RELENG_10 performance regression (was Re: 35-40% performance drop releng9 vs releng10 openvpn

[Konstantin Belousov]

On Sat, Mar 21, 2015 at 02:13:06PM -0400, Mike Tancsa wrote:
> On 3/21/2015 11:52 AM, John Baldwin wrote:
> 
> >> http://tancsa.com/time/
> >
> > Do you know why you are using the HPET instead of TSC for timestamping?
> 
> Hi,
> 
> I am not consciously making any time keep decisions.
> 
> kern.eventtimer.choice: HPET(550) HPET1(450) LAPIC(400) i8254(100) RTC(0)
> kern.timecounter.choice: TSC(800) HPET(950) ACPI-fast(900) i8254(0) 
> dummy(-1000000)
> 
> (The full hardware info is at the above url)
> 
> 
> > Using the TSC can make a non-trivial performance difference since userland
> > can calculate timestamps without using system calls when it is used.
> > (That is not related to this case, but switching to the TSC in general is
> > preferable.)
> >
> > There are a few generations of Intel CPUs where you can't mix deeper sleep
> > states with the TSC as timecounter, but those CPUs are getting to be a bit
> > older at this point.
> >
> 
> This one is an AMD
> CPU: AMD G-T40E Processor (1000.02-MHz K8-class CPU)
>    Origin="AuthenticAMD"  Id=0x500f20  Family=0x14  Model=0x2  Stepping=0
>  
> Features=0x178bfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,MMX,FXSR,SSE,SSE2,HTT>
>    Features2=0x802209<SSE3,MON,SSSE3,CX16,POPCNT>
>    AMD Features=0x2e500800<SYSCALL,NX,MMX+,FFXSR,Page1GB,RDTSCP,LM>
>    AMD 
> Features2=0x35ff<LAHF,CMP,SVM,ExtAPIC,CR8,ABM,SSE4A,MAS,Prefetch,IBS,SKINIT,WDT>
>    SVM: NP,NRIP,NAsids=8
>    TSC: P-state invariant, performance statistics

It seems to be a consequnce of the code from r222869.  The test_tsc()
does not trust the P-state invariant report and explicitely check for
the family.  Your CPU family is 0x14, while code only bumps TSC priority
for family 0x15+.

Currently, tsc_is_invariant is set when CPU reports AMDPM_TSC_INVARIANT,
or for some models.  Should we bump TSC timecounter priority is smp
test passed and AMDPM_TSC_INVARIANT is set  ?

For now, you could just set TSC as timecounter.