kern.sched.preemp_tresh

fernandel · 2026-07-13T19:04:07+0100

Hi!

I have long time in /etc/sysctl.conf kern.sched.preemp_thresh=224 but on 15-REELEASE-p1 doesn't works anymore.

Code:

 sysctl kern.sched.preemp_thresh
sysctl: unknown oid 'kern.sched.preemp_thresh'

Is it something with my system or it is removed.

Thank you.

Maxnix · 2026-07-13T19:12:11+0100

Since 15.1 you can choose between the ULE and 4BSD scheduler, so that sysctl MIB was renamed accordingly; now it's kern.sched.ule.preempt_thresh or kern.sched.4bsd.preempt_thresh, depending on which scheduler you prefer.

EDIT. Corrected mispelling.

NapoleonWils0n · 2026-07-13T19:54:52+0100

I used this for years as well

Code:

kern.sched.preempt_thresh=224

And noticed it disappeared in Freebsd 15

fernandel and Manix missed the t from preemp_thresh

it should be

Code:

kern.sched.preempt_thresh=224

and not

Code:

kern.sched.preemp_thresh=224

also this is wrong

Code:

sysctl kern.sched.ule.preemp_thresh

it should be

Code:

sysctl kern.sched.ule.preempt_thresh

which on my system outputs

Code:

kern.sched.ule.preempt_thresh: 40

And this doesnt exist for me

Code:

kern.sched.4bsd.preempt_thresh

This is the full output kern.sched

Code:

[i] Yes Master ? sysctl kern.sched
kern.sched.ule.always_steal: 0
kern.sched.ule.trysteal_limit: 2
kern.sched.ule.steal_thresh: 2
kern.sched.ule.steal_idle: 1
kern.sched.ule.balance_interval: 127
kern.sched.ule.balance: 1
kern.sched.ule.affinity: 1
kern.sched.ule.idlespinthresh: 157
kern.sched.ule.idlespins: 10000
kern.sched.ule.static_boost: 115
kern.sched.ule.preempt_thresh: 40
kern.sched.ule.interact: 30
kern.sched.ule.slice: 12
kern.sched.ule.quantum: 94488
kern.sched.topology_spec: <groups>
 <group level="1" cache-level="3">
  <cpu count="12" mask="fff,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11</cpu>
  <children>
   <group level="2" cache-level="2">
    <cpu count="2" mask="3,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">2, 3</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="30,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">4, 5</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">6, 7</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="300,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">8, 9</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c00,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">10, 11</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
  </children>
 </group>
</groups>

kern.sched.available: 4BSD,ULE
kern.sched.name: ULE
kern.sched.4bsd.runq_fuzz: 1
kern.sched.4bsd.ipiwakeup.useloop: 0
kern.sched.4bsd.ipiwakeup.usemask: 1
kern.sched.4bsd.ipiwakeup.delivered: 0
kern.sched.4bsd.ipiwakeup.requested: 0
kern.sched.4bsd.ipiwakeup.enabled: 1
kern.sched.4bsd.slice: 12
kern.sched.4bsd.quantum: 94488
kern.sched.preemption: 1
kern.sched.cpusetsizemin: 2
kern.sched.cpusetsize: 128

So after some digging around it looks

this is the new code you should use

Code:

kern.sched.ule.preempt_thresh=224

as a replacement for the old code

Code:

kern.sched.preempt_thresh=224

which should be added to

Code:

 /etc/sysctl.conf

some one correct me if im wrong

Maxnix · 2026-07-13T20:03:45+0100

NapoleonWils0n said:

I used this for years as well

Code:

kern.sched.preempt_thresh=224

And noticed it disappeared in Freebsd 15

fernandel and Manix missed the t from preemp_thresh

it should be

Code:

kern.sched.preempt_thresh=224

and not

Code:

kern.sched.preemp_thresh=224

also this is wrong

Code:

sysctl kern.sched.ule.preemp_thresh

it should be

Code:

sysctl kern.sched.ule.preempt_thresh

which on my system outputs

Code:

kern.sched.ule.preempt_thresh: 40

And this doesnt exist for me

Code:

kern.sched.4bsd.preempt_thresh

This is the full output kern.sched

Code:

[i] Yes Master ? sysctl kern.sched
kern.sched.ule.always_steal: 0
kern.sched.ule.trysteal_limit: 2
kern.sched.ule.steal_thresh: 2
kern.sched.ule.steal_idle: 1
kern.sched.ule.balance_interval: 127
kern.sched.ule.balance: 1
kern.sched.ule.affinity: 1
kern.sched.ule.idlespinthresh: 157
kern.sched.ule.idlespins: 10000
kern.sched.ule.static_boost: 115
kern.sched.ule.preempt_thresh: 40
kern.sched.ule.interact: 30
kern.sched.ule.slice: 12
kern.sched.ule.quantum: 94488
kern.sched.topology_spec: <groups>
 <group level="1" cache-level="3">
  <cpu count="12" mask="fff,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11</cpu>
  <children>
   <group level="2" cache-level="2">
    <cpu count="2" mask="3,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">2, 3</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="30,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">4, 5</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">6, 7</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="300,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">8, 9</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c00,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">10, 11</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
  </children>
 </group>
</groups>

kern.sched.available: 4BSD,ULE
kern.sched.name: ULE
kern.sched.4bsd.runq_fuzz: 1
kern.sched.4bsd.ipiwakeup.useloop: 0
kern.sched.4bsd.ipiwakeup.usemask: 1
kern.sched.4bsd.ipiwakeup.delivered: 0
kern.sched.4bsd.ipiwakeup.requested: 0
kern.sched.4bsd.ipiwakeup.enabled: 1
kern.sched.4bsd.slice: 12
kern.sched.4bsd.quantum: 94488
kern.sched.preemption: 1
kern.sched.cpusetsizemin: 2
kern.sched.cpusetsize: 128

So after some digging around it looks

this is the new code you should use

Code:

kern.sched.ule.preempt_thresh=224

as a replacement for the old code

Code:

kern.sched.preempt_thresh=224

which should be added to

Code:

 /etc/sysctl.conf

some one correct me if im wrong

D'oh! That's what happens when carelessly using copy/paste!

Thank you for the correction.

fernandel · 2026-07-13T20:23:32+0100

NapoleonWils0n said:

I used this for years as well

Code:

kern.sched.preempt_thresh=224

And noticed it disappeared in Freebsd 15

fernandel and Manix missed the t from preemp_thresh

it should be

Code:

kern.sched.preempt_thresh=224

and not

Code:

kern.sched.preemp_thresh=224

also this is wrong

Code:

sysctl kern.sched.ule.preemp_thresh

it should be

Code:

sysctl kern.sched.ule.preempt_thresh

which on my system outputs

Code:

kern.sched.ule.preempt_thresh: 40

And this doesnt exist for me

Code:

kern.sched.4bsd.preempt_thresh

This is the full output kern.sched

Code:

[i] Yes Master ? sysctl kern.sched
kern.sched.ule.always_steal: 0
kern.sched.ule.trysteal_limit: 2
kern.sched.ule.steal_thresh: 2
kern.sched.ule.steal_idle: 1
kern.sched.ule.balance_interval: 127
kern.sched.ule.balance: 1
kern.sched.ule.affinity: 1
kern.sched.ule.idlespinthresh: 157
kern.sched.ule.idlespins: 10000
kern.sched.ule.static_boost: 115
kern.sched.ule.preempt_thresh: 40
kern.sched.ule.interact: 30
kern.sched.ule.slice: 12
kern.sched.ule.quantum: 94488
kern.sched.topology_spec: <groups>
 <group level="1" cache-level="3">
  <cpu count="12" mask="fff,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11</cpu>
  <children>
   <group level="2" cache-level="2">
    <cpu count="2" mask="3,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">0, 1</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">2, 3</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="30,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">4, 5</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">6, 7</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="300,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">8, 9</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
   <group level="2" cache-level="2">
    <cpu count="2" mask="c00,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0">10, 11</cpu>
    <flags><flag name="THREAD">THREAD group</flag><flag name="SMT">SMT group</flag></flags>
   </group>
  </children>
 </group>
</groups>

kern.sched.available: 4BSD,ULE
kern.sched.name: ULE
kern.sched.4bsd.runq_fuzz: 1
kern.sched.4bsd.ipiwakeup.useloop: 0
kern.sched.4bsd.ipiwakeup.usemask: 1
kern.sched.4bsd.ipiwakeup.delivered: 0
kern.sched.4bsd.ipiwakeup.requested: 0
kern.sched.4bsd.ipiwakeup.enabled: 1
kern.sched.4bsd.slice: 12
kern.sched.4bsd.quantum: 94488
kern.sched.preemption: 1
kern.sched.cpusetsizemin: 2
kern.sched.cpusetsize: 128

So after some digging around it looks

this is the new code you should use

Code:

kern.sched.ule.preempt_thresh=224

as a replacement for the old code

Code:

kern.sched.preempt_thresh=224

which should be added to

Code:

 /etc/sysctl.conf

some one correct me if im wrong

It was copy/paste...sorry

NapoleonWils0n · 2026-07-13T20:24:51+0100

No problem mate happens to all of us

So to recap it looks like this is the new code

Code:

kern.sched.ule.preempt_thresh=224

for the old code

Code:

kern.sched.preempt_thresh=224

and this doesnt seem to exist

Code:

kern.sched.4bsd.preempt_thresh

I think we need SirDice to confirm that
or if he's not about Sir David Beckham instead

fernandel · 2026-07-13T20:30:24+0100

NapoleonWils0n said:
No problem mate happens to all of us

So to recap it looks like this is the new code

Code:

kern.sched.ule.preempt_thresh=224

for the old code

Code:

kern.sched.preempt_thresh=224

and this doesnt seem to exist

Code:

kern.sched.4bsd.preempt_thresh

I think we need SirDice to confirm that
or if he's not about Sir David Beckham instead

Yes, it works.

paulw · 2026-07-14T02:31:28+0100

In /boot/loader.conf, if you set:

Code:

kern.sched.name=4BSD

you can use the 4BSD scheduler instead of the ULE default. I do have a zfs BE for that. There is no corresponding preempt_thresh sysctl for that scheduler.

One of the 4BSD scheduler values you can change if you want is kern.sched.4bsd.slice.

For what it's worth with more modern cpus, I don't think there's any real need anyway to change the default preempt_thresh value for desktop use.

EDIT: This is a related forum discussion thread:

What is sysctl kern.sched.preempt_thresh