Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

[Keith Busch]

On Thu, Dec 22, 2022 at 02:15:20AM -0700, Jonathan Derrick wrote:
> On 12/22/22 12:26 AM, korantwork@xxxxxxxxx wrote:
> > 
> > However, the bypass mode could increase the interrupts costs in CPU.
> > We test 12 disks in the 6 CPU,
>
> Well the bypass mode was made to improve performance where you have >4
> drives so this is pretty surprising. With bypass mode disabled, VMD will
> intercept and forward interrupts, increasing costs.
> 
> I think Nirmal would want to to understand if there's some other factor
> going on here.

With 12 drives and only 6 CPUs, the bypass mode is going to get more irq
context switching. Sounds like the non-bypass mode is aggregating and
spreading interrupts across the cores better, but there's probably some
cpu:drive count tipping point where performance favors the other way.

The fio jobs could also probably set their cpus_allowed differently to
get better performance in the bypass mode.