On Mon, 19 May 2014, Madhavan Srinivasan wrote: > On Monday 19 May 2014 05:42 AM, Rusty Russell wrote: > > Hugh Dickins <hughd@xxxxxxxxxx> writes: > >> On Thu, 15 May 2014, Madhavan Srinivasan wrote: > >>> > >>> Hi Ingo, > >>> > >>> Do you have any comments for the latest version of the patchset. If > >>> not, kindly can you pick it up as is. > >>> > >>> > >>> With regards > >>> Maddy > >>> > >>>> Kirill A. Shutemov with 8c6e50b029 commit introduced > >>>> vm_ops->map_pages() for mapping easy accessible pages around > >>>> fault address in hope to reduce number of minor page faults. > >>>> > >>>> This patch creates infrastructure to modify the FAULT_AROUND_ORDER > >>>> value using mm/Kconfig. This will enable architecture maintainers > >>>> to decide on suitable FAULT_AROUND_ORDER value based on > >>>> performance data for that architecture. First patch also defaults > >>>> FAULT_AROUND_ORDER Kconfig element to 4. Second patch list > >>>> out the performance numbers for powerpc (platform pseries) and > >>>> initialize the fault around order variable for pseries platform of > >>>> powerpc. > >> > >> Sorry for not commenting earlier - just reminded by this ping to Ingo. > >> > >> I didn't study your numbers, but nowhere did I see what PAGE_SIZE you use. > >> > >> arch/powerpc/Kconfig suggests that Power supports base page size of > >> 4k, 16k, 64k or 256k. > >> > >> I would expect your optimal fault_around_order to depend very much on > >> the base page size. > > > > It was 64k, which is what PPC64 uses on all the major distributions. > > You really only get a choice of 4k and 64k with 64 bit power. > > > This is true. PPC64 support multiple pagesize and yes the default page > size of 64k, is taken as base pagesize for the tests. > > >> Perhaps fault_around_size would provide a more useful default? > > > > That seems to fit. With 4k pages and order 4, you're asking for 64k. > > Maddy's result shows 64k is also reasonable for 64k pages. > > > > Perhaps we try to generalize from two data points (a slight improvement > > over doing it from 1!), eg: > > > > /* 4 seems good for 4k-page x86, 0 seems good for 64k page ppc64, so: */ > > unsigned int fault_around_order __read_mostly = > > (16 - PAGE_SHIFT < 0 ? 0 : 16 - PAGE_SHIFT); Rusty's bimodal answer doesn't seem the right starting point to me. Shouldn't FAULT_AROUND_ORDER and fault_around_order be changed to be the order of the fault-around size in bytes, and fault_around_pages() use 1UL << (fault_around_order - PAGE_SHIFT) - when that doesn't wrap, of course! That would at least have a better chance of being appropriate for architectures with 8k and 16k pages (Itanium springs to mind). Not necessarily right for them, since each architecture may have different faulting overheads; but a better chance of being right than blindly assuming 4k or 64k pages for everyone. I'd be glad to see that change go into v3.15: what do you think, Kirill, are we too late to make such a change now? Or do you see some objection to it? > This may be right. But these are the concerns, will not this make other > arch to pick default without any tuning Wasn't FAULT_AROUND_ORDER 4 chosen solely on the basis of x86 4k pages? Did other architectures, with other page sizes, back that default? Clearly not powerpc. > and also this will remove the > compile time option to disable the feature? Compile time option meaning your FAULT_AROUND_ORDER in mm/Kconfig for v3.16? I'm not sure whether Rusty was arguing against that or not. I think we are all three concerned to have a more sensible default than what's there at present. I don't feel very strongly about your Kconfig option: I've no objection, if it were to default to byte order 16. Hugh -- 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>