ca37ab7fb9
First of all, I'm sorry about this ugly hack. I hope the following long explanation is enough to justify it. We have observed that, in some conditions, the results for dataset number 10 (pb) in the zippy benchmark can show a >20% regression on Skylake CPUs. In order to diagnose this, we profiled the benchmark looking at hot functions (99% of the time is spent on DecompressAllTags), then looked at the generated code to see if there was any difference. In order to discard a minor difference we observed in register allocation we replaced zippy.cc with a pre-built assembly file so it was the same in both variants, and we still were able to reproduce the regression. After discarding a regression caused by the compiler, we digged a bit further and noticed that the alignment of the function in the final binary was different. Both were aligned to a 16-byte boundary, but the slower one was also (by chance) aligned to a 32-byte boundary. A regression caused by alignment differences would explain why I could reproduce it consistently on the same CitC client, but not others: slight differences in the sources can cause the resulting binary to have different layout. Here are some detailed benchmark results before/after the fix. Note how fixing the alignment makes the difference between baseline and experiment go away, but regular 32-byte alignment puts both variants in the same ballpark as the original regression: Original (note BM_UCord_10 and BM_UDataBuffer_10 around the -24% line): BASELINE BM_UCord/10 2938 2932 24194 3.767GB/s pb BM_UDataBuffer/10 3008 3004 23316 3.677GB/s pb EXPERIMENT BM_UCord/10 3797 3789 18512 2.915GB/s pb BM_UDataBuffer/10 4024 4016 17543 2.750GB/s pb Aligning DecompressAllTags to a 32-byte boundary: BASELINE BM_UCord/10 3872 3862 18035 2.860GB/s pb BM_UDataBuffer/10 4010 3998 17591 2.763GB/s pb EXPERIMENT BM_UCord/10 3884 3876 18126 2.850GB/s pb BM_UDataBuffer/10 4037 4027 17199 2.743GB/s pb Aligning DecompressAllTags to a 32-byte boundary + 16 bytes (this patch): BASELINE BM_UCord/10 3103 3095 22642 3.569GB/s pb BM_UDataBuffer/10 3186 3177 21947 3.476GB/s pb EXPERIMENT BM_UCord/10 3104 3095 22632 3.569GB/s pb BM_UDataBuffer/10 3167 3159 22076 3.496GB/s pb This change forces the "good" alignment for DecompressAllTags which, if anything, should make benchmark results more stable (and maybe we'll improve some unlucky application!).
54 KiB
54 KiB