Thunderbolt 2 devices and 4K monitor on the same bus

[theSeb]

I know that his has been discussed before in detail, but I cannot remember those details and I haven't been successful in trying to search for the discussions. I thought that I understood how TB and TB2 works, but I no longer think so, so I would appreciate some thoughts on this.

Let's say I want to connect a 4K display to the Mac Pro and a Thunderbolt 2 enclosure with two SSDs in RAID 0. Anandtech's 2013 Mac Pro review tells me that daisy chaining the 4K display off the TB2 enclosure is not a good idea. Fair enough and I understand that so far.

Thunderbolt 2 bonds these channels together to enable 20Gbps in each direction. The total bi-directional bandwidth remains at 40Gbps, but a single device can now use the full 20Gbps. Storage performance should go up if you have enough drives/SSDs to saturate the interface, but more importantly you can now send 4K video over Thunderbolt. Given how big of a focus 4K support is for Apple this round, Thunderbolt 2 mates up nicely with the new Mac Pro.

So far I’ve been able to sustain 1.38GB/s of transfers (11Gbps) over Thunderbolt 2 on the Mac Pro. Due to overhead and PCIe 2.0 limits (16Gbps) you won’t be able to get much closer to the peak rates of Thunderbolt 2.

Here’s where the six Thunderbolt 2 and three TB2 controllers come into play. Although you can daisy chain a 4K display onto the back of a Thunderbolt 2 storage device, doing so will severely impact available write bandwidth to that device. Remember that there’s only 20Gbps available in each direction, and running a 3840 x 2160 24bpp display at 60Hz already uses over 14Gbps of bandwidth just for display. I measured less than 4Gbps of bandwidth (~480MB/s) available for writes to a Thunderbolt 2 device downstream from the Mac Pro if it had a 4K display plugged in to it. Read performance remained untouched since display data only flows from host to display, leaving a full 20Gbps available for reads. If you’re going to connect Thunderbolt 2 devices to the Mac Pro as well as a 4K display, you’ll want to make sure that they aren’t on the same chain.

Great. Not on the same chain. Got that. But what happens if I put the TB2 enclosure and the 4K display on the same bus, in other words, on TB bus 1, port 1 connected to the 4K display and port 3 connected to a TB2 enclosure, for example?

Then let's take this question and say I want to do the same on the latest iMac, which only has one TB bus (unless they have changed this for the riMac, which I don't think they have). Reading through the 2011 iMac review, I am left confuzzled just how much the above scenario will be handled.

Only four lanes are used by Intel's Thunderbolt controller, the remaining lanes are used for things like Bluetooth and WiFi. Do the math and you'll realize that four PCIe 2.0 lanes are only good for 20Gbps of bandwidth, plus DMI between the Z68 chipset and Sandy Bridge is limited to 20Gbps itself. A single Thunderbolt port is capable of 20Gbps of bandwidth (10Gbps in each direction), so that works out well (if you don't use any of the other PCIe devices in the system at the same time). While the 21.5-inch iMac has a single Thunderbolt port, the 27-inch model has two. That's a total of up to 40Gbps of bandwidth to Thunderbolt devices, but only 20Gbps to the controller itself. Don't be fooled by the presence of two Thunderbolt ports on the 27-inch iMac, you don't get any more bandwidth than you would on the 21.5-inch model - you can just hook up more displays.

 

[mintakax]

I know that his has been discussed before in detail, but I cannot remember those details and I haven't been successful in trying to search for the discussions. I thought that I understood how TB and TB2 works, but I no longer think so, so I would appreciate some thoughts on this.

Let's say I want to connect a 4K display to the Mac Pro and a Thunderbolt 2 enclosure with two SSDs in RAID 0. Anandtech's 2013 Mac Pro review tells me that daisy chaining the 4K display off the TB2 enclosure is not a good idea. Fair enough and I understand that so far.



Great. Not on the same chain. Got that. But what happens if I put the TB2 enclosure and the 4K display on the same bus, in other words, on TB bus 1, port 1 connected to the 4K display and port 3 connected to a TB2 enclosure, for example?

Then let's take this question and say I want to do the same on the latest iMac, which only has one TB bus (unless they have changed this for the riMac, which I don't think they have). Reading through the 2011 iMac review, I am left confuzzled just how much the above scenario will be handled.

I would also like to see this answered/discussed or maybe links to more info ?

 

[VirtualRain]

Each bus has a single TB2 controller which has a DP 1.2 input and a PCIe x4 bus interface on one side and a pair of TB2 ports on the other (20Gbps each). Daisy chaining obviously muxes both into one port and contention can occur if both are running near peak bandwidth.

Putting the display and PCIe device on separate ports (same bus) should eliminate contention and allow each to operate at full capacity.

What's not possible is running two 4K DP1.2 displays off one bus as there's only a single internal DP1.2 interface to the TB controller.

Similarly, running two devices that can both saturate a x4 PCIe bus on the same TB bus (controller) will likely create contention if both are used at the same time.

 

[theSeb]

Each bus has a single TB2 controller which has a DP 1.2 input and a PCIe x4 bus interface on one side and a pair of TB2 ports on the other (20Gbps each). Daisy chaining obviously muxes both into one port and contention can occur if both are running near peak bandwidth.

Putting the display and PCIe device on separate ports (same bus) should eliminate contention and allow each to operate at full capacity.

What's not possible is running two 4K DP1.2 displays off one bus as there's only a single internal DP1.2 interface to the TB controller.

Similarly, running two devices that can both saturate a x4 PCIe bus on the same TB bus (controller) will likely create contention if both are used at the same time.

I got it. Thank you.

 

[scottomfg]

Each bus has a single TB2 controller which has a DP 1.2 input and a PCIe x4 bus interface on one side and a pair of TB2 ports on the other (20Gbps each). Daisy chaining obviously muxes both into one port and contention can occur if both are running near peak bandwidth.

Putting the display and PCIe device on separate ports (same bus) should eliminate contention and allow each to operate at full capacity.

What's not possible is running two 4K DP1.2 displays off one bus as there's only a single internal DP1.2 interface to the TB controller.

Similarly, running two devices that can both saturate a x4 PCIe bus on the same TB bus (controller) will likely create contention if both are used at the same time.

Hey VirtualRain, that is fantastic news! We've all been stumped over in the iMac forum, and this certainly seems to be a definitive answer. Can Yyou point me to some source or documentation? I have no reason to not trust you, and I really, really want you to be right, but when it comes to plopping down $4k, I prefer certainty. Thanks!

 

[MacVidCards]

Hey VirtualRain, that is fantastic news! We've all been stumped over in the iMac forum, and this certainly seems to be a definitive answer. Can Yyou point me to some source or documentation? I have no reason to not trust you, and I really, really want you to be right, but when it comes to plopping down $4k, I prefer certainty. Thanks!

I wouldn't get too excited. I don't think the iMac has same setup. They had to use up something to get that 5K display going.

 

[scottomfg]

I wouldn't get too excited. I don't think the iMac has same setup. They had to use up something to get that 5K display going.

What he described wouldn't be affected in any way by the GPU having a unique connection to the 5k display, FYI. Please don't reply unless you actually have something real to add, thanks!

 

[MacVidCards]

What he described wouldn't be affected in any way by the GPU having a unique connection to the 5k display, FYI. Please don't reply unless you actually have something real to add, thanks!

Thanks for the tip. I was also telling you that Mac Pro and iMac have very different implementations of TB2. You have far fewer options and bandwidth to play with so what works on a nMP won't necessarily work on an iMac.

Thank You !