Mastering G1 Garbage Collection And Jvm Serviceability Tools Practice Tests

Question 1

An application experiences a total execution time $T_{total} = 1000$ ms. If the GC pause time $T_{pause}$ is $50$ ms and the application threads run for $T_{app} = 950$ ms, calculate the throughput $P$ defined as $P = \frac{T_{app}}{T_{total}}$. If a new configuration reduces $T_{pause}$ by $40\%$, what is the new throughput $P_{new}$?

Accepted Answer

0.97

Answer

0.95

Answer

0.96

Answer

0.98

Question 2

A heap of size $H = 16$ GB is divided into $N = 2048$ regions of equal size $S$. Determine the size of a single region $S$ in MB. If the G1 collector identifies a collection set (CSet) containing $k = 128$ regions that are $80\%$ garbage, calculate the total reclaimed memory $M_{reclaimed}$ in GB.

Accepted Answer

1.6 GB

Answer

0.8 GB

Answer

1.2 GB

Answer

2.0 GB

Question 3

In a system where the heap fragmentation ratio $F$ is modeled by $F = \frac{V_{free}}{V_{total}}$, if CMS GC results in $F_{CMS} = 0.25$ and G1 GC results in $F_{G1} = 0.05$ due to regional compaction, is it mathematically correct to state that G1 reduces fragmentation by $80\%$ compared to CMS?

Accepted Answer

True

Answer

False

Question 4

Consider two GC strategies: Parallel GC (throughput-oriented) and CMS (latency-oriented). If an application has a workload $W$ where $W_{throughput} > W_{latency}$, which of the following scenarios justify selecting Parallel GC over CMS?

Accepted Answer

Option A

Answer

Option B

Answer

Option C

Answer

Option D

Question 5

If the G1 collector performs a Mixed GC cycle, and the total number of regions is $N=1024$, and the evacuation time per region is $t_r = 2$ ms, calculate the total stop-the-world pause time $T_{STW}$ if the collection set size is $k=50$ regions. Provide the answer in milliseconds.

Accepted Answer

100

Question 6

Given a heap region of size $S = 2048$ KB, if a young collection reclaims $R = 1536$ KB of space with a total pause time of $T = 120$ ms, calculate the GC efficiency $E$ defined as $E = \frac{R}{T}$. Provide the result in KB/ms.

Accepted Answer

12.8

Question 7

A G1 collector is processing a region where the number of cross-region references $N$ has exceeded the threshold for a fine-grained RSet. If the system transitions to a coarse-grained RSet, which of the following best describes the impact on the memory footprint and scanning overhead?

Accepted Answer

Option A

Answer

Option B

Answer

Option C

Answer

Option D

Question 8

During concurrent marking, the SATB (Snapshot-At-The-Beginning) algorithm maintains a virtual snapshot. If an object $O$ is marked at time $t_0$ and a reference to it is deleted at $t_1$, where $t_0 < t_1 < t_{remark}$, determine if the object $O$ will be reclaimed in the current cycle.

Accepted Answer

No

Question 9

An object occupies $55\%$ of a region of size $R$. According to G1 humongous object handling, which statement is true regarding its allocation?

Accepted Answer

Option A

Answer

Option B

Answer

Option C

Answer

Option D

Question 10

If the promotion threshold is set to $P = 15$ and an object has survived $n = 16$ young collections, it is guaranteed to be moved to the Old generation.

Accepted Answer

True

Answer

False

Question 11

In a system with $N = 1024$ regions, if the RSet for region $i$ uses a sparse table that can hold $K = 32$ entries before coarsening, and the number of incoming references $R_i$ grows such that $R_i > K$, calculate the number of entries the sparse table will hold after coarsening occurs.

Accepted Answer

0

Question 12

Which of the following stages in the G1 concurrent marking cycle involve a stop-the-world pause?

Accepted Answer

Option A

Answer

Option B

Answer

Option C

Answer

Option D

Question 13

If the TAMS (Top-at-Mark-Start) value for a region is $T_{start}$ and the current top of the region is $T_{curr}$, calculate the amount of memory allocated in that region since marking began, given $T_{curr} = 0x1000$ and $T_{start} = 0x0400$.

Accepted Answer

3072

Question 14

In a G1 GC log, the parallel time is recorded as $T_{total} = 60.0$ ms with $N = 8$ worker threads. If the individual worker times are $t_1=55, t_2=58, t_3=52, t_4=60, t_5=53, t_6=57, t_7=54, t_8=51$ (all in ms), calculate the load imbalance factor defined as $I = \frac{t_{max} - t_{avg}}{t_{avg}}$. What is the value of $I$?

Accepted Answer

0.091

Answer

0.055

Answer

0.109

Answer

0.125

Question 15

A JVM heap has a total capacity of $C = 16$ GB. If the evacuation failure rate is observed when the free space $S_{free}$ drops below $S_{crit} = 0.05 	imes C$, and the current allocation rate is $R = 2$ GB/s, how many seconds $t$ remain before a potential to-space exhaustion occurs if the current free space is $S_{curr} = 1.2$ GB?

Accepted Answer

0.2 seconds

Answer

0.1 seconds

Answer

0.4 seconds

Answer

0.6 seconds

Question 16

When analyzing a core file using the Serviceability Agent, if the target JVM was built with a different build version than the SA tool, the SA will always correctly map all memory structures without requiring the $SA\_ALTROOT$ environment variable.

Accepted Answer

False

Answer

True

Question 17

Given a heap occupancy threshold $H_{occ}$ and an initiating heap occupancy percent $P = 45\%$, if the total heap size is $10$ GB and the current live data set is $4$ GB, calculate the remaining headroom $H_{rem}$ before the marking cycle is triggered.

Accepted Answer

0.5 GB

Answer

1.0 GB

Answer

4.5 GB

Answer

5.5 GB

Question 18

If String Deduplication reduces the memory footprint of a set of $N=10^6$ strings by sharing a character array of size $S=100$ bytes each, and the original overhead per string object is $O=24$ bytes, calculate the total memory saved $M$ in MB, assuming a deduplication efficiency of $E=80\%$.

Accepted Answer

99.2 MB

Question 19

Which of the following actions are appropriate when diagnosing a 'to-space exhausted' error in G1 GC?

Answer

Increase the heap headroom

Answer

Lower the InitiatingHeapOccupancyPercent

Answer

Increase the number of concurrent marking threads

Answer

Disable String Deduplication

Question 20

Using the SA Object Histogram, you observe that $N_1 = 5000$ instances of Class A consume $M_1 = 5$ MB and $N_2 = 1000$ instances of Class B consume $M_2 = 8$ MB. Calculate the average size $\bar{S}$ of an instance of Class B in bytes.

Accepted Answer

8192 bytes

Answer

1024 bytes

Answer

4096 bytes

Answer

16384 bytes

Mastering G1 Garbage Collection And Jvm Serviceability Tools Practice Tests

Practice Tests

Mastering G1 Garbage Collection and JVM Serviceability Tools

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