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How can I track down the remaining sources of jitter? 

I have reduced the jitter a lot, but there are still some odd areas of jitter in my base histogram that I can't track down.  On my base thread, you can see a few spikes out at -20us and 20us and also some spikes around -10us.  (I'm going to use the parallel port for driving my milling machine.)
 
I'm using an AMD FX-8350 cpus on a biostar A880GZ motherboard.  The AMD FX-8350 has 4 cores each with 2 threads, but I set nosmt, so linuxcnc is only using 2 cores.  I'm using the integrated graphics card.  It is ATI Radeon HD4250 Graphics .

I am running from a live usb drive with the following boot command.

    linux    /live/vmlinuz-6.1.0-30-rt-amd64 boot=live components quiet splash findiso=${iso_path} skew_tick=1 nosmt=force kthread_cpus=0-1 irqaffinity=0-1 rcu_nocb_poll rcu_nocbs=2-7 nohz=on nohz_full=2-7 isolcpus=2-7 intel_pstate=disable amd_pstate=disable idle=poll cpufreq.off=1 cpuidle.off=1 intel_idle.max_cstate=1 amd_idle.max_cstate=1 nowatchdog nosoftlockup
    initrd    /live/initrd.img-6.1.0-30-rt-amd64

When booted I also set

1. sudo sysctl kernel.timer_migration=0

2. sudo sysctl kernel.sched_rt_runtime_us=-1

(All these setings based on:  dantalion.nl/2024/09/29/linuxcnc-latency...arameter-tuning.html )

When I run 

watch -n 1 -d cat /proc/interrupts

I only see LOC (Local timer interrupts) on CPU2, CPU4, and CPU6, until I start the latency-histogram.  All other interrupts are on CPU0.  After starting the latency histogram I also see IWI (IRQ work interrupts) on CPU6.  (Note: I tried not isolating CPU6. Then the IWI interrupts stayed on CPU6, but the histogram looked worse.

I think I've disabled everything I can in the bios, but maybe I misunderstood some bios setting, and mistakenly turned something on.  Here's all my settings I think apply:
Advanced>PCI Subsystem Settings>
  PCI ROM Priority = Legacy ROM
  PCI Latency Timer = 32 PCI Bus Clocks
  VGA Palette Snoop = Disabled
  PERR# Generation = Disabled
  SERR# Generation = Disabled
Advanced>PCI Subsystem Settings>PCI Express Settings>
  Relaxed Ordering = Disabled
  Extended Tag = Disabled
  No Snoop = Enabled
  Maximum Payload = Auto
  Maximum Read Request = Auto
  ASPM Support = Disabled
  Extended Synch = Disabled
  Link Training Retry = 5
  Link Traing Timeout (uS) = 100
  Unpopulated Links = Keep Link ON
Advanced>ACPI Settings
  Enable Hibernation = Disabled
  ACPI Sleep State = Suspend Disabled
  Lock Legacy Resources = Disabled
  Wake System With Fixed Time = Disabled
  USB Device Wake Up From S3/S4 = Enabled
  PME Wake Up From S5 = Enabled
  PS2 Keyboard PowerOn = Any Key
  Stroke Key = Wake Key
  PS2 Mouse PowerOn = Enabled
Advanced>CPU Configuration
  Limit CPUID Maximum = Disabled
  PowerNow = Disabled
  C1E = Disabled
  SVM = Disabled
  Core C6 State = Disabled
  HPC Mode = Disabled
  core performance boost = Disabled
  HTC temperature limit = 70
  Core Leveling = Auto
       (I tried disabling, but it just reduces the amount of cores that I can use.)
Advanced>SATA Configuration
  All ports = Not Present
Advanced>USB Configuration
  USB Devices: = 1 Drive, 1 Mouse
  Legacy USB Support = Auto  (I tried Disabled, but then I can't boot from USB at all)
  EHCI Hand-off = Disabled
  USB transfer time-out = 20 sec
  Device reset time-out = 20 sec
  Device power-up delay = Auto
  Mass Storage Devices: Samsung Flash Drive FIT 1100 = Auto
Advanced>SMART FAN Control
  CPU Smart FAN = Disabled
Advanced>Super IO Configuration
  Super IO Chip = IT8728
  Restore AC Power Loss = Power Off
Advanced>Super IO Configuration> Serial Port 0 Configuration
  Serial Port = Disabled
Advanced>Super IO Configuration>Parallel Port Configuration
  Parallel Port = Enabled
  Device Settings =  I0=378h; IRQ=5;
  Change Settings = Auto
  Device Mode = Standard Parallel ...
Advanced>Super IO Configuration>CIR Controller Configuration
  CIR Controller = Disabled
Advanced>Chipset
  NB Power Management Features = Disabled
Advanced>Chipset>Internal Graphics Configuration
  Internal Graphics Mode = UMA
    UMA Frame Buffer Size = 1024MB
  GFX Engine Clock Override = Disabled
  Surround View = Auto
  FB Location = Above 4G
  AMD 880 HD Audo = Enabled
  Vbios Type = HDMI
Advanced>Chipset>PCI Express Configuration
  GPP Slots Power Limit, W = 25
Advanced>Chipset>PCI Express Configuration>Port #02 Features
  Gen2 High Speed Mode = Auto
  Link ASPM = Disabled
  Link Width = Auto
  Slot Power Limit, W = 75
Advanced>Chipset>PCI Express Configuration>Port #06 Features
  Gen2 High Speed Mode = Auto
  Link ASPM = Disabled
Advanced>Chipset>PCI Express Configuration>NB-SB Port Features
  NB-SB Link ASPM = L1
  NP NB-SB VC1 Traffic Support = Disabled
  Link Width = Auto
Advanced>Chipset>South Bridge Configuration
  SB CIM Version:  1.1.0.9
  SB Clock Spread Spectrum = Disabled
Advanced>Chipset>South Bridge Configuration>SB SATA Configuration
  OnChip SATA Channel = Disabled
  OnChip IDE mode = Legacy mode
  SATA-III Mode = Auto
Advanced>Chipset>South Bridge Configuration>SB USB Configuration
  (All enabled)
Advanced>Chipset>South Bridge Configuration>SB HD Azalia Configuration
  HD Audio Azalia Device = Disabled
  Azalia Snoop = Disabled
Advanced>Chipset>OnBoard Peripherals Configuration
  Realtek PCIE NIC = Disabled
  MAC ID not find

Under the Performance section I left everything default.

Any ideas where I can further reduce jitter?