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GPU and Thunderbolt 3 Bandwidth: Why the AMD Radeon Pro W5700X Might be a Better Choice than the AMD Radeon Pro Vega II: Display Stream Compression Support

As I write this, Apple still has not made available the AMD Radeon Pro W5700X graphics card option, but it might be worth waiting for. Here’s why.

As noted in previous posts, running a 5K display eats up nearly half of the Thunderbolt 3 write bandwidth, and a 6K display eats up about 75% of the write bandwidth—you’re stuck with a Thunderbolt 3 bus running at USB speeds (for writes).

About that Dual 6K Display Support on the Apple 2019 MacBook Pro 16-inch

Understanding Thunderbolt 3 Bandwidth

Display Stream Compression (DSC)

See also 2020 iMac 5K Display Stream Compression (DSC) and Its Role in Making Dual 6K Display Support Possible.

Along comes VESA standard Display Stream Compression (DSC), which can cut the bandwidth losses up to 75%, claiming visually undetectable compression of the video signal.

DSC is available with the AMD Radeon W5700X, but not available in the AMD Radeon Pro Vega II or AMD Radeon Pro Vega II Duo, as per Apple specifications, below. The W5700X can also support three 6K displays, whereas the Vega II can support only two 6K displays (not that I anticipate that being an issue for most of us!).

So (price difference aside), you’re left with a crummy choice, either (a) 50% greater GPU compute speed and a degraded Thunderbolt 3 bus, or (b) inferior GPU compute speed and a superior Thunderbolt 3 bus.

Thus except for hard-core video users who will probably use multiple GPUs anyway, the AMD Radeon W5700X looks to be a better overall choice if used as the single video card. A lot depends on whether you have high-performance needs on that bus and/or enough other Thunderbolt 3 busses available.

If the W5700X is installed versus the 580X then there are presumably 3 busses total, two on the card itself on one on the top of the Mac Pro (versus on on the card and one on the Mac Pro with the 580X base video card).

Supposing one buys the Mac Pro with the 580X and then adds the W5700X... presumably one has 4 busses (not five), because the Mac Pro ports on the case can only be mapped to one card or the other.

With four busses via two graphics cards, the DSC issue may be moot for users with one 5K or 6K display, since the other 3 busses area all unmolested. If so, then the Vega II is just as attractive, since one degraded Thunderbolt 3 bus out of 4 is just fine. But... if two 5K or 6K displays are added, then two busses are degraded, leaving two busses at full speed.

What I am likely to do

Pricing is a factor for sure—if the W5700X is around $1200 (half the cost of the Vega II), I might go with that. Along with the 580X I already have installed, three full-performance Thunderbolt 3 busses is enough.

However, I don’t want to regret dumbing down an already large investment in my workflow for the next 3-4 years, so this is steering me towards the Vega II GPU. Things like Gigapixel AI are painfully slow, so if the Vega II can run 50% faster than a W5700X (maybe, maybe not), then that is a huge bump up.

AMD Radeon Pro W5700X specifications, per Apple

  • 40 compute units, 2560 stream processors
  • 16GB of GDDR6 memory with 448GB/s memory bandwidth
  • Up to 9.4 teraflops single precision or 18.9 teraflops half precision
  • Four Thunderbolt 3 ports and one HDMI 2.0 port on card
  • Two DisplayPort connections routed to system to support internal Thunderbolt 3 ports
  • Support for Display Stream Compression (DSC)
  • Support for up to six 4K displays, three 5K displays, or three Pro Display XDRs
  • Full-height MPX Module fills an MPX bay and uses extra power and PCIe bandwidth

AMD Radeon Pro Vega II specifications, per Apple

Lacks support for Display Stream Compression!

  • 64 compute units, 4096 stream processors
  • 32GB of HBM2 memory with 1TB/s memory bandwidth
  • Up to 14.1 teraflops single precision or 28.3 teraflops half precision
  • Infinity Fabric Link connection enables two Vega II GPUs to connect at up to 84GB/s
  • Four Thunderbolt 3 ports and one HDMI 2.0 port on card
  • Two DisplayPort connections routed to system to support internal Thunderbolt 3 ports
  • Support for up to six 4K displays, three 5K displays, or two Pro Display XDRs.
  • Full-height MPX Module fills an MPX bay and uses extra power and PCIe bandwidth
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