On Fri 23-09-16 12:55:23, Vlastimil Babka wrote:

On 09/23/2016 10:26 AM, Michal Hocko wrote:

quoted

quoted

 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

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