[ambrop7]

Thanks for the info. Yes I suppose zero-copy can be made to work but surely one needs to go through a LOT of trouble to make it work.

I'm curious about sending data for TCP through, don't you need to have the original data available anyway, in case it needs to be retransmitted? Do the overlapped TX operations (on Windows) complete only once the the data has also been acked? Are you expected to do multiple overlapped operations concurrently to prevent bad performance due to waiting for ack of pending data?

[geocar]

Windows was designed for an era where context switches were around 80µsec (nowadays they're closer to 2µsec), so a lot of that hard work has already paid for itself.

> don't you need to have the original data available anyway, in case it needs to be retransmitted? Do the overlapped TX operations (on Windows) complete only once the the data has also been acked?

I don't know. My knowledge of Windows is almost twenty years old at this point.

If I recall correctly, the TCP driver actually makes a copy when it makes the packet checksums (since you have the cost of reading the pages anyway), but I think behaviour this is for compatibility with Winsock, and it could have used a copy-on-write page, or given a zero page in other situations.