Issues regarding ucc_coll_args::tag

[nirandaperera]

The docs mention that a collective tag is a 64-bit integer.

ucc/docs/doxygen/collectives.md

Line 148 in a7e46ee

to provide the “tag”, which is an unsigned 64-bit integer.

Butucc_coll_args.tag is a typedef uint16_t ucc_coll_id_t type. Do you know if this is a bug or intentional?

Also, should the tag be unique to the collective operation individually, or should it be unique to all collective operations globally?

[Sergei-Lebedev]

Hi @nirandaperera, I think it's bug in documentation. Currently "tag" exists only in API, no TLs in UCC support it and it would be difficult to implement 64 bit tags. @manjugv can you please take a look.

[nirandaperera]

@Sergei-Lebedev so does it mean that none of the tls currently support unordered collectives?

[Sergei-Lebedev]

@Sergei-Lebedev so does it mean that none of the tls currently support unordered collectives?

yes, it was added to API for future use.

[nirandaperera]

It seems like the tag value is used in the internal impl.
https://github.com/openucx/ucc/blob/master/src/components/tl/ucp/tl_ucp_coll.h#L339-L340

It seems to be copied to ucc_tl_ucp_task.tagged.tag value. And in the UCP impl, that seems to be used in creating the ucp tag. So, it seems to me that we can still use tagged/ unordered collectives, isn't it?

[nirandaperera]

https://github.com/openucx/ucc/blob/master/src/components/tl/ucp/tl_ucp_tag.h#L16
This tells that a ucp message id is 15 bits long. So, I guess that uint16 type seems to be correct in the underneath impl

[Sergei-Lebedev]

It seems like the tag value is used in the internal impl. https://github.com/openucx/ucc/blob/master/src/components/tl/ucp/tl_ucp_coll.h#L339-L340

It seems to be copied to ucc_tl_ucp_task.tagged.tag value. And in the UCP impl, that seems to be used in creating the ucp tag. So, it seems to me that we can still use tagged/ unordered collectives, isn't it?

Yes, this message tag is used to correctly schedule send/recv operations along with source rank and team id. TL UCP generates tag when user doesn't provide it. The idea is if user gives us a tag we don't generate but use whatever user gives us. Apparently it should work only with TL UCP.