Back to the Future: A Retroactive Study of Aspect Evolution in Operating System Code

1. Back to the Future: A Retroactive Study of Aspect Evolution in Operating System Code Sabine Hauert

2. Plan • Introduction • FreeBSD and it’s evolutions • Goals • Case studies • Page Daemon Activation • Prefetching • Disk Quotas • Blocking in Device Drivers • Summary • Conclusions and discussion

3. IntroductionFreeBSD • What is FreeBSD? • Free operating system • x86, amd64 compatible • Alpha/AXP, IA-64, PC-98 and UltraSPARC® architectures. • Derived from BSD, the version of UNIX ® developed at the University of California, Berkeley.

4. IntroductionFreeBSD evolution V2 212,000 LOC V3 357,000 LOC V4 474,000 LOC

5. IntroductionFreeBSD evolution V2 212,000 LOC • Device Drivers: • 581 new subdirectories and files • 100’000 LOC V3 357,000 LOC V4 474,000 LOC

6. IntroductionFreeBSD evolution V2 212,000 LOC • Page daemon activation sites: • 50% less V3 357,000 LOC V4 474,000 LOC

7. IntroductionFreeBSD evolution V2 212,000 LOC • Blocking in Device Drivers: • 50% more V3 357,000 LOC V4 474,000 LOC

8. IntroductionGoals • Analyze the impact of evolution on an “aspect-oriented” implementation of the OS. • Compare with original version • Localization • Configurability • Redundancy • Modularity

9. Case Studies Page Daemon Activation • Frees physical memory when the number available pages falls below some threshold. The daemon imposes overhead for determining which pages will be replaced and for writing them back to disk if necessary. • Speed is important • Context specific • Crosscuts operations that consume available pages

10. Case Studies Page Daemon Activation: Where can it occur? usr/src/sys/ Kern/ vfs_bio.c allocbuf() Vm/ swap_pager.c getpages() putpages() sync() io_done() vm_fault.c additional_pages() 1 vm_page.c unqueue() 1 alloc() 3 vm_wait() 1 vm_await() 1+ Vm/ swap_pager.c getpages() putpages() sync() io_done() vm_fault.c additional_pages() 1 vm_page.c unqueue() 1 alloc() 5 vm_wait() 1 Vm/ swap_pager.c getpages() 1 putpages() 3 sync() 1 io_done() 2 vm_fault.c additional_pages() 1 vm_page.c unqueue() 1 alloc() 5 vm_wait() 1

11. Locality System -wide view Context Functionality aspect page_daemon_wakeup { pointcut unqueuing_available_pages(vm_page_t m): execution(void vm_page_unqueue(m)) && cflow(execution(void vm_page_activate(vm_page_t)) || execution(void vm_page_wire(vm_page_t)) || execution(void vm_page_unmanage(vm_page_t)) || execution(void vm_page_deactivate(vm_page_t, int))); pointcut allocating_buffers(vm_object_t obj, vm_pindex_t pindex): execution(vm_page_t vm_page_lookup(obj, pindex)) && cflow(execution(int allocbuf(struct buf*, int))); around(vm_page_t m): unqueuing_available_pages(m) { int queue = m->queue; proceed(m); if (((queue - m->pc) == PQ_CACHE) && (pages_available() < vm_page_threshold())) pagedaemon_wakeup(); } around(vm_object_t obj, vm_pindex_t pindex): allocating_buffers(obj, pindex) { vm_page_t m = proceed(obj, pindex); if ((m != NULL) && !(m->flags & PG_BUSY) && ((m->queue - m->pc) == PQ_CACHE) && (pages_available() < vfs_page_threshold())) pagedaemon_wakeup(); return m; } … }

12. Case Studies Page Daemon Activation: Evolution • Changes are only made to the aspect • Delete/add poincuts and advice • Refactoring of threshold checks • Adding little helper methods

13. Case Studies Prefetching for mapped files • Prefetching is a heuristic designed to amortize costs by bringing in additional pages that may be required in the near future. • VM suggests pages for prefetching • File system decides • crosscuts virtual memory and file systems, coordinating high-level allocation and low-level de-allocation of prefetched pages.

14. Case Studies Prefetching for mapped files: Where can it occur? usr/src/sys/ Vm/ vm_fault.c vm_fault() 3 additiona_pages() 1 ufs/ufs/ ufs_readwrite.c ffs_getpages() 4+ ffs_read() 1 gnu/ext2fs/ ext2_vnops.c ext2_getpages() 3 ufs/ufs/ ufs_readwrite.c ffs_getpages() 3 ufs/ufs/ ufs_readwrite.c ffs_getpages() 3 - ffs_read()

15. aspect mapped_file_prefetching{ pointcut vm_fault_path(vm_map_t map): cflow(execution(int vm_fault(map,..))); pointcut getpages_path(vm_map_t map, vm_object_t obj, vm_page_t* plist, int n, in fpage): cflow(execution(int ffs_getpages(obj, plist, n, fpage) || execution(int vnode_leaf_pager_getpages(obj, plist,n,fpage))); before(vm_map_t map, vm_object_t obj, vm_page_t* plist, int n, int fpage): execution(int vnode_pager_getpages(obj, plist, n, fpage)) && vm_fault_path(map) { …plan and allocate prefetched pages…} after(vm_object_t obj, vm_page_t* plist, int n, int fpage, int valid): execution(valid check_valid(..)) && getpages_path(obj, plist, n, fpage) { …dealloc all prefetched pages…} after(vm_object_t obj, vm_page_t* plist, int n, int fpage, struct transfer_args* trans_args): execution(int calc_range(trans_args)) && getpages_path(obj, plist, len, fpage) { …dealloc non contiguous pages… } …}

16. Case Studies Prefetching for mapped files: evolution • V2->V3: optimized implementation of sequential mode prefetching to FFS. Use of the file system read-path for sequential mode. • Separation of the aspect in two aspects: • Normal sequential mode aspect • Optimized sequential aspect • V3->V4: remove modifications in V3. • Taking sequential aspect out of Makefile and recombining both aspects in the normal mode aspect VS. Editing of code within FFS operations for a non-AO system.

17. Case Studies Disk Quota • Designed to track disk utilization and enforce limits for all users and groups • Structure: set of low-level disk space related operations that are consistently monitoring/limiting all disk usage • crosscuts disk-consuming operations from multiple file systems

18. Case Studies Disk Quota: Where can it occur? usr/src/sys/ ufs/ffs/ vfs.c 3 inode.c 2 alloc.c 5 ufs/ffs/ vfs.c 3 inode.c 3+ alloc.c 5 ufs/ffs/ vfs.c 3 inode.c 3 alloc.c 5 balloc.c 1 + ufs/ffs/ vfs.c 3 inode.c 3 alloc.c 5 balloc.c 1 + ufs/ufs/ vnops.c 7 inode.c 3 ufs/ufs/ vnops.c 13 + inode.c 3 vfs.c 1 + ufs/ufs/ vnops.c 13 + inode.c 3 vfs.c 1 + gnu/ext2fs/ vfs.c 4 inode.c 2 alloc.c 3 vnops.c 8 + gnu/ext2fs/ vfs.c 4 inode.c 2 alloc.c 3 vnops.c 8 + gnu/ext2fs/ vfs.c 4 inode.c 2 alloc.c 3

19. Case Studies Disk Quota #ifdef QUOTA if (mp->mnt_flag & MNT_QUOTA) { int i; error = vflush(mp, 0, SKIPSYSTEM|flags); if (error) return (error); for (i = 0; i < MAXQUOTAS; i++) { if (ump->um_quotas[i] == NULLVP) continue; quotaoff(p, mp, i); } } #endif ... } #if QUOTA int i; #endif ... #if QUOTA if (mp->mnt_flag & MNT_QUOTA) { if ((error = vflush(mp, 0, SKIPSYSTEM|flags)) !=0) return (error); for (i = 0; i < MAXQUOTAS; i++) { if (ump->um_quotas[i] == NULLVP) continue; quotaoff(p, mp, i); } } #endif ... }

20. aspect disk_quota { pointcut flushfiles(register struct mount *mp, int flags, struct proc *p): execution(int ffs_flushfiles(mp, flags, p)) || execution(int ext2_flushfiles(mp, flags, p)); around(register struct mount *mp, int flags, struct proc *p): flushfiles(mp, flags, p) { register struct ufsmount *ump; ump = VFSTOUFS(mp); if (mp->mnt_flag & MNT_QUOTA) { int i; int error = vflush(mp, NULLVP, SKIPSYSTEM|flags); if (error) return (error); for (i = 0; i < MAXQUOTAS; i++) { if (ump->um_quotas[i] == NULLVP) continue; quotaoff(p, mp, i); } } return proceed(mp, flags, p); }

21. Case Studies Disk Quota: Evolution • Adding pointcuts and advice is needed to incrementally extend its configuration to include new functionality. • V2->V3: new feature for server which automatically assigned the ownership of a new file to be that of the enclosing directory.

22. Case Studies Blocking in device drivers • Explicitly surrender the CPU and block, allowing another process to run • Keep CPU busy while processes waits for device I/O • crosscuts all device-specific operations involved with I/O

23. Case Studies Blocking: Where can it occur?

24. Case Studies Blocking: tracking non-timeout tsleeps() aspect device_blocked_notimeout { pointcut block_in_device(void* ident, int priority, const char* wmesg, int timo): call(int tsleep(ident, priority, wmesg, timo)) && within(“/usr/src/sys/dev”); before(void* ident, int priority, const char* wmesg, int timo): block_in_device(ident, priority, wmesg, timo) { if (timo == 0) printf(“Blocking in device, no timeout: %s pid: %d\n”, wmesg, curproc->p_pid); } }

25. Case Studies Disk Quota: Evolution • No evolution needed!

27. Conclusion • Changeability • Textual locality => consistency • Configurability • Modifying aspects • Modifying makefile options • Redundancy • Quota aspect example • Extensibility • Device aspect

28. Discussion • What about the disadvantages? • How biased is the analysis? • How to define the limit between what should be in an aspect and what shouldn’t: core functionality vs. non core functionality. • Can we know how a system will evolve?