(resending as text, sorry for previous post which didn't make it to the ML)
On Wed, Jul 29, 2015 at 7:12 AM, Michel Lespinasse <wal...@google.com> wrote: > > On Wed, Jul 29, 2015 at 6:59 AM, Vladimir Davydov <vdavy...@parallels.com> > 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. > > My only concern about the interface is that it exposes the fact that the scan > is done by address - if the interface only showed per-memcg totals, it would > make it possible to change the implementation underneath if we somehow figure > out how to work around the mmap_sem issue in the future. I don't think that > is necessarily a blocker but this is something to keep in mind IMO. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
