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Amazon doesnt expose much of these statistics (how fast of ram do i get with a M3.large or a c3.med etc) . Does this mean real performance is for those who own their servers?

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RAM performance is the same for VMs and bare metal. And the ~1.5x performance difference between different grades of DRAM (e.g. 1333 vs. 1600 vs. 2133 MHz) is negligible compared to the massive cache-RAM and RAM-flash gaps.

And speaking of cache, lstopo (from the hwloc package) does work correctly under EC2.

It may be the same in the sense that hypervisors don't explicitly limit it, but on a multicore host you're sharing memory bandwidth with the other guests, in the common(?) case when the host has more cores than a guest. You can also experience increased latency when there is access contention.

True, if you want full and predictable memory/cache performance you need to rent a full-machine VM (usually 8xlarge) or bare metal.

It's easy enough to find out. Create an array of linked-list nodes, and have them point to each other randomly (hint: std::random_shuffle w/an array of indices). Write a routine that traverses the list N times. Time the routine for larger lists. You should see a jump as your list gets larger than each stage of cache.

Seems to me that would be very dependent on neighbours and their current workloads?

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