On Tuesday 20 May 2014 04:53 AM, Hugh Dickins wrote: > 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. Ok. > >> 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 Added it as one way to reset or disable the default value. But then I guess we decided on having FAULT_AROUND_ORDER as a variable which is more important than Kconfig option. > option: I've no objection, if it were to default to byte order 16. > Thanks for review With regards Maddy > 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>