On Thu, 23 Jul 2020 11:30:01 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote: > Recently 0day reported many strange performance changes (regression > or improvement), in which there was no obvious relation between > the culprit commit and the benchmark at the first look, and it causes > people to doubt the test itself is wrong. > > Upon further check, many of these cases are caused by the change > to the alignment of kernel text or data, as whole text/data of kernel > are linked together, change in one domain may affect alignments of > other domains. > > gcc has an option '-falign-functions=n' to force text aligned, and with > that option enabled, some of those performance changes will be gone, > like [1][2][3]. > > Add this option so that developers and 0day can easily find performance > bump caused by text alignment change, Would they use it this way, or would they simply always enable the option to reduce the variability? It makes sense, but is it actually known that this does reduce the variability? > as tracking these strange bump > is quite time consuming. Though it can't help in other cases like data > alignment changes like [4]. > > Following is some size data for v5.7 kernel built with a RHEL config > used in 0day: > > text data bss dec filename > 19738771 13292906 5554236 38585913 vmlinux.noalign > 19758591 13297002 5529660 38585253 vmlinux.align32 > > Raw vmlinux size in bytes: > > v5.7 v5.7+align32 > 253950832 254018000 +0.02% > > Some benchmark data, most of them have no big change: > > * hackbench: [ -1.8%, +0.5%] > > * fsmark: [ -3.2%, +3.4%] # ext4/xfs/btrfs > > * kbuild: [ -2.0%, +0.9%] > > * will-it-scale: [ -0.5%, +1.8%] # mmap1/pagefault3 > > * netperf: > - TCP_CRR [+16.6%, +97.4%] > - TCP_RR [-18.5%, -1.8%] > - TCP_STREAM [ -1.1%, +1.9%] What do the numbers in [] mean? The TCP_CRR results look remarkable? > [1] https://lore.kernel.org/lkml/20200114085637.GA29297@shao2-debian/ > [2] https://lore.kernel.org/lkml/20200330011254.GA14393@feng-iot/ > [3] https://lore.kernel.org/lkml/1d98d1f0-fe84-6df7-f5bd-f4cb2cdb7f45@xxxxxxxxx/ > [4] https://lore.kernel.org/lkml/20200205123216.GO12867@shao2-debian/ >
