* Alex Shi <alex.shi@xxxxxxxxxx> wrote: > On 12/12/2013 09:13 PM, Ingo Molnar wrote: > > > > * Mel Gorman <mgorman@xxxxxxx> wrote: > > > >> There was a large performance regression that was bisected to commit 611ae8e3 > >> (x86/tlb: enable tlb flush range support for x86). This patch simply changes > >> the default balance point between a local and global flush for IvyBridge. > >> > >> Signed-off-by: Mel Gorman <mgorman@xxxxxxx> > >> --- > >> arch/x86/kernel/cpu/intel.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c > >> index dc1ec0d..2d93753 100644 > >> --- a/arch/x86/kernel/cpu/intel.c > >> +++ b/arch/x86/kernel/cpu/intel.c > >> @@ -627,7 +627,7 @@ static void intel_tlb_flushall_shift_set(struct cpuinfo_x86 *c) > >> tlb_flushall_shift = 5; > >> break; > >> case 0x63a: /* Ivybridge */ > >> - tlb_flushall_shift = 1; > >> + tlb_flushall_shift = 2; > >> break; > > > > I'd not be surprised if other CPU models showed similar weaknesses > > under ebizzy as well. > > > > I don't particularly like the tuning aspect of the whole feature: the > > tunings are model specific and they seem to come out of thin air, > > without explicit measurements visible. > > > > In particular the first commit that added this optimization: > > > > commit c4211f42d3e66875298a5e26a75109878c80f15b > > Date: Thu Jun 28 09:02:19 2012 +0800 > > > > x86/tlb: add tlb_flushall_shift for specific CPU > > > > already had these magic tunings, with no explanation about what kind > > of measurement was done to back up those tunings. > > > > I don't think this is acceptable and until this is cleared up I think > > we might be better off turning off this feature altogether, or making > > a constant, very low tuning point. > > > > The original code came via: > > > > 611ae8e3f520 x86/tlb: enable tlb flush range support for x86 > > > > which references a couple of benchmarks, in particular a > > micro-benchmark: > > > > My micro benchmark 'mummap' http://lkml.org/lkml/2012/5/17/59 > > show that the random memory access on other CPU has 0~50% speed up > > on a 2P * 4cores * HT NHM EP while do 'munmap'. > > > > if the tunings were done with the micro-benchmark then I think they > > are bogus, because AFAICS it does not measure the adversarial case of > > the optimization. You have not replied to this concern of mine: if my concern is valid then that invalidates much of the current tunings. > > So I'd say at minimum we need to remove the per model tunings, and > > need to use very conservative defaults, to make sure we don't slow > > down reasonable workloads. > > I also hate to depends on mysterious hardware differentiation. But > there do have some changes in tlb/cache part on different Intel > CPU.(Guess HPA know this more). And the different shift value get > from testing not from air. :) As far as I could see from the changelogs and the code itself the various tunings came from nowhere. So I don't see my concerns addressed. My inclination would be to start with something like Mel's known-good tuning value below, we know that ebizzy does not regress with that setting. Any more aggressive tuning needs to be backed up with ebizzy-alike adversarial workload performance numbers. Thanks, Ingo (Patch totally untested.) =============> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c index dc1ec0d..c98385d 100644 --- a/arch/x86/kernel/cpu/intel.c +++ b/arch/x86/kernel/cpu/intel.c @@ -614,23 +614,8 @@ static void intel_tlb_flushall_shift_set(struct cpuinfo_x86 *c) case 0x61d: /* six-core 45 nm xeon "Dunnington" */ tlb_flushall_shift = -1; break; - case 0x61a: /* 45 nm nehalem, "Bloomfield" */ - case 0x61e: /* 45 nm nehalem, "Lynnfield" */ - case 0x625: /* 32 nm nehalem, "Clarkdale" */ - case 0x62c: /* 32 nm nehalem, "Gulftown" */ - case 0x62e: /* 45 nm nehalem-ex, "Beckton" */ - case 0x62f: /* 32 nm Xeon E7 */ - tlb_flushall_shift = 6; - break; - case 0x62a: /* SandyBridge */ - case 0x62d: /* SandyBridge, "Romely-EP" */ - tlb_flushall_shift = 5; - break; - case 0x63a: /* Ivybridge */ - tlb_flushall_shift = 1; - break; default: - tlb_flushall_shift = 6; + tlb_flushall_shift = 2; } } -- 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>