Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression

[Michal Hocko]

On Wed 05-12-18 10:43:43, Mel Gorman wrote:
> On Wed, Dec 05, 2018 at 10:08:56AM +0100, Michal Hocko wrote:
> > On Tue 04-12-18 16:47:23, David Rientjes wrote:
> > > On Tue, 4 Dec 2018, Mel Gorman wrote:
> > > 
> > > > What should also be kept in mind is that we should avoid conflating
> > > > locality preferences with THP preferences which is separate from THP
> > > > allocation latencies. The whole __GFP_THISNODE approach is pushing too
> > > > hard on locality versus huge pages when MADV_HUGEPAGE or always-defrag
> > > > are used which is very unfortunate given that MADV_HUGEPAGE in itself says
> > > > nothing about locality -- that is the business of other madvise flags or
> > > > a specific policy.
> > > 
> > > We currently lack those other madvise modes or mempolicies: mbind() is not 
> > > a viable alternative because we do not want to oom kill when local memory 
> > > is depleted, we want to fallback to remote memory.
> > 
> > Yes, there was a clear agreement that there is no suitable mempolicy
> > right now and there were proposals to introduce MPOL_NODE_RECLAIM to
> > introduce that behavior. This would be an improvement regardless of THP
> > because global node-reclaim policy was simply a disaster we had to turn
> > off by default and the global semantic was a reason people just gave up
> > using it completely.
> > 
> 
> The alternative is to define a clear semantic for THP allocation
> requests that are considered "light" regardless of whether that needs a
> GFP flag or not. A sensible default might be
> 
> o Allocate THP local if the amount of work is light or non-existant.
> o Allocate THP remote if one is freely available with no additional work
>   (maybe kick remote kcompactd)
> o Allocate base page local if the amount of work is light or non-existant
> o Allocate base page remote if the amount of work is light or non-existant
> o Do heavy work in zonelist order until a base page is allocated somewhere

I am not sure about the ordering without a deeper consideration but I
thin THP should reflect the approach we have for base bages.

> It's not something could be clearly expressed with either NORETRY or
> THISNODE but longer-term might be saner than chopping and changing on
> which flags are more important and which workload is most relevant. That
> runs the risk of a revert-loop where each person targetting one workload
> reverts one patch to insert another until someone throws up their hands
> in frustration and just carries patches out-of-tree long-term.

Fully agreed!

> I'm not going to prototype something along these lines for now as
> fundamentally a better compaction could cut out part of the root cause
> of pain.

Yes there is some ground work to be done first.

-- 
Michal Hocko
SUSE Labs