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Comment by edwinksl | Hacker News Reader

Would hyperthreading be helpful or harmful?

Hyper threading is not harmful per se. It lets your CPU make forward progress when it would otherwise be stalled waiting for something. My issue is that they call hyperthreads "vCPU" which makes it seem like you're getting a full core, while in reality you're getting 60% of a core at most.

vCPU is a different concept than hyperthreading logical cores, though. They're decoupled. (vCPU comes from virtualization software like Xen.)

They are, but what you are buying is a HT cpu core on aws.

Hyper threading often is harmful when you use it, because while it does let your CPU make forward progress, it does that at the expense of e.g. cache that is evicted.

Obviously depends on your workload, but on my highly parallel "standard" workloads, my experience is that you can get at most 15% more with hyperthreading on (e.g. 4 cores/8 threads) compared to off (4 cores/4 threads), whereas on the cache intensive loads, I get 20-30% LESS with hyperthreading on.

I have never encountered such an abnormal workload. This is also less likely to happen in Broadwell Xeon and up, where last level cache can be partitioned. And this is also less likely to happen on Google Cloud in particular, because Google uses high end CPUs with tons of cache.

If both core threads are memory (and cache) intensive, then you get effectively half the cache size and half the memory bandwidth. Partitioning may make eviction less random, but the cache size is still halved, regardless of how much "tons of cache" you start with.

Increasing cache has the net effect of increasing hit ratio, sometimes substantially. With 20MB per die this may change the calculation of where things drop off. I have found that I can't reliably predict how a chip will perform, so I just wrote a bunch of benchmarks and it takes me about half an hour to see if the chip performs better or worse than I thought it would. Google's Broadwell VMs perform very well.

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