正确地获取 iOS 应用占用的内存
2017-05-14
当我们想去获取 iOS 应用的占用内存时，通常我们能找到的方法是这样的，用 resident_size：
#import <mach/mach.h>
- (int64_t)memoryUsage {
int64_t memoryUsageInByte = 0;
struct task_basic_info taskBasicInfo;
mach_msg_type_number_t size = sizeof(taskBasicInfo);
kern_return_t kernelReturn = task_info(mach_task_self(), TASK_BASIC_INFO, (task_info_t) &taskBasicInfo, &size);
if(kernelReturn == KERN_SUCCESS) {
memoryUsageInByte = (int64_t) taskBasicInfo.resident_size;
NSLog(@"Memory in use (in bytes): %lld", memoryUsageInByte);
} else {
NSLog(@"Error with task_info(): %s", mach_error_string(kernelReturn));
}
return memoryUsageInByte;
}
但是测试的时候，我们会发现这个跟我们在 Instruments 里面看到的内存大小不一样，有时候甚至差别很大。更加准确的方式应该是用 phys_footprint：
#import <mach/mach.h>
- (int64_t)memoryUsage {
int64_t memoryUsageInByte = 0;
task_vm_info_data_t vmInfo;
mach_msg_type_number_t count = TASK_VM_INFO_COUNT;
kern_return_t kernelReturn = task_info(mach_task_self(), TASK_VM_INFO, (task_info_t) &vmInfo, &count);
if(kernelReturn == KERN_SUCCESS) {
memoryUsageInByte = (int64_t) vmInfo.phys_footprint;
NSLog(@"Memory in use (in bytes): %lld", memoryUsageInByte);
} else {
NSLog(@"Error with task_info(): %s", mach_error_string(kernelReturn));
}
return memoryUsageInByte;
}
关于 phys_footprint 的定义可以在 XNU 源码中，找到 osfmk/kern/task.c 里对于 phys_footprint 的注释：
/*
* phys_footprint
* Physical footprint: This is the sum of:
* + (internal - alternate_accounting)
* + (internal_compressed - alternate_accounting_compressed)
* + iokit_mapped
* + purgeable_nonvolatile
* + purgeable_nonvolatile_compressed
* + page_table
*
* internal
* The task's anonymous memory, which on iOS is always resident.
*
* internal_compressed
* Amount of this task's internal memory which is held by the compressor.
* Such memory is no longer actually resident for the task [i.e., resident in its pmap],
* and could be either decompressed back into memory, or paged out to storage, depending
* on our implementation.
*
* iokit_mapped
* IOKit mappings: The total size of all IOKit mappings in this task, regardless of
clean/dirty or internal/external state].
*
* alternate_accounting
* The number of internal dirty pages which are part of IOKit mappings. By definition, these pages
* are counted in both internal *and* iokit_mapped, so we must subtract them from the total to avoid
* double counting.
*/
注释里提到的公式计算的应该才是应用实际使用的物理内存。