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

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Date:   Fri, 23 Dec 2022 15:53:12 +0800
From:   Xinghui Li <korantwork@...il.com>
To:     Jonathan Derrick <jonathan.derrick@...ux.dev>
Cc:     nirmal.patel@...ux.intel.com, lpieralisi@...nel.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xinghui Li <korantli@...cent.com>
Subject: Re: [PATCH] PCI: vmd: Do not disable MSI-X remapping in VMD 28C0 controller

Jonathan Derrick <jonathan.derrick@...ux.dev> 于2022年12月22日周四 17:15写道：
>
>
>
> On 12/22/22 12:26 AM, korantwork@...il.com wrote:
> > From: Xinghui Li <korantli@...cent.com>
> >
> > Commit ee81ee84f873("PCI: vmd: Disable MSI-X remapping when possible")
> > disable the vmd MSI-X remapping for optimizing pci performance.However,
> > this feature severely negatively optimized performance in multi-disk
> > situations.
> >
> > In FIO 4K random test, we test 1 disk in the 1 CPU
> >
> > when disable MSI-X remapping:
> > read: IOPS=1183k, BW=4622MiB/s (4847MB/s)(1354GiB/300001msec)
> > READ: bw=4622MiB/s (4847MB/s), 4622MiB/s-4622MiB/s (4847MB/s-4847MB/s),
> > io=1354GiB (1454GB), run=300001-300001msec
> >
> > When not disable MSI-X remapping:
> > read: IOPS=1171k, BW=4572MiB/s (4795MB/s)(1340GiB/300001msec)
> > READ: bw=4572MiB/s (4795MB/s), 4572MiB/s-4572MiB/s (4795MB/s-4795MB/s),
> > io=1340GiB (1438GB), run=300001-300001msec
> >
> > 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.

We also find the more drives we tested, the more severe the
performance degradation.
When we tested 8 drives in 6 CPU, there is about 30% drop.

> I think Nirmal would want to to understand if there's some other factor
> going on here.

I also agree with this. The tested server is None io-scheduler.
We tested the same server. Tested drives are Samsung Gen-4 nvme.
Is there anything else you worried effecting test results?