Sunday, July 06

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Daily News Stuff 6 July 2025

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  • The Nvidia RTX 5090 - the fastest graphics card available - can lose up to 25% performance if it doesn't have full PCIe 5.0 x16 bandwidth.  (WCCFTech)

    For example, running at half speed - with either a PCIe x8 slot or PCIe 4.0 - it loses...  Basically nothing.  Maybe 1%.

    At a quarter of the bandwidth - so PCIe 3.0 - it loses 10% of its performance.

    If you drop all the way back to PCIe 2.0 you finally see that 25% performance loss.

    Meaning that PCIe 5.0 doesn't improve performance unless you don't have all 16 lanes available, even on a 5090.

    And if you're using it for a workload that resides mostly on the card, like AI processing, you hardly need anything.  There's more variability between test runs than between a single lane and a full x16 slot.

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Posted by: Pixy Misa at 06:26 PM | Comments (1) | Add Comment | Trackbacks (Suck)
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1 "If you drop all the way back to PCIe 2.0 you finally see that 25% performance loss."
Well, if you cut the bandwidth by a factor of 8:  the highest bar that's about 25% smaller than the full PCIe 5 x16 is the PCIe 4 x4.  So cut out 3/4 of the lanes, and then cut the bandwidth of what's left in half.
PCIe 4 x4 is PCIE 5 x2.
The good side of that is it means bifurcation would be nice, letting you have a second video card or a bunch of extra SSDs with minimal performance loss:  PCIe5 x8 lost about 160 points out of 12K.  I'd take that tradeoff.

Posted by: Rick C at Monday, July 07 2025 02:39 AM (1zWbY)

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