Hi Ilpo, On 11/20/2023 3:13 AM, Ilpo Järvinen wrote: > When reading memory in order, HW prefetching optimizations will > interfere with measuring how caches and memory are being accessed. This > adds noise into the results. > > Change the fill_buf reading loop to not use an obvious in-order access > using multiply by a prime and modulo. > > Using a prime multiplier with modulo ensures the entire buffer is > eventually read. 23 is small enough that the reads are spread out but > wrapping does not occur very frequently (wrapping too often can trigger > L2 hits more frequently which causes noise to the test because getting > the data from LLC is not required). > > It was discovered that not all primes work equally well and some can > cause wildly unstable results (e.g., in an earlier version of this > patch, the reads were done in reversed order and 59 was used as the > prime resulting in unacceptably high and unstable results in MBA and > MBM test on some architectures). > > Link: https://lore.kernel.org/linux-kselftest/TYAPR01MB6330025B5E6537F94DA49ACB8B499@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx> > --- I am not very comfortable with all the uncertainty involved in this patch. A consolation is that this is surely an improvement. Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx> Reinette
