hyperv/storvsc: tune storage performance by changing io max and channel selection

[honzhan_microsoft.com]

Summary

Storage performance IOPS or latency will degrade under multiple thread test with FIO, when comparing with Linux for Hyper-V. The possible reasons are:
(1) IO request queue is not as long as Linux.
(2) outgoing channels were not fully utilized.

So, this fix targets to improve the storage performance by 2.2X on Azure with RAID0 (12 SSD 1T disks) after enlarging the max io request queue, and considering both CPUID and LUN for channel selection.

Submitted by: Hongjiang Zhang <honzhan microsoft com>

[sepherosa_gmail.com]

sys/dev/hyperv/storvsc/hv_storvsc_drv_freebsd.c

139

Since the variable is u_int, we should use SYSCTL_UINT here.

723

We probably should combine this two lines. Since ch_sel is only used once here.

sys/dev/hyperv/vmbus/vmbus_chan.c

1333 ↗

(On Diff #19615)

I don't think we need this. vmbus_subchan_get() is already there.

sys/dev/hyperv/vmbus/vmbus_chanvar.h

92

This does not look correct :P

136

Probably put it in drivers instead of in channel. Not all drivers need this stat.