نظم تشغيل 2-المحاضرة العاشرة

Copy-on-Write
Recall that the fork( ) system call creates a child process that is a duplicate of its parent.
Traditionally, fork( ) worked by creating a copy of the parent’s address space for the child,
duplicating the pages belonging to the parent.
However, considering that many child processes invoke the exec() system call immediately after
creation, the copying of the parent’s address space may be unnecessary. Instead, we can use a
technique known as copy-on-write, which works by allowing the parent and child processes initially
to share the same pages. These shared pages are marked as copy-on-write pages, meaning that if
either process writes to a shared page, a copy of the shared page is created. Copy-on-write is
illustrated in Figures below, which show the contents of the physical memory before and after
process 1 modifies page C.
Copy-on-Write
Recall that the fork( ) system call creates a child process that is a duplicate of its parent.
Traditionally, fork( ) worked by creating a copy of the parent’s address space for the child,
duplicating the pages belonging to the parent.
However, considering that many child processes invoke the exec() system call immediately after
creation, the copying of the parent’s address space may be unnecessary. Instead, we can use a
technique known as copy-on-write, which works by allowing the parent and child processes initially
to share the same pages. These shared pages are marked as copy-on-write pages, meaning that if
either process writes to a shared page, a copy of the shared page is created. Copy-on-write is
illustrated in Figures below, which show the contents of the physical memory before and after
process 1 modifies page C.