获取Mac端的内存使用情况

最近遇到一个需求，要求实现Mac上的内存使用情况监测。之前没有做过这方面东西，查了一天资料把它给实现了。这个过程很坑，资料比较少，所以想着写下来，方便有需要的人使用。

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。