CSE120 Principles of Operating Systems. Prof Yuanyuan (YY) Zhou Advanced Memory Management

Transcription

1 CSE120 Principes of Operating Systems Prof Yuanyuan (YY) Zhou Advanced Memory Management

2 Advanced Functionaity Now we re going to ook at some advanced functionaity that the OS can provide appications using virtua memory tricks Shared memory Copy on Write Mapped fies 2 11/7/18 CSE 120

3 Sharing Private virtua address spaces protect appications from each other Usuay exacty what we want But this makes it difficut to share data (have to copy) Parents and chidren in a forking Web server or proxy wi want to share an in-memory cache without copying We can use shared memory to aow processes to share data using direct memory references Both processes see updates to the shared memory segment Process B can immediatey read an update by process A 3

4 Sharing (2) How can we impement sharing using page tabes? Have PTEs in both tabes map to the same physica frame Each PTE can have different protection vaues Must update both PTEs when page becomes invaid P1 s Page Tabe Physica Memory Page frame Page frame P2 s Page Tabe 4 How are we going to coordinate access to shared data?

5 Sharing (3) Can map shared memory at same or different virtua addresses in each process address space Different: 10 th virtua page in P1 and 7 th virtua page in P2 correspond to the 2 nd physica page Fexibe (no address space conficts), but pointers inside the shared memory segment are invaid Same: What happens if it points to data inside/outside the segment? 2 nd physica page corresponds to the 10 th virtua page in both P1 and P2 Less fexibe, but shared pointers are vaid 5

6 Copy on Write OSes spend a ot of time copying data System ca arguments between user/kerne space Entire address spaces to impement fork() Use Copy on Write (CoW) to defer arge copies as ong as possibe, hoping to avoid them atogether Instead of copying pages, create shared mappings of parent pages in chid virtua address space Shared pages are protected as read-ony in parent and chid Reads happen as usua Writes generate a protection faut, trap to OS, copy page, change page mapping in cient page tabe, restart write instruction How does this hep fork()? 6

7 Execution of fork() Parent process s page tabe Page 1 Page 2 Physica Memory Page 1 Page 2 Chid process s page tabe 7

8 fork() with Copy on Write Parent process s page tabe Page 1 Page 2 When either process modifies Page 1, page faut hander aocates new page and updates PTE in chid process Physica Memory Protection bits set to prevent either process from writing to any page Page 1 Page 2 Chid process s page tabe Under what circumstances such copies can be deferred forever? 8

9 Mapped Fies Mapped fies enabe processes to do fie I/O using oads and stores Instead of open, read into buffer, operate on buffer, Bind a fie to a virtua memory region (mmap() in Unix) PTEs map virtua addresses to physica frames hoding fie data Virtua address base + N refers to offset N in fie Initiay, a pages mapped to fie are invaid OS reads a page from fie when invaid page is accessed 9

10 Memory-Mapped Fies Pages are a invaid initiay P1 Physica Memory A read occurs A read occurs Page 1 Page 2 Fie Content 1 Fie Content 2 What happens if we unmap the memory? How do we know whether we need to write changes back to fie? 10

11 Writing Back to Fie OS writes a page to fie when evicted, or region unmapped If page is not dirty (has not been written to), no write needed Dirty bit trick (not protection bits) 11

12 Mapped Fies (2) Fie is essentiay backing store for that region of the virtua address space (instead of using the swap fie) Virtua address space not backed by rea fies aso caed Anonymous VM Advantages Uniform access for fies and memory (just use pointers) Less copying Drawbacks Process has ess contro over data movement OS handes fauts transparenty Does not generaize to streamed I/O (pipes, sockets, etc.) 12 11/7/18 CSE 120