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Google Explains Meltdown, Spectre Fix Impact On Cloud Services

Unlike many had predicted, Meltdown--the Intel-only vulnerability that is fixed by forcing the CPU to reload its TLB when running a kernel process--wasn’t the biggest headache for Google.

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  1. "Since we regular users don’t have this visibility into our software, we might end up being the hardest hit by the Meltdown/Spectre fixes."

    There have been a number of tests that show desktop performance will not be affected significantly. In doing some tests, the biggest difference in one test for a heavy Photoshop load only amounted to a tenth of a second difference. So almost all desktop users won't see much of a difference.

    However, the biggest hit is in all those disk benchmarks. In their synthetic tests, they are seeing 30-40% drops. Now that is going to be interesting to see how they figure out how to get accurate results. I feel a little sorry for the ones doing those benchmarks; perhaps they will need to keep an unpatched system (non-Internet connected, of course) to keep running them for a while.
  2. 2Be_or_Not2Be said:
    However, the biggest hit is in all those disk benchmarks. In their synthetic tests, they are seeing 30-40% drops. Now that is going to be interesting to see how they figure out how to get accurate results. I feel a little sorry for the ones doing those benchmarks; perhaps they will need to keep an unpatched system (non-Internet connected, of course) to keep running them for a while.

    Those synthetic tests aren't worth a fart, especially for gauging the performance hit for consumer workloads post-MD/Spectre. TH already tested storage and the hit was negligible. This is why they ran that article in the first place... in response to all the web outcry quoting synthetic storage bench results.

    http://www.tomshardware.com/news/microsoft-meltdown-patch-storage-performance,36236.html

    Obviously commercial workloads are going to be a different animal.
  3. Also, I know this should be obvious... but since Retpoline requires a recompile... I can't help but think gee that's not exactly a silver bullet. How is that superior to a microcode update??
  4. alextheblue said:
    Also, I know this should be obvious... but since Retpoline requires a recompile... I can't help but think gee that's not exactly a silver bullet. How is that superior to a microcode update??

    Branch prediction, itself, is so fundamental that it's probably not implemented in microcode. The performance/efficency benefit of hard-wiring it is probably too great.

    Maybe some CPUs have the ability to configure its behavior from BIOS, however. Or perhaps there are hacks you could do in the microcode for jump instructions, but maybe the microcode doesn't have enough flexibility or visibility to implement the same technique used by the compiler.
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