Quick Answer
The answer is to run echo 1 > /proc/sys/vm/drop_caches, which immediately frees up file cache without affecting running processes. This works because writing 1 to that file instructs the kernel to release pagecache—the memory used for caching disk reads—while leaving process memory and swap untouched. On the Linux Foundation Certified System Administrator LFCS exam, this scenario tests your understanding of memory management under pressure: when free -m shows high cache usage but low available memory, the trap is to think you need to kill processes or restart services. Instead, the safe, non-destructive operation is to use drop_caches, which reclaims memory for applications without disruption. Remember the mnemonic: “One to free the cache, no processes to smash.”
This LFCS practice question tests your understanding of operation of running systems. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 Linux system reports 'Out of memory' errors frequently. The administrator checks memory usage with 'free -m' and notices that most memory is used by file cache. Which command can the administrator run to immediately free up the cache without affecting running processes?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Clue: "immediately / without restart"
Why it matters: Time or reboot constraint — the correct answer must take effect right away without requiring a reboot or reload.
sysctl vm.drop_caches=1
Why wrong: sysctl expects a key=value pair but the correct way is to write to the proc file directly.
swapoff -a
Why wrong: Disabling swap does not free cache and may cause system instability.
echo 1 > /proc/sys/vm/drop_caches
Writing 1 to drop_caches frees pagecache.
kill -9 $(pidof some_process)
Why wrong: Killing processes is not appropriate; it would stop applications.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
echo 1 > /proc/sys/vm/drop_caches
Option C is correct because writing 1 to /proc/sys/vm/drop_caches instructs the kernel to free pagecache (file cache) without terminating any processes. This is a safe, non-destructive operation that reclaims memory used for caching disk I/O, which is exactly what the administrator needs when 'free -m' shows most memory consumed by cache.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
sysctl vm.drop_caches=1
Why it's wrong here
sysctl expects a key=value pair but the correct way is to write to the proc file directly.
swapoff -a
Why it's wrong here
Disabling swap does not free cache and may cause system instability.
echo 1 > /proc/sys/vm/drop_caches
Why this is correct
Writing 1 to drop_caches frees pagecache.
Clue confirmation
The clue words "which command", "immediately / without restart" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
kill -9 $(pidof some_process)
Why it's wrong here
Killing processes is not appropriate; it would stop applications.
Common exam traps
The trap here is that candidates confuse 'sysctl' with direct procfs writes, or think 'swapoff -a' frees memory, when in fact it only disables swap and does not reclaim file cache.
Detailed technical explanation
The /proc/sys/vm/drop_caches interface accepts values 1 (free pagecache), 2 (free dentries and inodes), and 3 (free all). Writing 1 only clears clean cache pages that are immediately reclaimable; dirty pages must be written back first (via 'sync') to be freed. In real-world scenarios, this is useful before benchmarking or after a large file operation, but the kernel will automatically reclaim cache under memory pressure, so manual intervention is rarely needed.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Operation of Running Systems — This question tests Operation of Running Systems — Read the scenario before looking for a memorised answer..
The correct answer is: echo 1 > /proc/sys/vm/drop_caches — Option C is correct because writing 1 to /proc/sys/vm/drop_caches instructs the kernel to free pagecache (file cache) without terminating any processes. This is a safe, non-destructive operation that reclaims memory used for caching disk I/O, which is exactly what the administrator needs when 'free -m' shows most memory consumed by cache.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "which command", "immediately / without restart". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This LFCS practice question is part of Courseiva's free Linux Foundation certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LFCS exam.
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