On Tue, 10 Jan 2017 16:15:27 -0800 (PST) David Rientjes <rientjes@xxxxxxxxxx> wrote: > There is no thp defrag option that currently allows MADV_HUGEPAGE regions > to do direct compaction and reclaim while all other thp allocations simply > trigger kswapd and kcompactd in the background and fail immediately. > > The "defer" setting simply triggers background reclaim and compaction for > all regions, regardless of MADV_HUGEPAGE, which makes it unusable for our > userspace where MADV_HUGEPAGE is being used to indicate the application is > willing to wait for work for thp memory to be available. > > The "madvise" setting will do direct compaction and reclaim for these > MADV_HUGEPAGE regions, but does not trigger kswapd and kcompactd in the > background for anybody else. > > For reasonable usage, there needs to be a mesh between the two options. > This patch introduces a fifth mode, "defer+madvise", that will do direct > reclaim and compaction for MADV_HUGEPAGE regions and trigger background > reclaim and compaction for everybody else so that hugepages may be > available in the near future. > > A proposal to allow direct reclaim and compaction for MADV_HUGEPAGE > regions as part of the "defer" mode, making it a very powerful setting and > avoids breaking userspace, was offered: > http://marc.info/?t=148236612700003. This additional mode is a > compromise. > > A second proposal to allow both "defer" and "madvise" to be selected at > the same time was also offered: http://marc.info/?t=148357345300001. > This is possible, but there was a concern that it might break existing > userspaces the parse the output of the defrag mode, so the fifth option > was introduced instead. > > This patch also cleans up the helper function for storing to "enabled" > and "defrag" since the former supports three modes while the latter > supports five and triple_flag_store() was getting unnecessarily messy. > > --- a/Documentation/vm/transhuge.txt > +++ b/Documentation/vm/transhuge.txt > @@ -110,6 +110,7 @@ MADV_HUGEPAGE region. > > echo always >/sys/kernel/mm/transparent_hugepage/defrag > echo defer >/sys/kernel/mm/transparent_hugepage/defrag > +echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag > echo madvise >/sys/kernel/mm/transparent_hugepage/defrag > echo never >/sys/kernel/mm/transparent_hugepage/defrag > > @@ -120,10 +121,15 @@ that benefit heavily from THP use and are willing to delay the VM start > to utilise them. > > "defer" means that an application will wake kswapd in the background > -to reclaim pages and wake kcompact to compact memory so that THP is > +to reclaim pages and wake kcompactd to compact memory so that THP is > available in the near future. It's the responsibility of khugepaged > to then install the THP pages later. > > +"defer+madvise" will enter direct reclaim and compaction like "always", but > +only for regions that have used madvise(MADV_HUGEPAGE); all other regions > +will wake kswapd in the background to reclaim pages and wake kcompactd to > +compact memory so that THP is available in the near future. > + It would be helpful if this text were to tell the reader why they may choose to use this option: runtime effects, advantages, when-to-use, when-not-to-use, etc. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>