Number of CQE's generated

[anandv1223]

Hello,

I am experimenting with liburing (kernel version 5.15.126).
I had a basic query.

I am trying to batch 128 reads on a particular fd (tap interface fd).

I get 128 SQE's, associate unique IOV's, prep_readv and do a single submit for the batch. Assuming there is only 1 packet to be read on the fd, how many CQE's will be generated? 1 CQE with bytes read for that single packet or 128 CQE's with only 1 having non-zero cqe->res while the other 127 have a 0 cqe->res because there was nothing to read ?

Apologize if this is based on incorrect understanding of io_uring operations.
Looking for some clarity.

Thanks,
-Anand

[isilence]

This approach is problematic, CQEs can get reordered relatively to actual request execution. E.g. if the fd has a byte range [A,B); [B;C), you can get cqe1 = [B,C), cqe2 = [A,B). And then can have various performance issues, and bearing the overhead of creating additional requests. You'll likely have better time using multishot requests instead.

In any case to answer the question, in most cases you'll get one request reading everything it can, say all 128 bytes, and all other requests will either continue waiting in the kernel or complete if they're executed with some nowait / nonblock flag set.

[anandv1223]

Thanks.
Is multishot supported in Linux kernel version 5.15.126 ?
https://man7.org/linux/man-pages/man3/io_uring_prep_recv_multishot.3.html (requires 6.10)
https://man7.org/linux/man-pages/man3/io_uring_prep_read_multishot.3.html (requires 6.7)