Re: [PATCH 5/5] mm: Stall movable allocations until kswapd progresses during serious external fragmentation event

[Mel Gorman]

On Tue, Nov 27, 2018 at 02:20:30PM +0100, Vlastimil Babka wrote:
> > This patch has a marginal rate on fragmentation rates as it's rare for
> > the stall logic to actually trigger but the small stalls can be enough for
> > kswapd to catch up. How much that helps is variable but probably worthwhile
> > for long-term allocation success rates. It is possible to eliminate
> > fragmentation events entirely with tuning due to this patch although that
> > would require careful evaluation to determine if it's worthwhile.
> > 
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> 
> The gains here are relatively smaller and noisier than for the previous
> patches. Also I'm afraid that once antifrag loses against the ultimate
> adversary workload (see the "Caching/buffers become useless after some
> time" thread), then this might result in adding stalls to a workload
> that has no other options but to allocate movable pages from partially
> filled unmovable blocks, because that's simply the majority of
> pageblocks in the system, and the stalls can't help the situation. If
> that proves to be true, we could revert, but then there's the new
> user-visible tunable... and that all makes it harder for me to decide
> about this patch :) If only we could find out early while this is in
> linux-mm/linux-next...
> 

Andrew, would you mind dropping this patch from mmotm please? I think
the benefit is marginal relative to the potential loss. If it turns out
we ever really do need it then hopefully there will be better data
supporting it.

Thanks.

-- 
Mel Gorman
SUSE Labs