| I'm struggling to think of any real-world networking scenario where cut-through switching for ethernet can provide any sort of benefit. Most data-center networks are involve aggregation/spine/core switches at higher speed than TOR/leaf switches - so the packet has to be buffered. Most every workload is going to involve port collisions (2 packets at the same time needing the same output port) - again a full buffering required (or drops and throughput reduction). Bigger networks start using vlans, mpls, ip routing at switches, and vxlan - requiring more lookups per-packet, larger "minimum chunks of packet seen before port is determined" sizes, etc reducing the value of cut-through. And so on. But i also struggle to find a scenario where a principle engineer at google is refusing to accept that cut-through switches exist, but rather, was refusing to accept that the scenario they're talking through with you would ever involve an effectively deployed cut-through switch (since that's basically never). |
When it works, it reduces latency and reduces buffer memory usage a smidge. This ASIC switch can't afford to buffer any packets, so cut through is the only option... AFAIK, most ethernet switches have at least some buffer memory...
> Most data-center networks are involve aggregation/spine/core switches at higher speed than TOR/leaf switches - so the packet has to be buffered.
If you have traffic flowing within a rack, you can cut through for that where the machines attach to the TOR switch at the same speed.
If you have traffic between racks, and it goes through a spine, it likely needs to buffer at the TORs, but it could cut through at the spine.
> Most every workload is going to involve port collisions (2 packets at the same time needing the same output port) - again a full buffering required (or drops and throughput reduction).
Depends on the workload and redundancy requirements. If you want to have dual link aggregation and no loss of throughput with a single lost link, you'll upgrade line speed when link utilization is at 50%, so maybe 50% of packets could be cut through (depending on burstiness). If latency is a key driver, you may have line speed much higher than throughput and have very little utilization.