Hi Andrew, Thanks for the review. On Wed, Jul 22, 2020 at 08:39:19PM -0700, Andrew Morton wrote: > 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 We've had concerns about side effects, like increased kernel size won't be accepted by embedded system, the possible i-cache usage/contention increase. And I've only done limited benchmark test, so I thought it may be safer to be off by default. Though my bold thought was it could be default on :) > It makes sense, but is it actually known that this does reduce the > variability? Yes, at lease for the strange performance bumps reported by 0day, like in [1][2][3]. > > 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? For each of the benchmark listed above, I took 2 or 3 test platforms and run it with different parameters. So each of the benchmark will have several cases run, and [] lists the lowest and highest result. For the netperf/TCP_CRR case, the lowest is +16.6% on a Skylake server with 16 testing threads, and highest is +97.4 on a Cascadelake server with 96 testing threads. Thanks, Feng > > [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/ > >
