Laptops General - Read Only

[splitframe]

Hi,

I myself own an XPS 15 9550 and noticed that I was not getting the full Thunderbolt 3 speed
over it's Thunderbolt 3 port. I expected speeds around 2700MB/s, because of the overhead with
the theoretical gross speed of 3870MB/s, but I only ever got 1400MB/s out of it.

I did some digging and found out that none of the XPS notebooks with Thunderbolt 3 are even
able to achieve the full TB3 speeds, because they are only connected with two PCI Express lanes (x2.3).

You can read more about this on the following links:

Dell Subreddit Post
Chipset Digging on Forum

Thunderbolt 3 and the promised speeds were the MAIN REASON I even bought this notebook.
I feel very cheated here.

[kwinzman]

Hey Robert,

I am considering to buy the Dell XPS 15 which comes out this month.

However I will only buy it if the Thunderbolt 3 is fixed.

Can you please urgently confirm if this will be fixed for this model and for the new XPS15 2017?

Best

Markus

[jamthiede]

They didn't fix it.  This is from a chat I just had with customer support.

9:31:25 PM   Agent TIP_John Du

The New XPS 15 9650 is with the same as 9550 with TB3 port with 2 PCIe lanes.

[Schmagagled]

Robert-P

At this point somebody should pass a message to Frank Azor at XPS/Alienware and suggest that the XPS 13 specs shown on the current (6 Feb 2016) 9360 model page read as follows:

"Leading-edge connectivity: The Thunderbolt™ 3 multi-use Type-C port allows you to charge your laptop, connect to multiple devices (including support for up to two 4K displays) and enjoy data transfers up to 40Gbps, 8 times that of a USB 3.0i."

Since this appears on the page describing the 9360, it suggests that the Thunderbolt 3 connection in the 9360 is capable of transmitting data up to 40GB when there is conclusive proof that it is only using two lanes (2X) and would never achieve those speeds.

Yes, the spec for Thunderbolt 3 says it will transmit data up to 40GB but it is more than disingenuous at this point to keep advertising that the 9360 will achieve those throughput speeds given it's architecture.

I've returned my 9360 and will await further developments before buying again.

MG

[James McMinn · Accepted solution]

The number of PCI lanes connected to the Thunderbolt 3 port has nothing do with the speed of the Thunderbolt 3 port itself. I know this sounds counter intuitive, but let me explain.

Each PCI lane provides 8Gbit/s of bandwidth. This means that even with 4 lanes, the maximum possible bandwidth over PCI is 32Gbit/s, much lower than the 40Gbit/s that Thunderbolt 3 is capable of.

Instead, Thunderbolt 3 acts as a single interface for a number of different protocols, PCI-e being only one of them. The Thunderbolt port is also connected to a separate set of 8 DisplayPort lanes which provide 34Gbit/s worth of bandwidth to drive two 4K displays at 60Hz. The Thunderbolt port is able to draw bandwidth from both the DP and PCI lanes at the same time (with priority given to DP), so long as the total bandwidth being used at any one time stays below 40Gbit/s. This is where the 40Gbit/s comes from - not the PCI-e bandwidth.

Yes, having only 2 PCI-e lanes connected to the TB3 port does mean there is less PCI-e bandwidth available, however it does not limit the maximum speed of the port itself, which is still capable of delivering 40Gbit/s when used for DisplayPort data, something which would be necessary to reach 40Gbit/s even with 4 PCI-e lanes.

Please see the document for a more detailed explanation: https://thunderbolttechnology.net/sites/default/files/HBD16235_Thunderbolt_TB_r05.pdf

[splitframe]

James McMinn is sadly right. It's technically in spec and there are scenarios where the port reaches the 40Gbit. The only two grievances I have with Dell is that they advertise with 40Gbit bi-directional. Which for the port/cable itself is still true, but since HDMI and DP are one way ( except for some minor display information data ) there are not protocols left to fulfill these 40Gbit/s. Other than a proprietary solution in alternate mode that is.

Second they really could have posted this in the technical details. Some people want a TB3 port for the sole purpose of connecting a super fast SSD. The fact that even the Precision models are neutered that way as workstations is laughable :(

[Schmagagled]

This is exactly what I was hoping to use the 9360 for. I wanted to connect it to an external TB3 enclosure and use it with a GTX Titan class GPU (and fast SSD) and be assured that I would not be bandwidth limited as I have seen reported elsewhere.

Can anybody confirm that the 9365 changed the number of lanes connected to the external TB3 port to 4 (instead of the 2 lanes in the case of the 9360).?

MG

[N1cksys]

Class action!

[hip2b2]

Would you know if this issue has been resolved in more recent XPS 13 9360 laptops?  I am still within the return period of my laptop (purchased at Costco so it is extended) and I hate being taken for a dope on general principal.

Thanks

hip

[splitframe]

There is an official list with the link speed of all models here: www.dell.com/.../thunderbolt-3-40gbps-data-transfer-rate

[samos1111]

> Would you know if this issue has been resolved in more recent XPS 13 9360 laptops?  I am still within the return period of my laptop (purchased at Costco so it is extended) and I hate being taken for a dope on general principal.

As explained by James McMinn, you can get the full 40Gbps TB3 speed when connecting two external 4K@60Hz displays over DP, plus some PCIe traffic. Unfortunately, PCIe traffic is indeed restricted to 2xPCIe (2x8Gbps). This restriction is practically bothersome only in quite specific use-cases. For instance, with external GPU (which are actually not officially supported), or if you would want to connect a fast NVMe SSD externally.

Mind also that TB3 sounds advanced and hyper-cross-compatible but is in practice still a half-backed technology with plenty of compatibility issues even among devices of the same vendor (recall TB15).