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Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】
From: |
Bob Chen |
Subject: |
Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】 |
Date: |
Mon, 7 Aug 2017 21:04:16 +0800 |
Besides, I checked the lspci -vvv output, no capabilities of Access Control
are seen.
2017-08-01 23:01 GMT+08:00 Alex Williamson <address@hidden>:
> On Tue, 1 Aug 2017 17:35:40 +0800
> Bob Chen <address@hidden> wrote:
>
> > 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
>
>
> Yes, that's what I expected. So the numbers make sense, the immediate
> sibling GPU would share bandwidth between the root port and upstream
> switch port, any other GPU should not double-up on any single link.
>
> > > > 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
>
> Looks right, a unidirectional test would create bidirectional data
> flows on the root port to upstream switch link and should be able to
> saturate that link. With the bidirectional test, that link becomes a
> bottleneck.
>
> > > > 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
>
> Yes, the IOMMU is not free, page table walks are occurring here. Are
> you using 1G pages for the VM? 2G? Does this platform support 1G
> super pages on the IOMMU? (cat /sys/class/iommu/*/intel-iommu/cap, bit
> 34 is 2MB page support, bit 35 is 1G). All modern Xeons should support
> 1G so you'll want to use 1G hugepages in the VM to take advantage of
> that.
>
> > > 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-
>
> As Michael notes, this is AER, ACS is Access Control Services. It
> should be another capability in lspci. Thanks,
>
> Alex
>
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, (continued)
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Bob Chen, 2017/08/08
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Alex Williamson, 2017/08/08
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Michael S. Tsirkin, 2017/08/08
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Bob Chen, 2017/08/22
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Alex Williamson, 2017/08/22
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Michael S. Tsirkin, 2017/08/22
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Bob Chen, 2017/08/29
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Alex Williamson, 2017/08/29
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Bob Chen, 2017/08/30
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Alex Williamson, 2017/08/30
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】,
Bob Chen <=
- Re: [Qemu-devel] About virtio device hotplug in Q35! 【外域邮件.谨慎查阅】, Alex Williamson, 2017/08/07