3 ms·

It's true that you can't do the same work 1000 times faster with 1000 cores, but what analysis like this misses is that often you can do 1000 times more work, w

It's true that you can't do the same work 1000 times faster with 1000 cores, but what analysis like this misses is that often you can do 1000 times more work, work that you wouldn't even attempt with a single core system.

tombert 7y ago
That's Gustafson's Law, right? [0] [0] https://en.wikipedia.org/wiki/Gustafson's_law https://en.wikipedia.org/wiki/Gustafson's_law
TheOtherHobbes 7y ago
See also Amdahl's Law for the trade-off between speed and parallelism in massive multicore architectures. https://en.wikipedia.org/wiki/Amdahl%27s_law https://en.wikipedia.org/wiki/Amdahl%27s_law
- tombert 7y ago
  I think the OP knows about Amdahl's Law, but even Ahmdahl's law only talks about fixed-sized problem spaces. The solution to that is to increase the size of the problem space, which is covered by Gustafson's Law. [0] So, basically, any particular problem will only go n-times faster thanks to Ahmdahl's law, but with the advent of more CPUs, we feel less restricted in our problems, and as a result work on bigger data sets. [0] https://en.wikipedia.org/wiki/Gustafson's_law https://en.wikipedia.org/wiki/Gustafson's_law
sgt101 7y ago
Yes - so if you put 1000 cores with 1000 caches on a chip you could run 1000 vm's - more or less, perhaps 10000 in fact? Why aren't there chips like this... I think because it would blow apart the server manufacturers business models, and Intel's business model as well.
- eesmith 7y ago
  Those caches take up space. Or rather, if you could have 1000 cores each with 16 MiB L3 on-die cache, then why don't we see machines now with 10 cores and 1.6 GiB on-die L3 cache? After all, there already exists plenty of software (including some I wrote) which is limited by memory bandwidth, or by latency. Also, all of those 1000 cores would compete for the same main-memory bandwidth, so are much more likely to be bandwidth starved.

tombert 7y ago

That's Gustafson's Law, right? [0] [0] https://en.wikipedia.org/wiki/Gustafson's_law https://en.wikipedia.org/wiki/Gustafson's_law

TheOtherHobbes 7y ago

See also Amdahl's Law for the trade-off between speed and parallelism in massive multicore architectures. https://en.wikipedia.org/wiki/Amdahl%27s_law https://en.wikipedia.org/wiki/Amdahl%27s_law

tombert 7y ago
I think the OP knows about Amdahl's Law, but even Ahmdahl's law only talks about fixed-sized problem spaces. The solution to that is to increase the size of the problem space, which is covered by Gustafson's Law. [0] So, basically, any particular problem will only go n-times faster thanks to Ahmdahl's law, but with the advent of more CPUs, we feel less restricted in our problems, and as a result work on bigger data sets. [0] https://en.wikipedia.org/wiki/Gustafson's_law https://en.wikipedia.org/wiki/Gustafson's_law

sgt101 7y ago

Yes - so if you put 1000 cores with 1000 caches on a chip you could run 1000 vm's - more or less, perhaps 10000 in fact? Why aren't there chips like this... I think because it would blow apart the server manufacturers business models, and Intel's business model as well.

eesmith 7y ago
Those caches take up space. Or rather, if you could have 1000 cores each with 16 MiB L3 on-die cache, then why don't we see machines now with 10 cores and 1.6 GiB on-die L3 cache? After all, there already exists plenty of software (including some I wrote) which is limited by memory bandwidth, or by latency. Also, all of those 1000 cores would compete for the same main-memory bandwidth, so are much more likely to be bandwidth starved.