Project 3

1. Project 3 • Purpose: understand how memory allocators provide virtual memory to programs. • Write a BiBOP style simple memory allocator • Page size = 4K • Segregated size classes of pages • Limits external fragmentation • Can be very fast • Each page also contains a header, containing metadata about the page • Implement basic malloc and free API in allocator.cc. • Compile into a shared library: libmymalloc.so CS377: Operating Systems

2. Input:- • Allocator will run as a layer between any program and OS. • This layer will handle memory related system calls, and in turn request memory from the OS using mmap. • Input is a sequence of calls to malloc and free. • Calls intercepted by a shim layer. • Output:- • No output CS377: Operating Systems

3. Implementation details:- • allocator will allocate memory in units of pages (4K) • Segregated by different sized: 23 to 210 bytes. • Larger objects specially handled. • Bibop header. CS377: Operating Systems

4. To allocate memory from OS, use the following code: #include <fcntl> #include <stdlib> #include <sys> int fd = open(“/dev/zero”, O_RDWR); void * ptr = mmap (NULL, sz, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0); CS377: Operating Systems

5. To return memory call munmap:- • munmap(ptr, size); • The BiBop header needs to be located at the beginning of a page/area that you allocated with mmap. To do this you can use a "placement new": bibop_ptr = new (page_ptr) BibopHeader; • This allocates a BibopHeader at page_ptr address. • Write allocator.cc corresponding to allocator.h CS377: Operating Systems

6. Allocating memory inside the allocator: • Allocate only for the user program • Do not call new or malloc directly or indirectly • Do not use STL which uses heap memory • Can use some global variables • Make your program space efficient • Extra or out of order calls marked as wrong CS377: Operating Systems

7. Choosing space to allocate:- • When allocating a new page, please place it at the head of the list of pages of that size. The next allocation will occur from that page. • When a page becomes full, move it to the head of a separate list of pages that are full. • When a page that was full has an object freed, move it to the head of the list of pages with space available. The next allocation will occur from that page. • You can chose any object inside the page to allocate. • Using two lists is not necessary for large objects CS377: Operating Systems

8. Compiling:- g++ -O2 -DNDEBUG -Wall -shared allocator.cc –o libmymalloc.so libshim.a –ldl • Running:- export LD_PRELOAD=./libmymalloc.so ./test • Your allocator will run on simple programs only. • Compiling as part of a program:- g++ -Wall -g -o test allocator.cc test.cc libshim.a -ldl CS377: Operating Systems

9. Testing:- • Some real programs: pwd, df export LD_PRELOAD=/your/directory/goes/here/libmymalloc.so /usr/bin/time benchmark 0.00user 0.00system 0:00.00elapsed 50%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+274minor)pagefaults 0swaps CS377: Operating Systems

10. Performance hints:- • Finding a BibopHeader for a previous allocation should be constant time. • Finding a free page to allocate for any particular size should occur in constant time. • If your allocator is no longer using a page, it should be returned to the operating system. • Only open /dev/zero once. CS377: Operating Systems

11. Submission:- ./submit377 2 allocator.cc • Helper files (allocator.h, libshim.a) can be found in – /courses/cs300/cs377/cs377.f2008/project3 CS377: Operating Systems