Crossfire x16+x4 vs x16+x16

[RaymondL]

Hey guys!

I currently have a r9 280x and im planning to crossfire it however my motherboard only has 1 pcie that is 16x and my other 1 is a pcie x4 slot i thought it was going to be ok since my motherboard does support crossfire but i was just wondering will i see alot of performance difference if i did use x16+x4 versus x16+x16? I was looking at linus's video called

"Radeon HD 6990 Bandwidth Comparison Test 16x vs 8x vs 4x 3DMark 11 Linus Tech Tips"

I saw there was not much difference in performance and i was wondering if it would just be better for me to get a new motherboard that has 2x x16 pcie slots?

Would i see alot of difference if i had a x16+x16 slots? Or could i just stick to x16+x4?

 

By the way my motherboard is a Gigabyte 970a-UD3P Rev 1

[iamdarkyoshi]

It depends on the cpu as well. I have a 2500 and a z77 chipset and I have the same limitations but it really doesnt effect much.

[NumLock21]

Mainstream boards cannot do x16/x16 unless it has a PLX chip. What you can get is a board that does x8/x8, but IMO it's a waste to buy a new board for CFX.

[RaymondL]

so regardless of crossfiring with a x16+x4 slot it wont affect performance much compared to x16+x16 slot?

[SkilledRebuilds]

so regardless of crossfiring with a x16+x4 slot it wont affect performance much compared to x16+x16 slot?

Actually it can greatly...mind you this was less/never much of an issue with older cards, but todays powerhouses,.......another story.

 

 

If.......the 2nd slot is PCIE 2.0 4x FROM THE CHIPSET, NOT THE CPU like the first PCIE slot is...The 'data transfer speed' needed is just not good enough on the 2nd slot.

/Caps for importance,...not yelling

I would have to say that past knowledge and articles,...

16x/4x crossover you have is the issue,.. and its also running at PCIE 2.0 4x, not PCIE 3.0 4x,...

 

The x4 lanes are not delivered from the CPU like the first 16x slot is, adding latency and performance penalties.

There are threads/forums out there that can explain it in more detail, but reports that some highend setups on such 16x/4x can actually decrease performance over a single card as well as on the other hand, only giving a marginal increase but with more stuttering which 'feels' like its worse than a single card can do.

 

It's not just frame pacing (crossfire smoothness) being the issue, as the 4x slot is the issue, no frame pacing fix's that AMD now include in their drivers will help you.

Invest in a motherboard than can do PCIE 3.0 16x/16x or PCIE 8x8 when in crossfire/sli.

 

Using a single card in the PCIE 4x slot won't show the issues either though...,.. as its the transfer of data (between two cards) and that PCIE 2.0 4x slot being too slow and no doubt is the cause of your issues.

 

/Im gunna go hunt for articles on this.

 

http://forums.whirlpool.net.au/archive/2269391

I took the plunge and upgraded my motherboard to a Gigabyte GA-Z97X-SLI . This motherboard supports both PCI-E slots in 8x in crossfire as well as a few other features I was after.

I jumped straight into BF4 and saw a massive performance increase. I'm pulling 80-120fps on ultra on triple monitors (5040x1050)

Also saw an improvment of around 1.5x in a quick 3DMark run of a single test. Haven't done too much serious testing.

So in conclusion, it was either the H87M chipset that couldn't handle crossfire very well, or the 4x secondary slot.

 

http://www.tomshardware.com/answers/id-2312541/crossfire-290-pcie-0x16-0x4-bottleneck-res.html

Games not playing nice with crossfire on my current set up are:

Farcry 3 - more dropped frames and stutter than actual showed frames, reported FPS not bad - unplayable

Counter Strike GO - stuttering with crossfire

Grid Autosport - global illumination results in flicking headlights (strobing with v-sync / scanning with out v-sync)

planetary annihilation - stuttering with crossfire

skyrim - poor performance / stuttering - unplayable

watchdogs - large stutters - similar to farcry 3 - unplayable

Assassin's creed 4 - lighting/shading bugs with fixed horizontal lines of 20-30 pixels being darker on certain surfaces - average performance

lichdom: battlmage - poor performance / stuttering - unplayable

Games playing nice:

tomb raider

crysis 3

crysis 2

metro last light (nvidia physx MUST be disabled (else 4fps lol))

Benchmarks (synthetics) all so far work perfectly...

In general I find I have more poor performing titles than good ones... But when it works... 4k gaming with the settings cranked is like nothing else - as soon as a single card capable of 4k gaming @ reasonable frame rates is released ill go right back to a single card - unless more effort is invested in making crossfire/sli more reliable and polished.

Hoping stuttering is caused by xDMA over chipset/southbridge (as opposed to cpu/pcie bus/northbridge) as such new mobo will resolve. I belive that its less a bandwidth issue and more a latency issue (it takes longer for the cards to communicate than it does to render a frame as such the data is lost??) as some engines do not have an issue yet others do.

 

 

There are way more out there to read...

In this picture, PCIE 2.0 4x is = to the PCIE 3.0 2x shown.

 

A noticeable drop in performance on a single card in 2x (I'd call it frame latency though)

I would not use the above picture normally, as its not indicative of your actual issues ^Single card, not SLI, but it does indicate the fact that those PCIE 2.0 x4 lanes are not in any way doing good towards multi-gpu scaling when using a 3.0 16x & 2.0 4x setup.

[iamdarkyoshi]

With a single hd7950 running unengine heaven on full everything at 4800x1200, I get an average of 15 fps. Enabling crossfire to the second card roughly doubles it and I know the second slot is NOT 16x as only half of the pins are even present in the slot.