On Thu, 20 Dec 2012 13:47:10 +0100 Michal Hocko <mhocko@xxxxxxx> wrote: > > > + */ > > > +#if defined(CONFIG_PREEMPT_COUNT) > > > +#define MAX_GATHER_BATCH_COUNT (UINT_MAX) > > > +#else > > > +#define MAX_GATHER_BATCH_COUNT (((1UL<<(30-PAGE_SHIFT))/MAX_GATHER_BATCH)) > > > > Geeze. I spent waaaaay too long staring at that expression trying to > > work out "how many pages is in a batch" and gave up. > > > > Realistically, I don't think we need to worry about CONFIG_PREEMPT here > > - if we just limit the thing to, say, 64k pages per batch then that > > will be OK for preemptible and non-preemptible kernels. > > I wanted the fix to be as non-intrusive as possible so I didn't want to > touch PREEMPT (which is default in many configs) at all. I am OK to a > single limit of course. non-intrusive is nice, but best-implementation is nicer. > > The performance difference between "64k" and "infinite" will be > > miniscule and unmeasurable. > > > > Also, the batch count should be independent of PAGE_SIZE. Because > > PAGE_SIZE can vary by a factor of 16 and you don't want to fix the > > problem on 4k page size but leave it broken on 64k page size. > > MAX_GATHER_BATCH depends on the page size so I didn't want to differ > without a good reason. There's a good reason! PAGE_SIZE can vary by a factor of 16, and if this results in the unpreemptible-CPU-effort varying by a factor of 16 then that's bad, and we should change things so the unpreemptible-CPU-effort is independent of PAGE_SIZE. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>