LFCS Operation of Running Systems Practice Question
This LFCS practice question tests your understanding of operation of running systems. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
[ 123.456789] INFO: task kworker/u:2:789 blocked for more than 120 seconds.
[ 123.456790] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 123.456791] kworker/u:2 D 0 789 2 0x00000000
[ 123.456792] Call Trace:
[ 123.456793] __schedule+0x2f0/0x800
[ 123.456794] schedule+0x28/0x80
[ 123.456795] io_schedule+0x16/0x40
[ 123.456796] wait_on_page_bit+0x10b/0x120
[ 123.456797] __filemap_fdatawait_range+0x100/0x150
[ 123.456798] filemap_fdatawait+0x1c/0x20
[ 123.456799] __sync_filesystem+0x4a/0x80
[ 123.456800] sync_one+0x2a/0x30
[ 123.456801] process_sync_work+0x22/0x50
[ 123.456802] worker_thread+0x4f/0x3f0
[ 123.456803] kthread+0x10b/0x140
Based on the exhibit, what is the most likely cause of the blocked task?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disk I/O bottleneck or hung storage
The exhibit shows a process in 'D' state (uninterruptible sleep), which typically indicates the process is waiting for I/O completion from a block device. When a task is blocked in this state for an extended period, it is most likely due to a disk I/O bottleneck or hung storage, as the kernel cannot interrupt this wait. CPU starvation (run queue) and memory leaks (OOM or swapping) produce different process states, making disk I/O the primary suspect.
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.
✗
CPU starvation
Why it's wrong here
The trace shows I/O wait, not CPU.
✗
Memory leak
Why it's wrong here
Not indicated in the I/O wait trace.
✓
Disk I/O bottleneck or hung storage
Why this is correct
The task is blocked on I/O, typical of a slow or failing disk.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Network congestion
Why it's wrong here
Trace does not involve network stack.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse a process in 'D' state (uninterruptible sleep, I/O wait) with a process that is simply sleeping or waiting on CPU, leading them to incorrectly choose CPU starvation or memory issues instead of recognizing the classic symptom of a disk I/O bottleneck.
Trap categories for this question
Command / output trap
The trace shows I/O wait, not CPU.
Detailed technical explanation
How to think about this question
The 'D' state (TASK_UNINTERRUPTIBLE) is used by the kernel to protect critical I/O operations from signals, ensuring data integrity during disk writes. Commands like `ps aux` or `/proc/<PID>/status` show 'D' for processes waiting on storage, and tools such as `iostat -x 1` or `iotop` can confirm high I/O wait times or device queue saturation. In real-world scenarios, a hung NFS server or a failing disk controller can cause multiple processes to enter 'D' state, potentially leading to system lockups if the storage does not recover.
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 practitioner preparing for the LFCS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
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.
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Disk I/O bottleneck or hung storage — The exhibit shows a process in 'D' state (uninterruptible sleep), which typically indicates the process is waiting for I/O completion from a block device. When a task is blocked in this state for an extended period, it is most likely due to a disk I/O bottleneck or hung storage, as the kernel cannot interrupt this wait. CPU starvation (run queue) and memory leaks (OOM or swapping) produce different process states, making disk I/O the primary suspect.
What should I do if I get this LFCS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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