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Before I buy this and start throwing nvme-hosting 16x PCIe cards in it- is it true that the slots have to share bandwidth and are limited to fewer than the number of lanes present? Or will I be able to put two 16x cards in this machine and get 16 full-speed PCIe 4.0 lanes out of each one, and on down the lanes?
Hi small.world,
The information you're seeking may be included in the support article Install PCIe cards in your Mac Pro (2023):
"When you install or remove PCIe cards, Mac Pro detects the cards in each slot and allocates bandwidth between the two pools to maximize throughput. Expansion Slot Utility lets you view and change how your cards are allocated...
If you want to manually assign the pool allocations for your cards, turn off Automatic Bandwith Configuration and choose a pool for each card. For example, you might want to give a single card 100 percent allocation of pool A.
Whenever you make changes to pool allocation, Expansion Slot Utility prompts you to save your changes and restart your Mac before they take effect."
More info is also outlined at PCIe cards you can install in your Mac Pro (2023).
Regards.
Hi small.world,
The information you're seeking may be included in the support article Install PCIe cards in your Mac Pro (2023):
"When you install or remove PCIe cards, Mac Pro detects the cards in each slot and allocates bandwidth between the two pools to maximize throughput. Expansion Slot Utility lets you view and change how your cards are allocated...
If you want to manually assign the pool allocations for your cards, turn off Automatic Bandwith Configuration and choose a pool for each card. For example, you might want to give a single card 100 percent allocation of pool A.
Whenever you make changes to pool allocation, Expansion Slot Utility prompts you to save your changes and restart your Mac before they take effect."
More info is also outlined at PCIe cards you can install in your Mac Pro (2023).
Regards.
I have come across some information that is not definitive, but is tangentially-related to your query in an Apple article about the Intel Mac Pro 2019 model.
PCI Express Slots
Eight PCI Express slots are enabled by the 64 PCI Express lanes provided by the Intel Xeon processor and a high-performance PCI Express switch fabric on the main logic board that includes a 96-lane[-capable] PCI Express switch.
editorial:
to provide Fewer lanes, even though they are at PCIe 4 speeds, opens Apple up to ridicule that their Apple-Silicon solution is not completive with Intel. So I suggest that what is provided is almost full coverage of all slots at top possible speeds, but just like in the Mac Pro 2019 model, falls ever-so-slightly short of that ideal. But speed bumped up to PCIe 4 speeds.
High-Performance PCI Express Switch Fabric
Mac Pro PCI Express expansion capabilities are enabled by four x16 root ports on the Intel Xeon W processor, providing 64 PCI Express lanes and a high- performance PCI Express switch fabric. All PCI Express slots and Thunderbolt controllers connect either directly or through this PCIe fabric.
PCIe slots 1 and 3, the primary MPX Module slots, each connect directly to an x16 root port on the Xeon processor. This provides maximum bandwidth to the two MPX Module bays, ideal for graphics performance.
This discussion starts on page 10 of this Technology Overview:
https://www.apple.com/mac-pro/pdf/Mac_Pro_White_Paper_Aug_2021.pdf
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4 lanes do not take up 88% of Pool B.
[quote]Many built-in components such as SATA, Ethernet, Wi-Fi, and Bluetooth are connected to the system over PCIe and allocated to Slot B. This means that a system will always show a percentage allocated to Slot B, even if no PCIe cards are installed.[/quote] - Install PCIe cards in your Mac Pro (2023) - Apple Support
I agree Apple's documentation seems to reasonably imply that pool A has at least 24 (16+8) lanes but can offer no verification.
Actually, it’s surprisingly vague. Nowhere on that page does it list how many lanes are available in those two “pools” or why it would even be necessary to do things in that way. This wouldn’t be necessary at all (PCIe lane allocation) if there were enough lanes for the actual hardware; so on the absolute top end of Macs, you have to limit the way you use cards because it doesn’t have enough PCIe lanes to run everything at optimum. But I would like a more official confirmation of this.
Is there anyone at Apple willing to say “this machine has a total number of” X “lanes available for its PCI cards”, which must be allocated from two pools of “X” lanes each”? I have not seen this in any specification anywhere.
Actually I sure would like to keep this public. This should not be a support question - it should be a listed spec. Anyone looking at these machines should know the answer to this question. If I designed a system around a Mac Pro that requires two 16x nvme card hosts and two 8x cards and a 4x card, and I get everything and then there aren’t enough lanes, it’s not going to be a question of “well, your use case is unusual” - it’s a case of leaving out important specifications. If my use case is unusual and nobody needs all those lanes, what’s the harm in saying how many are available?
Since I’d had my response deleted for being speculative, let me revise it.
Here’s a question: do I have full access to all PCIe lanes simultaneously, or do I have a limited number of lanes that I must allocate using the expansion tool? Apple support and sales have no answer for me, so I’m asking for people in the community with direct experience installing in these devices. The pictures I have seen of the expansion management tool indicate that 4 lanes take up 88 percent of the B pool of lanes. Another depiction of this tool on the Apple support site shows two cards, one 16x and one 8x, and the Apple IO card; and says this is “optimal”. I’m not going to draw any conclusions from this as that might be frowned upon, so I will ask if anyone knows verifiably if one can use 2 16x cards at full use of 16 lanes each in the 2023 Mac Pro?
Grant Bennet-Alder wrote:
editorial:
to provide Fewer lanes, even though they are at PCIe 4 speeds, opens Apple up to ridicule that their Apple-Silicon solution is not completive with Intel. So I suggest that what is provided is almost full coverage of all slots at top possible speeds, but just like in the Mac Pro 2019 model, falls ever-so-slightly short of that ideal. But speed bumped up to PCIe 4 speeds.
I'm not sure that we can infer too much about the 2023 Apple Silicon Mac Pro from the 2019 Intel ones.
The 2019 Intel ones could take up to 1.5 TB of RAM (and had slots for installing RAM). The 2023 Apple Silicon ones can be ordered with 192 GB of RAM, and you cannot add RAM after the fact.
The 2019 Intel ones could take PCIe graphics cards and MPX graphics modules (where the MPX modules were built around the use of a couple of PCIe slots). The 2023 Apple Silicon ones do not support graphics cards, as the OS apparently assumes that if it is running on Apple Silicon, it can always make whatever optimization may be enabled by storing all GPU data in unified memory.
If there is no support for graphics cards (which are typically one of the main consumers of PCIe lanes), can we assume that the new Mac Pros MUST support at least as many lanes as the old ones?!?!?
I have looked at a Mac Pro that has no cards in it other than the Apple IO card - and it indicated 88% usage, so it’s certainly likely that the 88% comes from other components plus that card. But there doesn’t seem to be any way to get better use of pool B outside of maybe removing the card and using a Thunderbolt hub. But where do those six TB ports get their (<4) lanes from, and why can’t one just not use the lanes there and instead use them internally? Unless the external ones are limited also; but I can’t speculate on that.
I’m wondering if there’s anyone present who can speak to the efficacy of PCI switching and how much impact it might have on, say, data retrieval from nvme-hosting drives. If it’s not a problem for use cases that involve lots of small random reads and not a lot in terms of long sequential reads, that would be good to know.
Hi small.world,
Based on the information you've provided, your best bet will be to Get Support directly from Apple from here on out. You can use the link here to get started.
Take care.
Well, it is tangential - maybe some things to infer from but also an utterly different architecture. Really surprising that there’s no information for people buying a $10,000 machine.
2023 Mac Pro PCIe speed limited?
