Re:Intel Publishes Microcode Patches, No Benchmarking or Com...
by GrayShade on Thursday August 23, 2018 @07:47AM
(#17826396)
I assumed the tests were were ran on the new microcode version, as it was released a week before the tests, but it seems that Ubuntu didn't ship that update yet.
Their security advisory says that
> Optimized L1 data cache flushing is available via intel-microcode updates. The updated kernels implement a software fallback cache flushing mechanism for processors that have not received microcode updates.
It looks like the kernel will say "VMX: conditional cache flushes" on the new microcode so according to the Phoronix screenshots, they were running the older version.
Maybe we'll see a new set of benchmarks.
I assume the microcode update could be either equal o slightly worse in terms of performance, as the CPU might need to flush more frequently.
Which is pretty sad, as the status of kernel+microcode updates is quite confusing already. Some mitigations can take advantage of new microcode updates, if the kernel is recent enough. How does the pure soft-workaround compare in terms of performance to the microcode-assisted one?
Note that, combining all workarounds for meltdown+spectre-v1/v2+l1tf can have a significant performance hit for some workloads which are not purely cpu-bound. On top of that HT is now looking like a bad idea to start with.
I'm pretty sure that for a server where there's a lot of I/O and virtualization going on, enabling all the patches and workarounds + disabling HT can take a massive cut in overall throughput.