On 16/10/2024 16:16, David Hildenbrand wrote: >> Performance Testing >> =================== >> >> I've run some limited performance benchmarks: >> >> First, a real-world benchmark that causes a lot of page table manipulation (and >> therefore we would expect to see regression here if we are going to see it >> anywhere); kernel compilation. It barely registers a change. Values are times, >> so smaller is better. All relative to base-4k: >> >> | | kern | kern | user | user | real | real | >> | config | mean | stdev | mean | stdev | mean | stdev | >> |-------------|---------|---------|---------|---------|---------|---------| >> | base-4k | 0.0% | 1.1% | 0.0% | 0.3% | 0.0% | 0.3% | >> | compile-4k | -0.2% | 1.1% | -0.2% | 0.3% | -0.1% | 0.3% | >> | boot-4k | 0.1% | 1.0% | -0.3% | 0.2% | -0.2% | 0.2% | >> >> The Speedometer JavaScript benchmark also shows no change. Values are runs per >> min, so bigger is better. All relative to base-4k: >> >> | config | mean | stdev | >> |-------------|---------|---------| >> | base-4k | 0.0% | 0.8% | >> | compile-4k | 0.4% | 0.8% | >> | boot-4k | 0.0% | 0.9% | >> >> Finally, I've run some microbenchmarks known to stress page table manipulations >> (originally from David Hildenbrand). The fork test maps/allocs 1G of anon >> memory, then measures the cost of fork(). The munmap test maps/allocs 1G of anon >> memory then measures the cost of munmap()ing it. The fork test is known to be >> extremely sensitive to any changes that cause instructions to be aligned >> differently in cachelines. When using this test for other changes, I've seen >> double digit regressions for the slightest thing, so 12% regression on this test >> is actually fairly good. This likely represents the extreme worst case for >> regressions that will be observed across other microbenchmarks (famous last >> words). Values are times, so smaller is better. All relative to base-4k: >> > > ... and here I am, worrying about much smaller degradation in these micro- > benchmark ;) You're right, these are pure micro-benchmarks, and while 12% does > sound like "much", even stupid compiler code movement can result in such changes > in the fork() micro benchmark. > > So I think this is just fine, and actually "surprisingly" small. And, there is > even a way to statically compile a page size and not worry about that at all. > > As discussed ahead of times, I consider this change very valuable. In RHEL, the > biggest issue is actually the test matrix, that cannot really be reduced > significantly ... but it will make shipping/packaging easier. > > CCing Don, who did the separate 64k RHEL flavor kernel. > Thanks, David! I'm planning to investigate and see if I can improve even on that 12%. I have a couple of ideas. But like you say, I don't think this should be a blocker to moving forwards.
