A number of groups have tried to develop extremely parallel processors but all seem to have gained little traction. There was the XPU 128, the Epiphany(http://www.adapteva.com/) and more recently the Xenon Phi and AMD Epyc. At one point I remember reading a article about sun developing an asynchronous CPU which would be interesting. All these processors run into the same set of problems. 1) x86 silicon is amazingly cheap. 2) supporting multiple CPUs cause more software support for each new CPU architecture. 3) very little software is capable of truly taking advantage of many parallel threads without really funky compilers and software design tools. 4) having designed a fancy CPU most companies try very hard to keep their proprietary knowledge all within their own control where the x86 instruction set must be just about open source now days. 5) getting motherboard manufacturers to take a chance on a new CPU is not an easy thing. My benchmark for processor success is: Does several of Asus,Supermicro,Tyan,Gigabyte et al make a motherboard for this CPU. Even people with deep pockets like DEC with their Alpha CPU and IBM with their Power CPUs have not been able to make a significant inroad into the commodity server world. Mips has had some luck with low to mid range systems for routers and storage systems but their server business is long gone with the death of SGI. Sun/Oracle has had some luck with the Sparc but not all that much outside their own use and I am just speculating but I would bet that Sun/Oracle sells more x86 systems than Sparc systems. ARM seems to be having some luck but I believe that luck is because of their popularity in the small computer systems world sliding into supporting larger systems and not by being designed for servers from the get go. I am a bit of a processor geek and have put lots of effort in the past into elegant processors that just seem to go nowhere. I would love to see some technologies other than the current von Neumann somewhat parallel SMP but I have a sad feeling that that will be a long time coming. With the latest screw-up from Intel and the huge exploit surface that is the Intel ME someone may be able to get some traction by coming up with a processor that is designed and verified for security. On 01/29/2018 05:36 PM, David Collier-Brown via talk wrote:
Kunle Olukotun didn't like systems that wasted their time stalled on loads and branches. He and his team at Afara Websystems therefor designed a non-speculating processor that did work without waits. It became the Sun T1.
Speed without speculating
The basic idea is to have more decoders than ALUs, so you can have lots of threads competing for an ALU. If, for example, thread 0 comes to a load, it will stall, so on the next instruction thread 1 gets the ALU, and runs... until it stalls and thread 2 get the ALU. Ditto for thread 3, and control goes back to thread 0, which has completed a multi-cycle fetch from cache and is ready to proceed once more.
That is the basic idea of the Sun T-series processors.
The strength is that the ALUs are never waiting for work. The weakness is that individual threads still have to wait for data to come from cache.
You can improve on that
Now imagine it isn't entire ALUs that are the available resources, its individual ALU component, like adders. Now the scenario becomes
* thread 0 stalls * thread 1 get an adder * thread 2 gets a compare (really a subtracter) * thread 3 gets a branch unit, and will probably need to wait in the next cycle * thread 4 gets an adder * thread 5 gets an FPU
... and so on. Each cycle, the hardware assigns as many ALU components as it has available to threads, all of which can run. Only the stalled threads are waiting, and they don't need ALU bits to do that.
Now more threads can run at the same time, the ALU components are (probabilistically) all busy, and we have increased capacity. But individual threads are still waiting for cache...
Do I feel lucky?
In principle, we could allocate two adders to thread 5, one doing the current instruction and another doing a subsequent, non-dependent instruction. It's not speculative, but it is out-of-order. That makes some threads twice as fast when doing non-interacting calculations. Allocate it three adders and it's three times as fast.
If we're prepared to have more ALU components than decoders, decode deeply and we have enough of each to be likely to be able to find lots of non-dependent instructions, then we can be executing multiple instructions at once in multiple streams, and probabilistically get /startlingly/ better performance.
I can see a new kind of optimizing compiler, too: one which tries to group non-dependent instructions together.
Conclusion
Is this what happens in a T5? That's a question for a hardware developer: I have no idea... yet
Links:
https://en.wikipedia.org/wiki/Kunle_Olukotun
https://en.wikipedia.org/wiki/Afara_Websystems
https://web.archive.org/web/20110720050850/http://www-hydra.stanford.edu/~ku...
-- David Collier-Brown, | Always do right. This will gratify System Programmer and Author | some people and astonish the rest davecb@spamcop.net | -- Mark Twain
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