Device to host bandwidth limiting factor
 
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sefable
(@sefable)
Active Member
Joined: 9 months ago
 

Hello,

I have a GTX 1080 ( https://egpu.io/gigabyte-aorus-gtx-1080-gaming-box-review-pocket-rocket/).

I would like to know if anyone could explain what is the limiting factor that leads to a device to host bandwidth of 2.6GiB/s as benchmark in the above link?

Thunderbolt 3 with PCIe 3 x4 is 32Gib/s so we should in theory be able to achieve 4GiB/s.

Also, on the same page, the GTX 1070 shows 6GiB/s of memory read, how is this possible?

Thanks for any insights.

Cheers

This topic was modified 8 months ago

To do: Create my signature with system and expected eGPU configuration information to give context to my posts. I have no builds.

.

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itsage
(@itsage)
Illustrious Member Admin
Joined: 4 years ago
 

@sefable Here is the Thunderbolt 3 Overview document by Intel.

 

external graphics card builds
best laptops for external GPU
eGPU enclosure buyer's guide

 
2020 13" MacBook Pro [10th,4C,G] + RTX 2080 Ti @ 32Gbps-TB3 (AORUS Gaming Box) + Win10 2004 [build link]  


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sefable
(@sefable)
Active Member
Joined: 9 months ago
 

Thanks for the documentation, it's a well written one. 

But I can't find the answer to my question. 

To do: Create my signature with system and expected eGPU configuration information to give context to my posts. I have no builds.

.

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joevt
(@joevt)
Noble Member
Joined: 3 years ago
 

@sefable

There is no clear explanation. Thunderbolt 3 is limited to 22 Gbps (2750 MB/s) because reasons...

Even if you try to raid both Thunderbolt ports of a Thunderbolt 3 controller, you only get about 23 Gbps.

When USB4 arrives, we'll see if the same is true for that. At least with USB4, the spec is open and available. In the USB4 spec, it mentions a tradeoff between latency and throughput. Maybe this also applies to Thunderbolt 3 (it refers to PCIe tunnelling which Thunderbolt supports, and also USB4 tunnelling which of course is new to USB4).

DisplayPort tunnelling has different behaviour because a certain bandwidth is required (depends on resolution and color depth). 

Mac mini (2018), Mac Pro (Early 2008), MacBook Pro (Retina, 15-inch, Mid 2015), GA-Z170X-Gaming 7, Sapphire Pulse Radeon RX 580 8GB GDDR5, Sonnet Echo Express III-D, Trebleet Thunderbolt 3 to NVMe M.2 case


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