Open Radar

 
16-Jan-2012 07:07 PM Alexander Potapenko:
Summary: Process virtual memory cleanup takes quite a lot of time

Steps to Reproduce:
=====================
$ cat p.c
#include <sys/mman.h>
int main() {
 void *t = mmap(0, 0x00000fffffffffffUL, PROT_READ| PROT_WRITE,
MAP_ANON | MAP_PRIVATE | MAP_NORESERVE, -1, 0);
}
=====================

runs for half a second on my 64-bit Snow Leopard machine:
=====================
$ time ./p

real    0m0.545s
user    0m0.000s
sys     0m0.544s
=====================

According to Shark, most of the time is spent in pmap_remove at exit time:
=====================
# Report 5 - session_008.mshark - Time Profile of PID #98012
SharkProfileViewer
# Generated from the visible portion of the outline view
+ 59.9%, pmap_remove, mach_kernel
| + 59.9%, vm_map_lookup_locked, mach_kernel
| | + 59.9%, vm_map_remove, mach_kernel
| | | + 59.9%, task_terminate_internal, mach_kernel
| | | | + 59.9%, exit1, mach_kernel
| | | | | + 59.9%, exit, mach_kernel
| | | | | |   59.9%, unix_syscall64, mach_kernel
- 12.0%, pmap64_pde, mach_kernel
- 10.7%, pmap64_pdpt, mach_kernel
- 5.0%, vm_map_lookup_locked, mach_kernel
- 2.7%, zalloc_canblock, mach_kernel
- 2.2%, pmap_pde, mach_kernel
- 1.8%, lck_mtx_lock_spin, mach_kernel
- 1.6%, vm_map_entry_insert, mach_kernel
- 1.0%, lck_mtx_unlock_darwin10, mach_kernel
- 0.5%, hw_lock_to, mach_kernel
=====================

Expected Results:
The program runs for milliseconds, as this should be safe to drop all the mappings on process shutdown.

Actual Results:
The program runs for half a second.

Regression:

Notes:
AddressSanitizer (http://code.google.com/p/address-sanitizer/) maps such a huge amount of memory to shadow the address space of the process being debugged.
The shutdown time for the process being tested affects the testing experience a bit.

16-Jan-2012 07:08 PM Alexander Potapenko:
$ uname -a
Darwin hostname 10.8.0 Darwin Kernel Version 10.8.0: Tue Jun  7 16:33:36 PDT 2011; root:xnu-1504.15.3~1/RELEASE_I386 i386