Question

I m trying to use async io on linux. As far as i know there re 3 options:

kernel calls (io_submit and friends)
libRT - uses threads in user space
libRTKAIO - wrapper of kernel calls which does not use threads

I m using the last option, and i see, that in my unit test that runs a lot of async io requests in multiple threads, /proc/sys/fs/aio-nr is never higher than 1024. I wonder where lays such limitation.

I ve set /proc/sys/fs/aio-max-nr to 16M, so it s not an issue.

A related question (also mine) aio_write on linux with rtkaio is sometimes long

Answer 1

There s a system call named "io_setup". In librtkaio this call is receiving 1024 as an argument. This, leads to the constant 1024 as i described and hurts performance if there re more than 1024 ios in parallel. Note that 1024 is hardcoded.

Only calling lio_listio in blocking mode will allow you to raise this 1024 bar.

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