Copy-on-write technique in Operating Systems - Step-by-Step Execution

Concept Flow - Copy-on-write technique

Process A and B share same memory page

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Process B tries to write to shared page

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OS detects write attempt

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OS creates a copy of the page for Process B

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Process B writes to its own copy

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Process A continues using original page

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Both processes have separate pages after write

Two processes share the same memory page until one tries to write, then the OS copies the page so each has its own.

Execution Sample

Operating Systems

Process A and B share page P
Process B writes to page P
OS copies page P to P_B
Process B writes to P_B
Process A uses original P

Shows how copy-on-write delays copying memory until a write happens.

Analysis Table

Step	Action	Memory State	Result
1	Process A and B share page P	P shared by A and B	No copy, memory shared
2	Process B attempts to write to P	P shared by A and B	Write detected, triggers copy
3	OS copies page P to P_B for Process B	P owned by A, P_B owned by B	Separate copies created
4	Process B writes to P_B	P unchanged, P_B modified	B's write isolated
5	Process A continues using original P	P unchanged	A unaffected by B's write
6	End	Separate pages for A and B	Copy-on-write complete

💡 Process B's write triggers copy, so both have separate pages after step 5

State Tracker

Memory Page	Start	After Step 3	After Step 4	Final
P (Process A)	Shared	Owned by A	Unchanged	Owned by A
P_B (Process B)	N/A	New copy created	Modified by B	Owned by B

Key Insights - 3 Insights

Why is memory not copied when processes first share the page?

Because both processes only read the page initially, so OS saves memory by sharing until a write occurs (see step 1 in execution_table).

What triggers the OS to make a copy of the memory page?

A write attempt by one process to the shared page triggers the OS to copy it (see step 2 and 3 in execution_table).

Does the original process's memory page change after the other process writes?

No, the original process keeps using the original page unchanged (see step 5 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the OS create a new copy of the memory page?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Check the 'Action' and 'Result' columns in execution_table rows for step 3.

According to variable_tracker, what is the state of Process B's memory page after step 4?

A Shared with Process A

B New copy modified by Process B

C Unchanged original page

D No memory page assigned

💡 Hint

Look at the 'P_B (Process B)' row under 'After Step 4' in variable_tracker.

If Process B never writes to the shared page, what would happen according to the concept flow?

A Process A loses access to the page

B OS copies the page anyway

C Processes continue sharing the same page

D Both processes crash

💡 Hint

Refer to the first step in concept_flow where sharing continues until a write occurs.

Concept Snapshot

Copy-on-write delays copying shared memory pages until a write occurs.
Processes share pages to save memory.
When a write happens, OS copies the page for the writing process.
This keeps memory use efficient and isolated after writes.
Used in OS for process creation and memory management.

Full Transcript

Copy-on-write is a technique where two processes initially share the same memory page to save space. When one process tries to write to that page, the operating system detects this and creates a separate copy of the page for that process. This way, the writing process modifies its own copy, while the other process continues using the original page unchanged. This method avoids unnecessary copying until it is needed, making memory use more efficient. The execution steps show the shared state, the write attempt, the copying action, and the final separate pages for each process.