On Fri 23-09-16 12:55:23, Vlastimil Babka wrote: > On 09/23/2016 10:26 AM, Michal Hocko wrote: > >> include/linux/compaction.h | 5 +++-- > >> mm/compaction.c | 44 +++++++++++++++++++++++--------------------- > >> mm/internal.h | 1 + > >> mm/vmscan.c | 6 ++++-- > >> 4 files changed, 31 insertions(+), 25 deletions(-) > > > > This is much more code churn than I expected. I was thiking about it > > some more and I am really wondering whether it actually make any sense > > to check the fragidx for !costly orders. Wouldn't it be much simpler to > > just put it out of the way for those regardless of the compaction > > priority. In other words does this check makes any measurable difference > > for !costly orders? > > I've did some stress tests and sampling > /sys/kernel/debug/extfrag/extfrag_index once per second. The lowest > value I've got for order-2 was 0.705. The default threshold is 0.5, so > this would still result in compaction considered as suitable. > > But it's sampling so I might not got to the interesting moments, most of > the time it was -1.000 which means the page should be just available. > Also we would be changing behavior for the user-controlled > vm.extfrag_threshold, so I'm not entirely sure about that. Does anybody depend on that or even use it out there? I strongly suspect this is one of those dark corners people even do not know they exist... > I could probably reduce the churn so that compaction_suitable() doesn't > need a new parameter. We could just skip compaction_suitable() check > from compact_zone() on the highest priority, and go on even without > sufficient free page gap? Whatever makes the code easier to understand. Please do not take me wrong I do not want to push back on this too hard I just always love to get rid of an obscure heuristic which even might not matter. And as your testing suggests this might really be the case for !costly orders AFAIU. -- Michal Hocko SUSE Labs -- 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>