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PCI-e Bandwidth question

A

All Taken

macrumors 6502a
Original poster
Dec 28, 2009
790
13
UK
I have a 2012 3.33Ghz 6 core Mac Pro.

I know that the PCI-e slots on my machine are in order of appearance:

4x
4x
16x
16x

Do the top two slots share bandwidth? i.e 8x card in top 4x slot - will it run at 8x bandwidth if the second 4x slot is free?

If I have two Apple 5770's installed in the 16x slots this will cover up the third slot from the bottom as the height is restricting access. If I wanted to use an 8x third party RAID card could I theoretically utilise the combined bandwidth of the top two PCI-e slots? I could only plug the card into the top slot as that would be the only one free but would the Mac Pro allocate 8x bandwidth to that slot with the third from the bottom being empty?

I've tried to explain it in three different ways above so hopefully someone has the answer.

Thanks
 
Concorde Rules said:
Immaterial anyway as X58 (5520) is limited to 36 lanes anyway.

So two x16 lanes = 32, with the final two slots fighting over the last 4... (http://ark.intel.com/products/36783/Intel-5520-IO-Hub).

Easy way to test the slot however, put one of the GPUs in slot 3 and see what happens...
Click to expand...

Ah I see. They do share bandwidth just not in the way I had hoped. If you have 1 16x card in slot one (bottom), and an 8x card in the top I wonder if bandwidth is allocated at 8x that way?

If the bottom two 16x slots are used and a 4x card used in slot three, also another 4x card used in the final slot, where does the magical 'extra' (40 in total) bandwidth come from?
 
All Taken said:
Ah I see. They do share bandwidth just not in the way I had hoped. If you have 1 16x card in slot one (bottom), and an 8x card in the top I wonder if bandwidth is allocated at 8x that way?

If the bottom two 16x slots are used and a 4x card used in slot three, also another 4x card used in the final slot, where does the magical 'extra' (40 in total) bandwidth come from?
Click to expand...

As far as I know, the top two slots go through a PCIe switch as those lanes run through the ICH which only has 4 PCIe lanes. So there's really only 2GB/s of bandwidth available to the top two slots... total. So although a pair of cards in those slots will each negotiate a 4 lane connection, the switch/ICH is bottlenecking everything.

What's worse, I just put an eight lane (x8) Highpoint 2720 in my MP (based on your recommendation - Thanks!) and there's something besides the 4 lane (2GB/s) limit bottlenecking performance on the top slots. With a few SSD's in RAID0, the max sequential reads I get in the top slots is around 1GB/s whereas if I move it to one of the x16 slots, I can get about 1.4GB/s which is about the limit of the drives on the card.
 
VirtualRain said:
As far as I know, the top two slots go through a PCIe switch as those lanes run through the ICH which only has 4 PCIe lanes. So there's really only 2GB/s of bandwidth available to the top two slots... total. So although a pair of cards in those slots will each negotiate a 4 lane connection, the switch/ICH is bottlenecking everything.

What's worse, I just put an eight lane (x8) Highpoint 2720 in my MP (based on your recommendation - Thanks!) and there's something besides the 4 lane (2GB/s) limit bottlenecking performance on the top slots. With a few SSD's in RAID0, the max sequential reads I get in the top slots is around 1GB/s whereas if I move it to one of the x16 slots, I can get about 1.4GB/s which is about the limit of the drives on the card.
Click to expand...

Tough call, I'm actually asking this question as I was going to run two graphics card and my 2720 in the top slot hopefully with 8x performance. I'm now limited to one 16x card and the 2720, do you like the card overall barring the 4x issue?
 
All Taken said:
Tough call, I'm actually asking this question as I was going to run two graphics card and my 2720 in the top slot hopefully with 8x performance. I'm now limited to one 16x card and the 2720, do you like the card overall barring the 4x issue?
Click to expand...

Yeah, the card is great. I have a similar dilemma... I have two recent Apple Cinema Displays with mini-display port connectors each driven by a GT120 in the bottom two slots. So if I want to get full performance from the 2720, I need to update these 2 graphics cards with a single card with dual mini display ports. This is not a big deal as the 5770 can be had for cheap, it's just an unfortunate added cost to getting this setup optimized.
 
VirtualRain said:
Yeah, the card is great. I have a similar dilemma... I have two recent Apple Cinema Displays with mini-display port connectors each driven by a GT120 in the bottom two slots. So if I want to get full performance from the 2720, I need to update these 2 graphics cards with a single card with dual mini display ports. This is not a big deal as the 5770 can be had for cheap, it's just an unfortunate added cost to getting this setup optimized.
Click to expand...

Get a PC 6870 and flash it? They have two mDP with DVI and HDMI.

Unfortunately that is the way it is with X58.
 
Concorde Rules said:
Get a PC 6870 and flash it? They have two mDP with DVI and HDMI.

Unfortunately that is the way it is with X58.
Click to expand...

Thanks, yeah... and it seems that 10.8.3 might have drivers for 7xxx series cards so I might just wait a bit. I'd really prefer an Nvidia card, but they don't appear to make any with dual MDP ports.
 
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