LPIC-2 Linux Kernel and System Startup Practice Question
This LPIC-2 practice question tests your understanding of linux kernel and system startup. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company runs a high-frequency trading application that requires extremely low latency. The system administrator has compiled a custom Linux kernel with various real-time patches and tuned kernel parameters. After deploying the new kernel, the application performance degrades significantly. The administrator suspects that kernel preemption settings are causing context switch overhead. Which of the following actions should the administrator take to diagnose and optimize the kernel preemption model?
Use sysctl to set 'kernel.preempt_model' to 'none'
Why wrong: There is no sysctl parameter for preemption model.
B
Add 'preempt=none' to the kernel command line
Why wrong: The preemption model is not a kernel command-line parameter; it is compiled into the kernel.
C
Check /proc/sched_debug for preemption counters and adjust the kernel's preemption model by recompiling with CONFIG_PREEMPT_NONE instead of CONFIG_PREEMPT
The preemption model (none, voluntary, full) is a compile-time config; /proc/sched_debug reveals preemption activity.
D
Change the kernel preemption model at runtime by writing to /sys/kernel/preempt_control
Why wrong: No such runtime control exists; preemption model is set at compile time.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check /proc/sched_debug for preemption counters and adjust the kernel's preemption model by recompiling with CONFIG_PREEMPT_NONE instead of CONFIG_PREEMPT
Option C is correct because the kernel preemption model is a compile-time configuration, not a runtime parameter. To change from CONFIG_PREEMPT (full preemption) to CONFIG_PREEMPT_NONE (no forced preemption), the administrator must recompile the kernel with the appropriate configuration. Checking /proc/sched_debug can reveal preemption-related counters and context switch statistics, helping confirm that excessive preemption is causing overhead.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use sysctl to set 'kernel.preempt_model' to 'none'
Why it's wrong here
There is no sysctl parameter for preemption model.
✗
Add 'preempt=none' to the kernel command line
Why it's wrong here
The preemption model is not a kernel command-line parameter; it is compiled into the kernel.
✓
Check /proc/sched_debug for preemption counters and adjust the kernel's preemption model by recompiling with CONFIG_PREEMPT_NONE instead of CONFIG_PREEMPT
Why this is correct
The preemption model (none, voluntary, full) is a compile-time config; /proc/sched_debug reveals preemption activity.
Related concept
Read the scenario before looking for a memorised answer.
✗
Change the kernel preemption model at runtime by writing to /sys/kernel/preempt_control
Why it's wrong here
No such runtime control exists; preemption model is set at compile time.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume kernel parameters can be changed at runtime via sysctl or /sys files, but the preemption model is a static compile-time choice unless the kernel is built with CONFIG_PREEMPT_DYNAMIC, which is not indicated in this scenario.
Trap categories for this question
Command / output trap
The preemption model is not a kernel command-line parameter; it is compiled into the kernel.
Detailed technical explanation
How to think about this question
The kernel preemption model is determined at compile time by the CONFIG_PREEMPT_* options (e.g., CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, CONFIG_PREEMPT). With CONFIG_PREEMPT (full preemption), the kernel can be preempted almost anywhere, increasing context switches and latency for low-latency trading workloads that benefit from deterministic scheduling. The /proc/sched_debug file provides detailed scheduler statistics, including preemption counts and context switch rates, which can help diagnose overhead. Recompiling with CONFIG_PREEMPT_NONE disables forced preemption, reducing context switches but increasing kernel latency for I/O-bound tasks.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Linux Kernel and System Startup — This question tests Linux Kernel and System Startup — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check /proc/sched_debug for preemption counters and adjust the kernel's preemption model by recompiling with CONFIG_PREEMPT_NONE instead of CONFIG_PREEMPT — Option C is correct because the kernel preemption model is a compile-time configuration, not a runtime parameter. To change from CONFIG_PREEMPT (full preemption) to CONFIG_PREEMPT_NONE (no forced preemption), the administrator must recompile the kernel with the appropriate configuration. Checking /proc/sched_debug can reveal preemption-related counters and context switch statistics, helping confirm that excessive preemption is causing overhead.
What should I do if I get this LPIC-2 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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