get or calculate the value of Win32PrioritySeparation
Win32PrioritySeparation is a registry value that allows you to adjust how Windows distributes CPU time between foreground processes and background processes.
This value determines the processor's time optimization strategy, affecting the duration and relative priority of foreground and background threads.
But what is it exactly?
Win32PrioritySeparation is a 6-bit mask (AABBCC), where each pair of bits controls a different aspect of the processor's timing optimization strategy
highest bits (AA) specify the duration of the interval This can be short or long (short or long)
the middle bits (BB) specify the length of the interval This can be variable or fixed (variable or fixed)
The first 4 bits divide their values into this quantum table:
| Dur/Leng | Short | Long |
|---|---|---|
| Variable | 06 12 18 | 12 24 36 |
| Fixed | 18 18 18 | 36 36 36 |
lowest bits (CC) specify the processor time optimization strategy that should be distributed between the foreground and background threads
this can be:
Equal and fixed (1:1). Foreground and background threads get the same processor time at fixed intervals.
2:1 ratio. Foreground threads get twice as much processor time as background threads.
3:1 ratio. Foreground threads get three times more processor time than background threads.
priority separation can vary depending on the value you specify, a fixed quantum would completely nullify priority separation between threads
The best known way to manage this is by going to sysdm.cpl>settings>advanced
Background Services 2(2), default, default, 3:1
bitmask = 000010
foregroundquantum = 36 units
backgroundquantum = 36 units
apparently this has nothing to do with 3:1
in windows server, because the interval is fixed that overrides the PsPrioritySeparation of the quanta,
although it is also taken as a boost for the current priority of foreground processes
lkd> dt _KPROCESS ffffa78530811080 -n QuantumReset nt!_KPROCESS //dwm
+0x281 QuantumReset : 36 '$'
lkd> dt _KPROCESS ffffa7852c0ec080 -n QuantumReset nt!_KPROCESS //windbg
+0x281 QuantumReset : 36 '$'
lkd> db PspForegroundQuantum 13
fffff801`1fb63574 24 24 24
lkd> dd PsPrioritySeparation 11
fffff801`1fb2c9d8 00000002
Programs 26(38), short, variable, 3:1
bitmask = 010110
foregroundquantum = 18 units
backgroundquantum = 6 units
here it seems we can see a 3:1, similarly the quanta are fixed but the PsPrioritySeparation is applied, because the length is variable
lkd> dt _KPROCESS ffffa78530811080 -n QuantumReset nt!_KPROCESS //dwm
+0x281 QuantumReset : 6
lkd> dt _KPROCESS ffffa7852c0ec080 -n QuantumReset nt!_KPROCESS //windbg
+0x281 QuantumReset : 18
lkd> db PspForegroundQuantum 13
fffff801`1fb63574 06 0c 12
lkd> dd PsPrioritySeparation 11
fffff801`1fb2c9d8 00000002
thanks to the information from: https://learn.microsoft.com/en-us/previous-versions/cc976120(v=technet.10)?redirectedfrom=MSDN
It can be stated that the maximum supported value is 3F (111111) If any value exceeds the maximum, only the 6 least significant bits (LSB) can be read, which in short would be the 6 bits that start from right to left.
