Blackmagic eGPU Thunderbolt Bandwidth

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William McGough

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Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 3:28 am

I was very intrigued to see the Blackmagic eGPU announcement today (and, of course, disappointed that it only has an RX 580 inside).

However, what surprised me the most was the fact that the enclosure includes USB ports and a downstream Thunderbolt 3 port due to the potential for that decision to impose a substantial performance hit (which is why many TB3 eGPU enclosures like the Sonnet one I own do not include any ports beyond the 1 required upstream TB3 port):

My (albeit amateur) understanding is that PCI Express lanes are discrete (cannot be sub-divided). Therefore, when you add any accessory (even a simple USB hub) to a Thunderbolt 3 enclosure, you lose one entire PCIe lane (out of four) that could be used to connect the GPU, even if the USB Hub only uses a small percentage of the bandwidth afforded by that lane. If this is the case, that would seem to represent a substantial loss (25% of total bandwidth) especially considering that Thunderbolt 3's 4 PCIe lanes are already only 25% of the lanes that would be used to connect a desktop GPU (16) (or 50% of the 8 typically used for laptop GPUs). And when you consider that you would be giving up 25% of your already limited bandwidth to a GPU just for the sake of a few USB ports, this would seem to be a fairly bad trade.

I was wondering if any of the Blackmagic staff (or anyone else for that matter) could clarify:
1. Do the USB Ports on the Blackmagic eGPU utilize 1 of the 4 PCIe lanes of the Thunderbolt 3 connection thereby limiting the GPU to 3 PCIe lanes? (Or does the USB hub use a different part of the Thunderbolt protocol ride outside of the PCIe lanes?)
2. If the USB hub does use one of the PCIe lanes, does it relinquish that lane to the GPU when no USB devices are connected?

Bonus questions:
3. When daisy-chaining a Thunderbolt 3 or DisplayPort display off of the Blackmagic eGPU, does that display have its signal routed directly from the GPU downstream to the display (as would be the case when connecting via the HDMI port) or is it routed back to the host computer before being redirected back down the Thunderbolt connection to the display?
4. Any shot at a Vega 16GB model? (It is 2018, after all!)

I am very glad to see so many companies, especially Blackmagic and Apple embracing and investing in eGPU technologies. I absolutely love being able to use a powerful GPU at home, unplug a cable, and bring a compact laptop with me on the road (as opposed to copying files between my desktop and laptop every time I traveled – which is what i used to do). Now if only we could get some modern GPUs on the Mac side!
Resolve Studio × Micro Panel, 2016 MacBook Pro × eGPU, Canon C200, Pocket Cinema Camera × Metabones, Eizo CG248-4K
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Andrew Kolakowski

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 11:00 am

Definitely stay away from daisy-chaining when it comes to eGPU.

I'm also bit surprised that there are 4 x USB ports as this has to take some bandwidth. TB3 has as far as I understand data link and DP link. I don't think you can (without some special approach) use DP link for sending data, eg USB.

For most source files Resolve has to decode it on CPU (sometimes GPU) and send data to GPU. This means eg 4K or other resolution data has to travel over TB to eGPU. When processed on GPU then it has to come back to CPU for final encode. This means data does travel quite a bit over TB3 and in case of 4K (or higher resolution)+ high fps we are hitting TB3 bandwidth (around 3.5GBytes/sec in reality).
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Xtreemtec

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 11:12 am

I guess the USB 3 ports and downstream Thunderbolt port are more there for when you are not running the eGPU.. So for copying files on your MAC, connecting some usb devices like printers and stuff..

Your machine is not constantly needing the eGPU if youare not running resolve or gaming..
But to prevent you from unpugging and replugging the device constantly you have some expansion ports to work with that are on your desk relativily easy to acces..

I wouldn't run Resolve and have the content on a USB3 or thunderbolt drive connected to the eGPU.. that would not be wise or even work.. ;)
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Andrew Kolakowski

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 11:13 am

Yes, this makes sense.
Although I can already see people connecting external drives to eGPU box and importing/exporting data to it while working in Resolve :)
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Howard Roll

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 8:01 pm

Does external storage really require that much bandwidth? Even 4.6K raw is only about 4 Gbps or 10%. Drop down to any flavor of 4.6K compression or HD and the requirement becomes much smaller. Use optimized media and it becomes a fraction of that.

Additionally the 13" MacBooks only have a single TB3 bus, plug your storage into the machine or the eGPU, it doesn't make any difference.
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Andrew Kolakowski

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostFri Jul 13, 2018 10:29 pm

External storage no, but when data is processed on GPU then it's 32bit float, so this starts to demand quite serious bandwidth. Single 4K frame is 100 MBytes. I don't know if Resolve converts it to something more manageable before sending to final export stage (where it's compressed on CPU to your chosen codec). If not than 4K 50p is above 5GBytes/sec so already way higher than TB3 data link bandwidth.
Remember that data which is processed on GPU/CPU is not anymore RAW (or compressed RAW) data, but 32bit float one with RGB or maybe even 4x32bit float channels (YRGB). This is why GPUs need x8 or x16 PCI-E connection and it's internal memory works at eg. 500GBytes/sec. (TB3 data channels is about 3.5GBytes/s in reality).
Last edited by Andrew Kolakowski on Sat Jul 14, 2018 10:34 am, edited 1 time in total.
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rick.lang

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSat Jul 14, 2018 5:17 am

With just 4K DCI straining the current and near future capabilities of Thunderbolt 3, what’s happening next? I don’t know if there’s a TB 4 coming that ‘joins’ two TB 3 together to provide 8 lanes and/or is able to double the throughput to 80 Gb/s.


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Greg Lee

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSat Jul 14, 2018 10:50 pm

"Disappointed that it only has an RX 580 inside)."

It's a simple price issue. Sonnet has an eGPU model with an RX580, and another model with a Vega.

The RX580 one is $550. The Vega one is $1,300.

https://egpu.io/external-gpu-buyers-guide-2018/
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Uli Plank

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 4:28 am

We need to put this into relation:
It doesn't make much sense to combine a MBP with a super-duper GPU.
Neither is it advisable if you want to work with DR to combine two different cards when connecting this to an iMac, and you don't get more than a 580 there. OK, there is still the overpriced iMac Pro, but the eGPU doesn't come in space-grey, so it's completely useless ;-)

Maybe BM is waiting for another real MacPro (if it ever comes) to offer a more powerful model.
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rick.lang

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 1:52 pm

‘In for a penny, in for a pound.’ Once you make the initial investment in a product line, a company tends to make greater and greater investments. The market at some point, rewards you or not. So it may be waiting for new technology that will make the product more compelling than the initial offering.

Case in point: the original URSA line, followed by the very compelling URSA Mini 4.6K/Pro. We are eagerly waiting to see if the Pocket4K, follows the same trajectory and I think it will.

So this year’s BMD eGPU 580 may be a reasonably successful new product line for a small slice of use cases and early adoptees, to be followed by the eGPU Vega99 in another year or two that’s a perfect mate for a future host. Clearly price, horsepower, bandwidth, and heat management are four balls to juggle to achieve a perfect mate. Balancing those factors today in a decent initial offering Is easier to do with smaller balls. Some of the negative comments here may be a reaction to the size of the initial balls, but keep some perspective: this will appeal to some, with more to come.


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Chris Chiasson

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 5:01 pm

Uli Plank wrote:We need to put this into relation:
It doesn't make much sense to combine a MBP with a super-duper GPU.
Neither is it advisable if you want to work with DR to combine two different cards when connecting this to an iMac, and you don't get more than a 580 there. OK, there is still the overpriced iMac Pro, but the eGPU doesn't come in space-grey, so it's completely useless ;-)

Maybe BM is waiting for another real MacPro (if it ever comes) to offer a more powerful model.


I think it makes sense. Some people don’t want a $5000 desktop AND a $3000 laptop, and would rather just turn a MacBook Pro into a powerhouse, when they’re back at the desk. Some don’t care about being under power while on the go, because they aren’t doing serious rendering until their home. All they need is a $500 to $1000 external monitor, an external harddrive for backups, and a $800 eGPU setup to speed up rendering. That alone saves like $3000 for a dedicated desktop.

And though it’s not advisable, there’s still some benefits to using two GPUs at once. This guy used an eGPU and his iMac Pro to Superpower Resolve for some rendering for example.

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Andrew Kolakowski

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 5:14 pm

Laptop will be never be a workstation replacement for sure. eGPU will help, but then you hit CPU bottleneck (laptops are still huge compromise there also).
This AMD 580 "shines" so much only against laptops with Intel GPUs. When you have better GPUs then difference is not so crazy big.
Someone may argue that for the price of high spec MacBook Pro+ eGPU you can get workstation+simple laptop which overall will give you way more power and real usability. It's all very relative. I also don't think that eGPU is going attract serious Resolve users. It's for those who do rather simple/enthusiast things and "normally" have just a Mackbook Pro at home (slower or faster one).

This is to "simple" test. He's using HD timeline which eGPU should be absolutely fine with. When we move to 4K+ then things may don't look so well. You will start hitting limited speed of TB3 for GPU needs.
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Chris Chiasson

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 5:23 pm

Andrew Kolakowski wrote:Laptop will be never be a workstation replacement for sure. eGPU will help, then you hit CPU bottleneck.

This is to "simple" test. He is using HD timeline which eGPU should be absolutely fine with. When we move to 4K+ then things may don't look so well. You will start hitting limited speed of TB3 for GPU needs.


Who said someone working at home needs a full workstation, and is editing 200 hours of 4K footage every day? Let’s get some context here. If someone needs a full workstation to edit 4K non stop, why would they need a high power laptop with 32gig of ram, 6 core processor, and 4tb of storage? If someone needs both of these, it’s because they’re making so much money, yet working non stop that even in the toilet they’re processing and editing 4K. Which I don’t think is many. The setup I’m describing is for light editing work. And for light editing, you don’t need a dedicated desktop AND a superspec laptop. That’s the group I’m descriptive, which a laptop and eGPU makes perfect sense.

As for the test, the way I see it, we’re only gonna get there if we keep pushing. Eventually Thunderbolt 4 may come out, and elimate bottle necks we currently have. But to get there, we gotta start here.
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Andrew Kolakowski

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 5:31 pm

Main problem is simple- most Apple laptops are not suited for Resolve at all. They cost serious money and then you are "forced" to buy external GPU (which takes whole investment into really big numbers for many). Even then you are still compromising. It will help those with Intel GPUs for sure, but you end up paying huge money for ability to run Resolve on your MacBook Pro.
Lets see real tests how eGPU helps with 4K timelines (with typical sources), as these are the main issues for most current users.
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rick.lang

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostSun Jul 15, 2018 6:14 pm

In the 9to5Mac tests, the eGPU 580 is only slightly slower than the eGPU Vega64. There must be other bottlenecks with his 2017 MacBook constraining the performance of the eGPU Vega 64. Nonetheless this means the BMD eGPU 580 is the value purchase for that computer.


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JPOwens

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Re: Blackmagic eGPU Thunderbolt Bandwidth

PostMon Jul 16, 2018 4:33 pm

Andrew Kolakowski wrote:I also don't think that eGPU is going attract serious Resolve users. It's for those who do rather simple/enthusiast things and "normally" have just a Mackbook Pro at home (slower or faster one).


Concur. At least out of the gate... Its "only" one GPU, and not upgradeable, so it's basically the cost of a stand-alone card albeit in its own housing-- a "terminal" investment as far as future projects are concerned. The take-away for me is that PCIe external enclosures (Cubix, et al) will have to respond with a Thunderbolt-enabled enclosure to get beyond where many of us are - stuck with 2010 MacPro 5,1 towers. Kluged for the last time?

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