Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

On Thu, Mar 19, 2015 at 04:05:46PM -0700, Linus Torvalds wrote:

On Thu, Mar 19, 2015 at 3:41 PM, Dave Chinner [off-list ref] wrote:

quoted

My recollection wasn't faulty - I pulled it from an earlier email.
That said, the original measurement might have been faulty. I ran
the numbers again on the 3.19 kernel I saved away from the original
testing. That came up at 235k, which is pretty much the same as
yesterday's test. The runtime,however, is unchanged from my original
measurements of 4m54s (pte_hack came in at 5m20s).

Ok. Good. So the "more than an order of magnitude difference" was
really about measurement differences, not quite as real. Looks like
more a "factor of two" than a factor of 20.

Did you do the profiles the same way? Because that would explain the
differences in the TLB flush percentages too (the "1.4% from
tlb_invalidate_range()" vs "pretty much everything from migration").

The runtime variation does show that there's some *big* subtle
difference for the numa balancing in the exact TNF_NO_GROUP details.

TNF_NO_GROUP affects whether the scheduler tries to group related processes
together. Whether migration occurs depends on what node a process is
scheduled on. If processes are aggressively grouped inappropriately then it
is possible there is a bug that causes the load balancer to move processes
off a node (possible migration) with NUMA balancing trying to pull it back
(another possible migration). Small bugs there can result in excessive
migration.

-- 
Mel Gorman
SUSE Labs