I assume 1-3 hops of modern switches without congestion. Given 100Gb/s lanes these numbers are possible if you get all the bottlenecks out of the way. The moment you hit a deep queue the latency explodes.

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jiggawatts 7 months ago | parent [-]

So, are you talking about theoretical latencies here based on bandwidths and cable lengths, or actual measured latencies end-to-end between hosts?

I know that "in principle" the physics of the cabling allows single digit microseconds, but I've never seen it anywhere near that low even with cross-over cables with zero switches in-path!

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eqvinox 7 months ago | parent [-]

You need high bandwidth links (time to get the entire packet across starts to matter), run on bare metal (or have very well working HW virtualisation support), and tune NIC parameters and OS processing appropriately. But it's practically achievable.

Switches in these scenarios (e.g. 25GE DC targeted) are pretty predictable and add <1μs (unless misconfigured)

	▲	jiggawatts 7 months ago \| parent [-]
		> But it's practically achievable. I've never seen this in practice. Maaaaybe with Infiniband and custom-written apps that use a proprietary SDK. I'd love to see references to actual benchmarks.