Bug 254040 - AMD 5950X hyperthreading strange performance swings
Summary: AMD 5950X hyperthreading strange performance swings
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: 12.2-RELEASE
Hardware: amd64 Any
: --- Affects Some People
Assignee: freebsd-bugs (Nobody)
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2021-03-05 15:45 UTC by Dennis Noordsij
Modified: 2021-03-05 15:45 UTC (History)
0 users

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Description Dennis Noordsij 2021-03-05 15:45:51 UTC
I plan to upgrade our server to a Ryzen 9 5950X system, 16 cores, 3400MHz base frequency, 128GB RAM, and ran into an issue while testing.

For reference I will use a very simple command:

  dd if=/dev/zero bs=1M count=1000 | bzip2 - | wc

which, using a Linux rescue system (i.e. nothing else running) consistently and repeatedly completes in about:

  1048576000 bytes (1.0 GB, 1000 MiB) copied, 4.67561 s, 224 MB/s



Now for FreeBSD 12.2-RELEASE, on a basic boot not running anything except ssh:

if I _disable_ hyperthreading using machdep.hyperthreading_allowed=0, I get the following approximate result consistently and repeatedly:

  dd if=/dev/zero bs=1M count=1000 | bzip2 - | wc
  1048576000 bytes transferred in 4.874335 secs (215121876 bytes/sec)
 
Slightly slower than Linux, not sure if this is in how bzip2 is compiled etc, but nothing that worries me. 



However, if I _enable_ hyperthreading, i.e. the default I started with, then I will get:

  dd if=/dev/zero bs=1M count=1000 | bzip2 - | wc
  1048576000 bytes transferred in 4.887522 secs (214541450 bytes/sec)
  1048576000 bytes transferred in 7.507138 secs (139677190 bytes/sec)
  1048576000 bytes transferred in 6.227179 secs (168386989 bytes/sec)
  1048576000 bytes transferred in 7.590263 secs (138147516 bytes/sec)
  1048576000 bytes transferred in 7.421037 secs (141297776 bytes/sec)
  1048576000 bytes transferred in 4.922986 secs (212995935 bytes/sec)
  1048576000 bytes transferred in 4.945138 secs (212041827 bytes/sec)
  1048576000 bytes transferred in 7.671600 secs (136682828 bytes/sec)
  1048576000 bytes transferred in 7.673428 secs (136650273 bytes/sec)

i.e. very consistently varying results with relatively large differences in commands executed immediately after one another (and no other load whatsoever).


I'm curious why this is happening.

I am not running powerd or touched any of the cpu settings.



Booting _without_ hyperthreading:

CPU: AMD Ryzen 9 5950X 16-Core Processor             (3393.70-MHz K8-class CPU)                                                          
  Origin="AuthenticAMD"  Id=0xa20f10  Family=0x19  Model=0x21  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=0x7ed8320b<SSE3,PCLMULQDQ,MON,SSSE3,FMA,CX16,SSE4.1,SSE4.2,MOVBE,POPCNT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND>                 
  AMD Features=0x2e500800<SYSCALL,NX,MMX+,FFXSR,Page1GB,RDTSCP,LM>                                                                       
  AMD Features2=0x75c237ff<LAHF,CMP,SVM,ExtAPIC,CR8,ABM,SSE4A,MAS,Prefetch,OSVW,IBS,SKINIT,WDT,TCE,Topology,PCXC,PNXC,DBE,PL2I,MWAITX,<b30>>                                                                                                                                      
  Structured Extended Features=0x219c97a9<FSGSBASE,BMI1,AVX2,SMEP,BMI2,ERMS,INVPCID,PQM,PQE,RDSEED,ADX,SMAP,CLFLUSHOPT,CLWB,SHA>         
  Structured Extended Features2=0x40068c<UMIP,PKU,VAES,VPCLMULQDQ,RDPID>                                                                 
  Structured Extended Features3=0x10                                                                                                     
  XSAVE Features=0xf<XSAVEOPT,XSAVEC,XINUSE,XSAVES>                                                                                      
  AMD Extended Feature Extensions ID EBX=0x111ef657<CLZERO,IRPerf,XSaveErPtr>                                                            
  SVM: NP,NRIP,VClean,AFlush,DAssist,NAsids=32768                                                                                        
  TSC: P-state invariant, performance statistics                                                                                         
real memory  = 137434759168 (131068 MB)                                                                                                  
avail memory = 133793423360 (127595 MB)                                                                                                  
Event timer "LAPIC" quality 600                                                                                                          
ACPI APIC Table: <ALASKA A M I >                                                                                                         
FreeBSD/SMP: Multiprocessor System Detected: 16 CPUs                                                                                     
FreeBSD/SMP: 1 package(s) x 2 cache groups x 8 core(s) x 2 hardware threads                                                              
FreeBSD/SMP Online: 1 package(s) x 2 cache groups x 8 core(s)                                                                            
                                                                                                                                         
                                                                                                                                         
                                                                                                                                         
# sysctl dev.cpu.0                                                                                                                       dev.cpu.0.cx_method: C1/hlt C2/io                                                                                                        
dev.cpu.0.cx_usage_counters: 11922 0                                                                                                     
dev.cpu.0.cx_usage: 100.00% 0.00% last 43430us                                                                                           
dev.cpu.0.cx_lowest: C1                                                                                                                  
dev.cpu.0.cx_supported: C1/1/1 C2/2/18                                                                                                   
dev.cpu.0.freq_levels: 3400/3740 2800/2800 2200/1980                                                                                     
dev.cpu.0.freq: 3400                                                                                                                     
dev.cpu.0.%parent: acpi0                                                                                                                 
dev.cpu.0.%pnpinfo: _HID=ACPI0007 _UID=0                                                                                                 
dev.cpu.0.%location: handle=\_SB_.PLTF.C000                                                                                              
dev.cpu.0.%driver: cpu                                                                                                                   
dev.cpu.0.%desc: ACPI CPU                                                                                                                
                                           

Booting _with_ hyperthreading:

CPU: AMD Ryzen 9 5950X 16-Core Processor             (3393.69-MHz K8-class CPU)                                                          
  Origin="AuthenticAMD"  Id=0xa20f10  Family=0x19  Model=0x21  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=0x7ed8320b<SSE3,PCLMULQDQ,MON,SSSE3,FMA,CX16,SSE4.1,SSE4.2,MOVBE,POPCNT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND>                 
  AMD Features=0x2e500800<SYSCALL,NX,MMX+,FFXSR,Page1GB,RDTSCP,LM>                                                                       
  AMD Features2=0x75c237ff<LAHF,CMP,SVM,ExtAPIC,CR8,ABM,SSE4A,MAS,Prefetch,OSVW,IBS,SKINIT,WDT,TCE,Topology,PCXC,PNXC,DBE,PL2I,MWAITX,<b30>>                                                                                                                                      
  Structured Extended Features=0x219c97a9<FSGSBASE,BMI1,AVX2,SMEP,BMI2,ERMS,INVPCID,PQM,PQE,RDSEED,ADX,SMAP,CLFLUSHOPT,CLWB,SHA>         
  Structured Extended Features2=0x40068c<UMIP,PKU,VAES,VPCLMULQDQ,RDPID>                                                                 
  Structured Extended Features3=0x10                                                                                                     
  XSAVE Features=0xf<XSAVEOPT,XSAVEC,XINUSE,XSAVES>                                                                                      
  AMD Extended Feature Extensions ID EBX=0x111ef657<CLZERO,IRPerf,XSaveErPtr>                                                            
  SVM: NP,NRIP,VClean,AFlush,DAssist,NAsids=32768                                                                                        
  TSC: P-state invariant, performance statistics                                                                                         
real memory  = 137434759168 (131068 MB)                                                                                                  
avail memory = 133793423360 (127595 MB)                                                                                                  
Event timer "LAPIC" quality 600                                                                                                          
ACPI APIC Table: <ALASKA A M I >                                                                                                         
FreeBSD/SMP: Multiprocessor System Detected: 32 CPUs                                                                                     
FreeBSD/SMP: 1 package(s) x 2 cache groups x 8 core(s) x 2 hardware threads                                                              
                                                                                                                                         
                                                                                                                                         
# sysctl dev.cpu.0                                                                                                                       
dev.cpu.0.cx_method: C1/hlt C2/io                                                                                                        
dev.cpu.0.cx_usage_counters: 3232 0                                                                                                      
dev.cpu.0.cx_usage: 100.00% 0.00% last 65311us                                                                                           
dev.cpu.0.cx_lowest: C1                                                                                                                  
dev.cpu.0.cx_supported: C1/1/1 C2/2/18                                                                                                   
dev.cpu.0.freq_levels: 3400/3740 2800/2800 2200/1980                                                                                     
dev.cpu.0.freq: 3400                                                                                                                     
dev.cpu.0.%parent: acpi0                                                                                                                 
dev.cpu.0.%pnpinfo: _HID=ACPI0007 _UID=0                                                                                                 
dev.cpu.0.%location: handle=\_SB_.PLTF.C000                                                                                              
dev.cpu.0.%driver: cpu                                                                                                                   
dev.cpu.0.%desc: ACPI CPU 



zenstates.py reports:

# ./zenstates.py -l
P0 - Enabled - FID = 88 - DID = 8 - VID = 48 - IDD = 22( / 1 )
             - Ratio = 34.00 - vCore = 1.10000
P1 - Enabled - FID = 8C - DID = A - VID = 58 - IDD = 1C( / 1 )
             - Ratio = 28.00 - vCore = 1.00000
P2 - Enabled - FID = 84 - DID = C - VID = 68 - IDD = 16( / 1 )
             - Ratio = 22.00 - vCore = 0.90000
P3 - Disabled
P4 - Disabled
P5 - Disabled
P6 - Disabled
P7 - Disabled
Core Performance Boost - Enabled
C6 State - Package - Disabled
C6 State - Core - Enabled


FWIW if I disable core performance boost the varying execution times shift from ~4.8 and ~7.8 to to ~6.5 and ~10.8 seconds respectively, i.e. same behaviour just slower.

I was hoping someone could explain why this is happening (note it doesn't happen on Linux), if it is expected, and/or how it can be worked around or fixed, or where the problem would be (p-states?). 

Happy to test anything.


PS - I tried a 13-BETA2 rescue boot which has HT enabled and it behaves exactly the same.