* Matteo Rizzo <matteorizzo@xxxxxxxxxx> wrote: > On Mon, 18 Sept 2023 at 19:39, Ingo Molnar <mingo@xxxxxxxxxx> wrote: > > > > What's the split of the increase in overhead due to SLAB_VIRTUAL=y, between > > user-space execution and kernel-space execution? > > > > Same benchmark as before (compiling a kernel on a system running the patched > kernel): > > Intel Skylake: > > LABEL | COUNT | MIN | MAX | MEAN | MEDIAN | STDDEV > ---------------+-------+----------+----------+----------+----------+-------- > wall clock | | | | | | > SLAB_VIRTUAL=n | 150 | 49.700 | 51.320 | 50.449 | 50.430 | 0.29959 > SLAB_VIRTUAL=y | 150 | 50.020 | 51.660 | 50.880 | 50.880 | 0.30495 > | | +0.64% | +0.66% | +0.85% | +0.89% | +1.79% > system time | | | | | | > SLAB_VIRTUAL=n | 150 | 358.560 | 362.900 | 360.922 | 360.985 | 0.91761 > SLAB_VIRTUAL=y | 150 | 362.970 | 367.970 | 366.062 | 366.115 | 1.015 > | | +1.23% | +1.40% | +1.42% | +1.42% | +10.60% > user time | | | | | | > SLAB_VIRTUAL=n | 150 | 3110.000 | 3124.520 | 3118.143 | 3118.120 | 2.466 > SLAB_VIRTUAL=y | 150 | 3115.070 | 3127.070 | 3120.762 | 3120.925 | 2.654 > | | +0.16% | +0.08% | +0.08% | +0.09% | +7.63% These Skylake figures are a bit counter-intuitive: how does an increase of only +0.08% user-time - which dominates 89.5% of execution, combined with a +1.42% increase in system time that consumes only 10.5% of CPU capacity, result in a +0.85% increase in wall-clock time? There might be hidden factors at work in the DMA space, as Linus suggested? Or perhaps wall-clock time is dominated by the single-threaded final link time of the kernel, which phase might be disproportionately hurt by these changes? (Stddev seems low enough for this not to be a measurement artifact.) The AMD Milan figures are more intuitive: > AMD Milan: > > LABEL | COUNT | MIN | MAX | MEAN | MEDIAN | STDDEV > ---------------+-------+----------+----------+----------+----------+-------- > wall clock | | | | | | > SLAB_VIRTUAL=n | 150 | 25.480 | 26.550 | 26.065 | 26.055 | 0.23495 > SLAB_VIRTUAL=y | 150 | 25.820 | 27.080 | 26.531 | 26.540 | 0.25974 > | | +1.33% | +2.00% | +1.79% | +1.86% | +10.55% > system time | | | | | | > SLAB_VIRTUAL=n | 150 | 478.530 | 540.420 | 520.803 | 521.485 | 9.166 > SLAB_VIRTUAL=y | 150 | 530.520 | 572.460 | 552.825 | 552.985 | 7.161 > | | +10.86% | +5.93% | +6.15% | +6.04% | -21.88% > user time | | | | | | > SLAB_VIRTUAL=n | 150 | 2373.540 | 2403.800 | 2386.343 | 2385.840 | 5.325 > SLAB_VIRTUAL=y | 150 | 2388.690 | 2426.290 | 2408.325 | 2408.895 | 6.667 > | | +0.64% | +0.94% | +0.92% | +0.97% | +25.20% > > > I'm not exactly sure why user time increases by almost 1% on Milan, it > could be TLB contention. The other worrying aspect is the increase of +6.15% of system time ... which is roughly in line with what we'd expect from a +1.79% increase in wall-clock time. Thanks, Ingo
