I/O sched priorities

[microchip8]

Hi,

Are there any plans in adding support for I/O scheduling classes/priorities so one can use ionice to set them?

[firelzrd]

Hi.

Thank you for asking.
ionice and fair scheduling has been one of the possible improvement ideas.

It can be achieved by introducing I/O contexts (like in BFQ), splitting dl_tree and associating them to each ioc, with vtime-like fair queueing mechanism by whatever metric (either block sizes or latencies), and making fill_batch_queues() to consider it.

I've been considering it for a while, but still I'm not sure whether the benefit is more than the harm from the overhead it adds.
Obviously it's expected to add a little more worst-case latency of important requests as the count of contending I/O processes increase, resulting in relatively spoiled advantage that ADIOS had.

Kyber for example, seems to intentionally ignore anything alike, and focuses on queue size control for best throughput with lowest overhead.

[microchip8]

Hi,

Thanks for explaining. The added overhead will most likely be in the tens of microseconds, like it is for BFQ. I was using Adios for some time but then I needed scheduling priorities and the only I/O sched in Linux at the moment that supports them is BFQ so I switched to it. I don't have any issues with BFQ nor do I notice the overhead, still it would be nice to have another scheduler that supports classes/priorities as well.

And then up for submission in the Linux kernel, no? :)