Difference between UCX zero-copy protocol and rendezvous protocol

[JiakunYan]

I am working on a paper involving UCX experiment results. I want to understand the difference between the zero-copy protocol and the rendezvous protocol so I can have a clear view what the following environment variables actually change
UCX_ZCOPY_THRESH
UCX_RNDV_THRESH

I would really appreciate the explanation!

[JiakunYan]

Another side question: is there a quick way to query the actual value of those thresholds if I set them to auto?

[changchengx]

check ucx_info tool to get the default value on the running system.
besides RNDV protocol, eager protocol also supports ZCOPY

[JiakunYan]

Thank you! Could you let me know what arguments I should pass to ucx_info? I tried ucx_info -c but it just told me the variable was set to auto. Is there a way to know which value this auto actually selects?

I dug into the source code for a little bit. ZCOPY (for ibverbs) just means to use iovec to transfer the messages, is it correct?

[changchengx]

ucx_info --help.

[JiakunYan]

1. ucx_info --help did not really tell me much regarding this question. I tried some flags but failed to find the information I wanted. Could you point me to the right argument?

2. I would really appreciate it if you could elaborate what you think or point me to the right places of the source code!

[yosefe]

1. Pls run the application with UCX_PROTO_INFO=y to get the information about protocols used at runtime for the specific application
2. Yes, zcopy uses iovec the the SW layers (UCT and ibverbs)

[changchengx]

zcopy uses iovec, does the iov(here) mean the WQE with multiple SGEs?

[yosefe]

zcopy uses iovec, does the iov(here) mean the WQE with multiple SGEs?

If needed, yes.

[changchengx]

I agree with this. I mistook it as UCP datatype IOV here for the original question.

[JiakunYan]

Thank you for your help!