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SubjectRe: [PATCH 2/3] x86: mm: Change tlb_flushall_shift for IvyBridge
On 12/12/2013 09:13 PM, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@suse.de> 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@suse.de>
>> ---
>> 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.
>
> 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. :)

>
> ( In theory madvise() could give us information about the usage
> pattern of the vma - but in practice madvise() is rarely used and I
> doubt ebizzy or other real-world apps are using it, so it's
> meaningless. )
>
> Thanks,
>
> Ingo
>


--
Thanks
Alex


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