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

[Jonathan Derrick <jonathan.derrick@linux.dev>]

As the bypass mode seems to affect performance greatly depending on the specific configuration,
it may make sense to use a moduleparam to control it

I'd vote for it being in VMD mode (non-bypass) by default.

On 12/27/2022 7:19 PM, Xinghui Li wrote:
> Jonathan Derrick <jonathan.derrick@linux.dev> 于2022年12月28日周三 06:32写道：
>>
>> The bypass mode should help in the cases where drives irqs (eg nproc) exceed
>> VMD I/O irqs. VMD I/O irqs for 28c0 should be min(63, nproc). You have
>> very few cpus for a Skylake system with that many drives, unless you mean you
>> are explicitly restricting the 12 drives to only 6 cpus. Either way, bypass mode
>> is effectively VMD-disabled, which points to other issues. Though I have also seen
>> much smaller interrupt aggregation benefits.
> 
> Firstly，I am sorry for my words misleading you. We totally tested 12 drives.
> And each drive run in 6 CPU cores with 8 jobs.
> 
> Secondly, I try to test the drives with VMD disabled,I found the results to
> be largely consistent with bypass mode. I suppose the bypass mode just
> "bypass" the VMD controller.
> 
> The last one，we found in bypass mode the CPU idle is 91%. But in remapping mode
> the CPU idle is 78%. And the bypass's context-switchs is much fewer
> than the remapping
> mode's. It seems the system is watiing for something in bypass mode.