3 ms·

For all the siblings describing various forms of register banking, you're actually literally describing SMT in which one core executes two or more tasks at the

by brigade 10y ago

For all the siblings describing various forms of register banking, you're actually literally describing SMT in which one core executes two or more tasks at the same time, dividing the register set and other processor resources between the threads and effectively switching between the tasks every cycle. Modern x86, Power, and SPARC CPUs implement this, as do basically all GPUs for a while now, all for very good reasons. Which is to say, it's quite easy for low-level architectural knowledge to become obsolete :p

noobiemcfoob 10y ago
> Which is to say, it's quite easy for low-level architectural knowledge to become obsolete Truer words never spoken! It doesn't help that the industry has interests in keeping advances trade secrets. Knowledge becomes highly siloed and outmodded rather quickly.
bogomipz 10y ago
I'm familiar with the term "register banking" is this the same as SMT? i.e if a functional unit is free I can execute another instruction there that would otherwise be waiting?
nkurz 10y ago
Which is to say, it's quite easy for low-level architectural knowledge to become obsolete Great insight. It's also a reminder that "best practice" is for a given set of assumptions, and it's good to periodically revisit whether these assumptions still hold. If the assumption is that the speed of register access is the limiting factor, it's probably good intuition to avoid anything that would slow this down. If instead the situation has changed so that memory access is hundreds of times slower than memory access (rather than the about the same), giving up some speed of register access for increased parallelism might be a good tradeoff. It's interesting to think of hyperthreading[1] as being an alternative to adding layers of cache, with GPU's being at the extreme. It's also interesting to ask why 2x hyperthreading has been chosen by Intel, while Power8 allows up to 8x. Is this just market differentiation, or is there something architectural that makes this the right choice for each? And does support for hyperthreading come at a cost in clock speed, or is this already too constrained by heat and power? [1] While I like avoiding made-up marketing terms, I'm using 'hyperthreading' rather than "SMT" here to distinguish multiple register sets sharing a single core from multiple cores sharing a single cache, and from multiple processors sharing access to RAM (SMP). Is there a separate acronym to distinguish "multicore simultaneous multi threading"?

by brigade 10y ago

noobiemcfoob 10y ago

> Which is to say, it's quite easy for low-level architectural knowledge to become obsolete Truer words never spoken! It doesn't help that the industry has interests in keeping advances trade secrets. Knowledge becomes highly siloed and outmodded rather quickly.

bogomipz 10y ago

I'm familiar with the term "register banking" is this the same as SMT? i.e if a functional unit is free I can execute another instruction there that would otherwise be waiting?

nkurz 10y ago

Which is to say, it's quite easy for low-level architectural knowledge to become obsolete Great insight. It's also a reminder that "best practice" is for a given set of assumptions, and it's good to periodically revisit whether these assumptions still hold. If the assumption is that the speed of register access is the limiting factor, it's probably good intuition to avoid anything that would slow this down. If instead the situation has changed so that memory access is hundreds of times slower than memory access (rather than the about the same), giving up some speed of register access for increased parallelism might be a good tradeoff. It's interesting to think of hyperthreading[1] as being an alternative to adding layers of cache, with GPU's being at the extreme. It's also interesting to ask why 2x hyperthreading has been chosen by Intel, while Power8 allows up to 8x. Is this just market differentiation, or is there something architectural that makes this the right choice for each? And does support for hyperthreading come at a cost in clock speed, or is this already too constrained by heat and power? [1] While I like avoiding made-up marketing terms, I'm using 'hyperthreading' rather than "SMT" here to distinguish multiple register sets sharing a single core from multiple cores sharing a single cache, and from multiple processors sharing access to RAM (SMP). Is there a separate acronym to distinguish "multicore simultaneous multi threading"?