要获取Mac的内存情况，需要用到vm_statistics64_data_t或者 vm_statistics_data_t，前者对应64位的机器后者是32位的机器。由于现在大部分电脑都是64位的了，所以直接看vm_statistics64_data_t的实现。
typedef struct vm_statistics64 vm_statistics64_data_t;
可以看到它其实是一个名为vm_statistics64的结构体，我们再看看这个vm_statistics64的实现。
/*
* vm_statistics64
*
* History:
* rev0 - original structure.
* rev1 - added purgable info (purgable_count and purges).
* rev2 - added speculative_count.
* ----
* rev3 - changed name to vm_statistics64.
* changed some fields in structure to 64-bit on
* arm, i386 and x86_64 architectures.
* rev4 - require 64-bit alignment for efficient access
* in the kernel. No change to reported data.
*
*/
struct vm_statistics64 {
natural_t free_count; /* # of pages free */
natural_t active_count; /* # of pages active */
natural_t inactive_count; /* # of pages inactive */
natural_t wire_count; /* # of pages wired down */
uint64_t zero_fill_count; /* # of zero fill pages */
uint64_t reactivations; /* # of pages reactivated */
uint64_t pageins; /* # of pageins */
uint64_t pageouts; /* # of pageouts */
uint64_t faults; /* # of faults */
uint64_t cow_faults; /* # of copy-on-writes */
uint64_t lookups; /* object cache lookups */
uint64_t hits; /* object cache hits */
uint64_t purges; /* # of pages purged */
natural_t purgeable_count; /* # of pages purgeable */
/*
* NB: speculative pages are already accounted for in "free_count",
* so "speculative_count" is the number of "free" pages that are
* used to hold data that was read speculatively from disk but
* haven't actually been used by anyone so far.
*/
natural_t speculative_count; /* # of pages speculative */
/* added for rev1 */
uint64_t decompressions; /* # of pages decompressed */
uint64_t compressions; /* # of pages compressed */
uint64_t swapins; /* # of pages swapped in (via compression segments) */
uint64_t swapouts; /* # of pages swapped out (via compression segments) */
natural_t compressor_page_count; /* # of pages used by the compressed pager to hold all the compressed data */
natural_t throttled_count; /* # of pages throttled */
natural_t external_page_count; /* # of pages that are file-backed (non-swap) */
natural_t internal_page_count; /* # of pages that are anonymous */
uint64_t total_uncompressed_pages_in_compressor; /* # of pages (uncompressed) held within the compressor. */
} __attribute__((aligned(8)));
里面定义了OSX各种类型的内存，我们需要对这些值进行处理就可以获得正确的内存情况。
总的内存: (active_count + inactive_count + speculative_count + wire_count + compressor_page_count ＋ free_count) * 4096
使用的内存: (active_count + inactive_count + speculative_count + wire_count + compressor_page_count － purgeable_count － external_page_count) * 4096
应用内存: internal_page_count * 4096
联动内存: wire_count * 4096
已压缩: compressor_page_count * 4096
已缓存文件: (purgeable_count + external_page_count) * 4096
内存压力: (vm_stat.wire_count + vm_stat.compressor_page_count) / 总的内存 * 100%
以上是一些常用的内存情况的数值。这里乘以4096是由平台决定，64位就需要乘以4096。我们可以通过以下方式获取vm_statistics64_data_t的值。
- (void)memoryUsage
{
vm_size_t page_size;
mach_port_t mach_port;
mach_msg_type_number_t count;
vm_statistics64_data_t vm_stat;
mach_port = mach_host_self();
count = HOST_VM_INFO64_COUNT;
if (KERN_SUCCESS == host_page_size(mach_port, &page_size) &&
KERN_SUCCESS == host_statistics64(mach_port, HOST_VM_INFO64,
(host_info64_t)&vm_stat, &count))
{
// 仅仅打印了内存压力，你可以打印其他的值
// 这里的page_size就是4096，是由平台决定的，64位机器的值就是4096
// 使用long long仅仅是害怕数值太大，不是必要的
long long useAndCache = (vm_stat.active_count + vm_stat.inactive_count + vm_stat.speculative_count + vm_stat.wire_count + vm_stat.compressor_page_count) * page_size;
long long cache = (vm_stat.purgeable_count + vm_stat.external_page_count) * page_size;
long long used = useAndCache - cache;
long long free = useAndCache + vm_stat.free_count - used;
long long pressure = (vm_stat.wire_count + vm_stat.compressor_page_count) * page_size;
NSLog(@"memory_pressure: %.2f%%",pressure * 1.0 / (free + used) * 100);
}
}
我们可以通过命令来进行测试，打开终端输入memory_pressure -p 40，系统会使电脑的内存压力保持在60%，这样就可以验证是否正确了。更简单的方法是装一个iStat，可以直观看到内存的使用情况。
最后需要注意的是，上面获取到的值都是byte为单位的，记得转换为自己需要的单位。
