On Tue, Feb 23, 2016 at 03:04:23PM +0000, Mel Gorman wrote:

In many benchmarks, there is an obvious difference in the number of
allocations from each zone as the fair zone allocation policy is removed
towards the end of the series. For example, this is the allocation stats
when running blogbench that showed no difference in headling performance

                          mmotm-20160209   nodelru-v2
DMA allocs                           0           0
DMA32 allocs                   7218763      608067
Normal allocs                 12701806    18821286
Movable allocs                       0           0

According to the mmotm numbers, your DMA32 zone is over a third of
available memory, yet in the nodelru-v2 kernel sees only 3% of the
allocations. That's an insanely high level of aging inversion, where
the lifetime of a cache entry is again highly dependent on placement.

The fact that this doesn't make a performance difference in the
specific benchmarks you ran only proves just that: these specific
benchmarks don't care. IMO, benchmarking is not enough here. If this
is truly supposed to be unproblematic, then I think we need a reasoned
explanation. I can't imagine how it possibly could be, though.

If reclaim can't guarantee a balanced zone utilization then the
allocator has to keep doing it. :( As far as I'm concerned, the
original reason for the fair zone allocator still applies.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>