Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】

[Bob Chen]

2017-08-01 13:46 GMT+08:00 Alex Williamson <address@hidden>:

> On Tue, 1 Aug 2017 13:04:46 +0800
> Bob Chen <address@hidden> wrote:
>
> > Hi,
> >
> > This is a sketch of my hardware topology.
> >
> >           CPU0         <- QPI ->        CPU1
> >            |                             |
> >     Root Port(at PCIe.0)        Root Port(at PCIe.1)
> >        /        \                   /       \
>
> Are each of these lines above separate root ports?  ie. each root
> complex hosts two root ports, each with a two-port switch downstream of
> it?
>

Not quite sure if root complex is a concept or a real physical device ...

But according to my observation by `lspci -vt`, there are indeed 4 Root
Ports in the system. So the sketch might need a tiny update.


          CPU0         <- QPI ->        CPU1

           |                             |

      Root Complex(device?)      Root Complex(device?)

         /    \                       /    \

    Root Port  Root Port         Root Port  Root Port

       /        \                   /        \

    Switch    Switch             Switch    Switch

     /   \      /  \              /   \     /   \

   GPU   GPU  GPU  GPU          GPU   GPU  GPU   GPU



>
> >     Switch    Switch             Switch    Switch
> >      /   \      /  \              /   \    /    \
> >    GPU   GPU  GPU  GPU          GPU   GPU GPU   GPU
> >
> >
> > And below are the p2p bandwidth test results.
> >
> > Host：
> >    D\D     0      1      2      3      4      5      6      7
> >      0 426.91  25.32  19.72  19.72  19.69  19.68  19.75  19.66
> >      1  25.31 427.61  19.74  19.72  19.66  19.68  19.74  19.73
> >      2  19.73  19.73 429.49  25.33  19.66  19.74  19.73  19.74
> >      3  19.72  19.71  25.36 426.68  19.70  19.71  19.77  19.74
> >      4  19.72  19.72  19.73  19.75 425.75  25.33  19.72  19.71
> >      5  19.71  19.75  19.76  19.75  25.35 428.11  19.69  19.70
> >      6  19.76  19.72  19.79  19.78  19.73  19.74 425.75  25.35
> >      7  19.69  19.75  19.79  19.75  19.72  19.72  25.39 427.15
> >
> > VM：
> >    D\D     0      1      2      3      4      5      6      7
> >      0 427.38  10.52  18.99  19.11  19.75  19.62  19.75  19.71
> >      1  10.53 426.68  19.28  19.19  19.73  19.71  19.72  19.73
> >      2  18.88  19.30 426.92  10.48  19.66  19.71  19.67  19.68
> >      3  18.93  19.18  10.45 426.94  19.69  19.72  19.67  19.72
> >      4  19.60  19.66  19.69  19.70 428.13  10.49  19.40  19.57
> >      5  19.52  19.74  19.72  19.69  10.44 426.45  19.68  19.61
> >      6  19.63  19.50  19.72  19.64  19.59  19.66 426.91  10.47
> >      7  19.69  19.75  19.70  19.69  19.66  19.74  10.45 426.23
>
> Interesting test, how do you get these numbers?  What are the units,
> GB/s?
>



A p2pBandwidthLatencyTest from Nvidia CUDA sample code. Units are
GB/s. Asynchronous read and write. Bidirectional.

However, the Unidirectional test had shown a different result. Didn't fall
down to a half.

VM:
Unidirectional P2P=Enabled Bandwidth Matrix (GB/s)
   D\D     0      1      2      3      4      5      6      7
     0 424.07  10.02  11.33  11.30  11.09  11.05  11.06  11.10
     1  10.05 425.98  11.40  11.33  11.08  11.10  11.13  11.09
     2  11.31  11.28 423.67  10.10  11.14  11.13  11.13  11.11
     3  11.30  11.31  10.08 425.05  11.10  11.07  11.09  11.06
     4  11.16  11.17  11.21  11.17 423.67  10.08  11.25  11.28
     5  10.97  11.01  11.07  11.02  10.09 425.52  11.23  11.27
     6  11.09  11.13  11.16  11.10  11.28  11.33 422.71  10.10
     7  11.13  11.09  11.15  11.11  11.36  11.33  10.02 422.75

Host:
Unidirectional P2P=Enabled Bandwidth Matrix (GB/s)
   D\D     0      1      2      3      4      5      6      7
     0 424.13  13.38  10.17  10.17  11.23  11.21  10.94  11.22
     1  13.38 424.06  10.18  10.19  11.20  11.19  11.19  11.14
     2  10.18  10.18 422.75  13.38  11.19  11.19  11.17  11.17
     3  10.18  10.18  13.38 425.05  11.05  11.08  11.08  11.06
     4  11.01  11.06  11.06  11.03 423.21  13.38  10.17  10.17
     5  10.91  10.91  10.89  10.92  13.38 425.52  10.18  10.18
     6  11.28  11.30  11.32  11.31  10.19  10.18 424.59  13.37
     7  11.18  11.20  11.16  11.21  10.17  10.19  13.38 424.13



>
> > In the VM, the bandwidth between two GPUs under the same physical switch
> is
> > obviously lower, as per the reasons you said in former threads.
>
> Hmm, I'm not sure I can explain why the number is lower than to more
> remote GPUs though.  Is the test simultaneously reading and writing and
> therefore we overload the link to the upstream switch port?  Otherwise
> I'd expect the bidirectional support in PCIe to be able to handle the
> bandwidth.  Does the test have a read-only or write-only mode?
>
> > But what confused me most is that GPUs under different switches could
> > achieve the same speed, as well as in the Host. Does that mean after
> IOMMU
> > address translation, data traversing has utilized QPI bus by default?
> Even
> > these two devices do not belong to the same PCIe bus?
>
> Yes, of course.  Once the transaction is translated by the IOMMU it's
> just a matter of routing the resulting address, whether that's back
> down the I/O hierarchy under the same root complex or across the QPI
> link to the other root complex.  The translated address could just as
> easily be to RAM that lives on the other side of the QPI link.  Also, it
> seems like the IOMMU overhead is perhaps negligible here, unless the
> IOMMU is actually being used in both cases.
>


Yes, the overhead of bandwidth is negligible, but the latency is not as
good as we expected. I assume it is IOMMU address translation to blame.

I ran this twice with IOMMU on/off on Host, the results were the same.

VM:
P2P=Enabled Latency Matrix (us)
   D\D     0      1      2      3      4      5      6      7
     0   4.53  13.44  13.60  13.60  14.37  14.51  14.55  14.49
     1  13.47   4.41  13.37  13.37  14.49  14.51  14.56  14.52
     2  13.38  13.61   4.32  13.47  14.45  14.43  14.53  14.33
     3  13.55  13.60  13.38   4.45  14.50  14.48  14.54  14.51
     4  13.85  13.72  13.71  13.81   4.47  14.61  14.58  14.47
     5  13.75  13.77  13.75  13.77  14.46   4.46  14.52  14.45
     6  13.76  13.78  13.73  13.84  14.50  14.55   4.45  14.53
     7  13.73  13.78  13.76  13.80  14.53  14.63  14.56   4.46

Host:
P2P=Enabled Latency Matrix (us)
   D\D     0      1      2      3      4      5      6      7
     0   3.66   5.88   6.59   6.58  15.26  15.15  15.03  15.14
     1   5.80   3.66   6.50   6.50  15.15  15.04  15.06  15.00
     2   6.58   6.52   4.12   5.85  15.16  15.06  15.00  15.04
     3   6.80   6.81   6.71   4.12  15.12  13.08  13.75  13.31
     4  14.91  14.18  14.34  12.93   4.13   6.45   6.56   6.63
     5  15.17  14.99  15.03  14.57   5.61   3.49   6.19   6.29
     6  15.12  14.78  14.60  13.47   6.16   6.15   3.53   5.68
     7  15.00  14.65  14.82  14.28   6.16   6.15   5.44   3.56



>
> In the host test, is the IOMMU still enabled?  The routing of PCIe
> transactions is going to be governed by ACS, which Linux enables
> whenever the IOMMU is enabled, not just when a device is assigned to a
> VM.  It would be interesting to see if another performance tier is
> exposed if the IOMMU is entirely disabled, or perhaps it might better
> expose the overhead of the IOMMU translation.  It would also be
> interesting to see the ACS settings in lspci for each downstream port
> for each test.  Thanks,
>
> Alex
>


How to display GPU's ACS settings? Like this?

[420 v2] Advanced Error Reporting
UESta: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP-
ECRC- UnsupReq- ACSViol-
UEMsk: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP-
ECRC- UnsupReq- ACSViol-
UESvrt: DLP+ SDES+ TLP- FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+
ECRC- UnsupReq- ACSViol-