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

[Vladimir Davydov <vdavydov-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

On Wed, Jul 29, 2015 at 05:08:55PM +0200, Michal Hocko wrote:
> 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-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
> > > 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?

Because it might be a part of a workload's working set, in which case
evicting it will make the workload lag.

Thanks,
Vladimir