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Date:   Tue, 27 Dec 2022 16:32:17 -0600
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Subject: Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0
 controller
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To:     Xinghui Li <korantwork@gmail.com>, Keith Busch <kbusch@kernel.org>
Cc:     nirmal.patel@linux.intel.com, lpieralisi@kernel.org,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Xinghui Li <korantli@tencent.com>
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From:   Jonathan Derrick <jonathan.derrick@linux.dev>
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On 12/23/2022 2:02 AM, Xinghui Li wrote:
> Keith Busch <kbusch@kernel.org> 于2022年12月23日周五 05:56写道：
>>
>> 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.
> 
> We found that tunning the interrupt aggregation can also bring the
> drive performance back to normal.
> 
>> The fio jobs could also probably set their cpus_allowed differently to
>> get better performance in the bypass mode.
> 
> We used the cpus_allowed in FIO to fix 12 dirves in 6 different CPU.
> 
> By the way, sorry for emailing twice, the last one had the format problem.

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.