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Comment by timw4mail | Hacker News Reader

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timw4mail·

I'm kind of disappointed all the tests were without ReBAR. I'm curious as to what kind of difference that makes.

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Apologies for the confusion, the tests were run with ReBAR. I've updated the article to reflect that

Shouldn't affect the conclusion for anything besides the PCIe copy to/from GPU tests.

It would make zero difference, except, maybe, for the PCIe bandwidth test. Because all other tests were stressed interfaces that reside within the GPU silicon itself or between the GPU and the DRAM.

Even the numbers of PCIe bandwidth test might not change much, even it's trying to test medium size memory to memory block transfers.

You are probably aware, but ReBAR has a huge effect on these card, so much that it is a requirement to get stable and acceptable FPS in games. I would assume that means the effect would be huge also in those smaller benchmarks.

edit: But was it really off in general, or only for the PCI-e 3.0 part of the article?

ReBAR only matters for transfers across the PCIe link, so for most of the tests in the article it's completely irrelevant. And even for the PCIe transfer tests, ReBAR should only affect the portions of the tests that are accessing more GPU memory than the default BAR size used for 32-bit compatibility.

Considering how much the Arc series loves to share system memory, I'm not so sure.

Intel's having to find and fix every place their drivers assumed that the CPU's RAM and the GPU's RAM were the same pool of memory. As a result, it's completely unsurprising that they're struggling with performance in real applications and games, but it doesn't provide a reason to expect microbenchmarks such as these to run afoul of an irrelevant problem.

These GPUs practically require ReBAR, performance without it drops significantly.

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