Re: [PATCH -mm v9 0/8] idle memory tracking

[Michal Hocko]

On Wed 29-07-15 17:45:39, Vladimir Davydov wrote:
> On Wed, Jul 29, 2015 at 07:12:13AM -0700, Michel Lespinasse wrote:
> > On Wed, Jul 29, 2015 at 6:59 AM, Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> > wrote:
> > >> I guess the primary reason to rely on the pfn rather than the LRU walk,
> > >> which would be more targeted (especially for memcg cases), is that we
> > >> cannot hold lru lock for the whole LRU walk and we cannot continue
> > >> walking after the lock is dropped. Maybe we can try to address that
> > >> instead? I do not think this is easy to achieve but have you considered
> > >> that as an option?
> > >
> > > Yes, I have, and I've come to a conclusion it's not doable, because LRU
> > > lists can be constantly rotating at an arbitrary rate. If you have an
> > > idea in mind how this could be done, please share.
> > >
> > > Speaking of LRU-vs-PFN walk, iterating over PFNs has its own advantages:
> > >  - You can distribute a walk in time to avoid CPU bursts.
> > >  - You are free to parallelize the scanner as you wish to decrease the
> > >    scan time.
> > 
> > There is a third way: one could go through every MM in the system and scan
> > their page tables. Doing things that way turns out to be generally faster
> > than scanning by physical address, because you don't have to go through
> > RMAP for every page. But, you end up needing to take the mmap_sem lock of
> > every MM (in turn) while scanning them, and that degrades quickly under
> > memory load, which is exactly when you most need this feature. So, scan by
> > address is still what we use here.
> 
> Page table scan approach has the inherent problem - it ignores unmapped
> page cache. If a workload does a lot of read/write or map-access-unmap
> operations, we won't be able to even roughly estimate its wss.

That page cache is trivially reclaimable if it is clean. If it needs
writeback then it is non-idle only until the next writeback. So why does
it matter for the estimation?

-- 
Michal Hocko
SUSE Labs
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