think the stupidity of your post just snuffed out everything.

The reason flock of chickens are a failure and only useful for niche markets is not because highly threaded situations don't exist - it's because single-threaded performance still matters a lot. Even in threaded situations, Amdahl kills you, but then when there is little threading at all, flock-of-chickens just look really bad.

So it doesn't matter if you can do really really well in some situations, because you do so abysmally poorly in many real-world situations.

The reason SMT is not stupid is because it doesn't have that issue. If you start with a CPU that does well at single-threaded apps, and then add SMT, it generally won't do worse at the single-threaded ones (assuming you did your SMT well - there are trade-offs you can make in the design that does make the SMT design slightly less powerful on ST). But it will do slightly better when you have lots of threads.

See the difference? SMT can give you a reasonable combination of both worlds. If you have a load where a lot of cores would help, SMT helps. If you don't have that load, you still don't look like a road-kill in the fast lane of single-threaded loads.

Your post and your subject line were stupid. If you want to look less like a moron, stop posting, or at least admit when you were being an idiot.

That said, right now SMT looks bad mainly due to the security concerns of timing attacks between threads. Pretty much all of the Spectre variants tend to work across SMT threads because of shared resources (and not sharing resources makes a lot of the SMT trade-offs look much worse).

But that SMT issue has absolutely nothing to do with the performance reasons people liked SMT.

Linus
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