Improve outlier latency by implementing batch size for bioq_disksort

[imp]

Summary

Implement maximum batch size for the disksort algorithm (also called
max car size in the literature). This limits the number of requests
that are batched together before starting a new batch. It has little
effect on the average service time, but can have a large effect on the
worst service time.

Submitted by: emax@

[kib]

IMO the batch size should be per-queue, but this is probably fine for now.

sys/kern/subr_disk.c

28

I do not like that this knob is under debug OID, is it really debugging facility ?

Also, it makes sense to allow the knob to be managed from the very beginning of the system lifetime, mark it as CTLFAG_RWTUN perhaps.

[imp]

This landed in r335066 / a971acbc25f7b9edfcc7f6367bfc69a64296b5da