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

Back to the Future: A Retroactive Study of Aspect

Evolution in Operating System Code

Back to the Future: A Retroactive Study of Aspect

Evolution in Operating System Code

Sabine HauertSabine Hauert

PlanPlan

Introduction FreeBSD and it’s evolutions Goals

Case studies Page Daemon Activation Prefetching Disk Quotas Blocking in Device Drivers

Summary Conclusions and discussion

Introduction FreeBSD and it’s evolutions Goals

Case studies Page Daemon Activation Prefetching Disk Quotas Blocking in Device Drivers

Summary Conclusions and discussion

IntroductionFreeBSD

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.

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.

IntroductionFreeBSD evolution


V2

212,000 LOC

V3

357,000 LOC

V4

474,000 LOC



V2

212,000 LOC

V3

357,000 LOC

V4

474,000 LOC

Device Drivers:

- 581 new subdirectories

and files

- 100’000 LOC



V2

212,000 LOC

V3

357,000 LOC

V4

474,000 LOC

Page daemon activation sites:

- 50% less



V2

212,000 LOC

V3

357,000 LOC

V4

474,000 LOC

Blocking in Device Drivers:

- 50% more

IntroductionGoals

IntroductionGoals

Analyze the impact of evolution on an “aspect-oriented” implementation of the OS.

Compare with original version Localization Configurability Redundancy Modularity

Analyze the impact of evolution on an “aspect-oriented” implementation of the OS.

Compare with original version Localization Configurability Redundancy Modularity

Case Studies Page Daemon Activation

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

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

Case Studies Page Daemon Activation: Where can it occur?

Case Studies Page Daemon Activation: Where can it occur?

Vm/

swap_pager.cgetpages() 1putpages() 3sync() 1io_done() 2

vm_fault.cadditional_pages() 1

vm_page.cunqueue() 1alloc() 5vm_wait() 1

usr/src/sys/

Kern/ vfs_bio.c

allocbuf()

Vm/

swap_pager.cgetpages()getpages()putpages()putpages()sync()sync()io_done()io_done()


vm_page.cunqueue() 1alloc() 5vm_wait() 1

Vm/

swap_pager.cgetpages()getpages()putpages()putpages()sync()sync()io_done()io_done()


vm_page.cunqueue() 1alloc() 3vm_wait() 1vm_await() 1+

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; }

… }

Locality

Functionality

Context

System-wide view

Case Studies Page Daemon Activation: Evolution

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

Changes are only made to the aspect Delete/add poincuts and advice Refactoring of threshold checks Adding little helper methods

Case Studies Prefetching for mapped files

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.

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.

Case Studies Prefetching for mapped files: Where can it occur?

Case Studies Prefetching for mapped files: Where can it occur?

Vm/ vm_fault.c vm_fault() 3 additiona_pages() 1

gnu/ext2fs/ ext2_vnops.c ext2_getpages() 3

ufs/ufs/ ufs_readwrite.c ffs_getpages() 3

usr/src/sys/

ufs/ufs/ ufs_readwrite.c ffs_getpages() 4+ ffs_read() 1

ufs/ufs/ ufs_readwrite.c ffs_getpages() 3 - ffs_read() ffs_read()

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… }…}

Case Studies Prefetching for mapped files: evolution

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.

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.

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

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

Case Studies Disk Quota: Where can it occur?

Case Studies Disk Quota: Where can it occur?

usr/src/sys/

ufs/ufs/ vnops.c 7 inode.c 3

gnu/ext2fs/ vfs.c 4 inode.c 2 alloc.c 3

ufs/ffs/ vfs.c 3 inode.c 2 alloc.c 5

ufs/ufs/ vnops.c 13 + inode.c 3 vfs.c 1 +

ufs/ffs/ vfs.c 3 inode.c 3+ alloc.c 5

gnu/ext2fs/ vfs.c 4 inode.c 2 alloc.c 3 vnops.c 8 +

ufs/ffs/ vfs.c 3 inode.c 3 alloc.c 5 balloc.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 +

ufs/ffs/ vfs.c 3 inode.c 3 alloc.c 5 balloc.c 1 +



#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 ... }

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); }

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.

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.

Case Studies Blocking in device drivers

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

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

Case Studies Blocking: Where can it occur?

Case Studies Blocking: Where can it occur?

version LOC devices

(subdirs)

Calls to tsleep()

Tsleep() w/ no timeout

2 8400 7 5 2

3 46900 27 55 11

4 114800 69 110 22

Case Studies Blocking: tracking non-timeout tsleeps()

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); }

}



No evolution needed! No evolution needed!

concern Major evolution Structural challenge

Original/aspect Benefits

Page daemon

wakeup

Revamping of code it crosscuts: VM and buffer cache

Multiple context specific thresholds

Scattered activation

/

Textually localized

Independent development & localized change

Prefetching for mapped files

Change in design of sequential mode

New subsystem interaction along execution paths

Internal to function

/

Explicit control flow

Explicit subsystem interaction & pluggability in makefile

Disk quota New functionality in code it crosscuts:

UFS,FFS,EXT2

Configurability

And sharing across file systems

#ifdefs w/ redundant code

/

Explicit sharing

Pointcut configurability

& reduced redundancy

Device blocking New device drivers added to the system

Consistency across rapidly growing diversity

Individualized devices /

Centralized assessment

Comprehensive coverage & further extensibility modularized

ConclusionConclusion

Changeability Textual locality => consistency

Configurability Modifying aspects Modifying makefile options

Redundancy Quota aspect example

Extensibility Device aspect

Changeability Textual locality => consistency

Configurability Modifying aspects Modifying makefile options

Redundancy Quota aspect example

Extensibility Device aspect

DiscussionDiscussion

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?

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?

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

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