golang获取服务内存信息

以linux服务为例子，正常情况下，要获取服务内存信息可以通过相关的命令，例如top、ps等命令。这些监控内存使用情况的方法，一般需要编写脚本，执行脚本后将执行结果发送给对应的监控服务，从而达到监控的效果。但是golang自带的包却有一个runtime包，可以轻松获取服务运行时候的各种包括内存使用情况的信息。
使用linux命令，一般情况下只能看服务使用了多少内存。但是服务内存具体的使用情况缺无法获取。golang的runtime包可以做到获取服务总共使用主机多少内存，也可以获取服务已经申请了多少内存，以及内存的分布。个人觉得golang的runtime包功能很强大，可以获取很多服务运行信息，后续要仔细学习。这里先重点来看下跟内存相关的接口，本博客是基于GO1.9 windows64版本的代码和运行环境进行分析。
runtime中和内存使用情况相关的结构体为runtime.MemStats，这个结构定义了golang运行过程中所有内存相关的信息，在源代码中定义如下：

// A MemStats records statistics about the memory allocator. 记录内存分配器的信息
type MemStats struct {
    // General statistics.

    // Alloc is bytes of allocated heap objects.
    // 堆空间分配的字节数
    // This is the same as HeapAlloc (see below).
    Alloc uint64

    // TotalAlloc is cumulative bytes allocated for heap objects.
    //
    // TotalAlloc increases as heap objects are allocated, but
    // unlike Alloc and HeapAlloc, it does not decrease when
    // objects are freed. 从服务开始运行至今分配器为分配的堆空间总和
    TotalAlloc uint64

    // Sys is the total bytes of memory obtained from the OS.
    //
    // Sys is the sum of the XSys fields below. Sys measures the
    // virtual address space reserved by the Go runtime for the
    // heap, stacks, and other internal data structures. It's
    // likely that not all of the virtual address space is backed
    // by physical memory at any given moment, though in general
    // it all was at some point. 服务现在使用的内存
    Sys uint64

    // Lookups is the number of pointer lookups performed by the
    // runtime.
    //
    // This is primarily useful for debugging runtime internals. 被runtime监视的指针数
    Lookups uint64

    // Mallocs is the cumulative count of heap objects allocated. 服务malloc的次数
    // The number of live objects is Mallocs - Frees.
    Mallocs uint64

    // Frees is the cumulative count of heap objects freed. 服务回收的heap objects
    Frees uint64

    // Heap memory statistics.
    //
    // Interpreting the heap statistics requires some knowledge of
    // how Go organizes memory. Go divides the virtual address
    // space of the heap into "spans", which are contiguous
    // regions of memory 8K or larger. A span may be in one of
    // three states:
    //
    // An "idle" span contains no objects or other data. The
    // physical memory backing an idle span can be released back
    // to the OS (but the virtual address space never is), or it
    // can be converted into an "in use" or "stack" span.
    //
    // An "in use" span contains at least one heap object and may
    // have free space available to allocate more heap objects.
    //
    // A "stack" span is used for goroutine stacks. Stack spans
    // are not considered part of the heap. A span can change
    // between heap and stack memory; it is never used for both
    // simultaneously.

    // HeapAlloc is bytes of allocated heap objects.
    //
    // "Allocated" heap objects include all reachable objects, as
    // well as unreachable objects that the garbage collector has
    // not yet freed. Specifically, HeapAlloc increases as heap
    // objects are allocated and decreases as the heap is swept
    // and unreachable objects are freed. Sweeping occurs
    // incrementally between GC cycles, so these two processes
    // occur simultaneously, and as a result HeapAlloc tends to
    // change smoothly (in contrast with the sawtooth that is
    // typical of stop-the-world garbage collectors).
    //服务分配的堆内存
    HeapAlloc uint64

    // HeapSys is bytes of heap memory obtained from the OS.
    //
    // HeapSys measures the amount of virtual address space
    // reserved for the heap. This includes virtual address space
    // that has been reserved but not yet used, which consumes no
    // physical memory, but tends to be small, as well as virtual
    // address space for which the physical memory has been
    // returned to the OS after it became unused (see HeapReleased
    // for a measure of the latter).
    //
    // HeapSys estimates the largest size the heap has had.
    //系统分配的堆内存
    HeapSys uint64

    // HeapIdle is bytes in idle (unused) spans.
    //
    // Idle spans have no objects in them. These spans could be
    // (and may already have been) returned to the OS, or they can
    // be reused for heap allocations, or they can be reused as
    // stack memory.
    //
    // HeapIdle minus HeapReleased estimates the amount of memory
    // that could be returned to the OS, but is being retained by
    // the runtime so it can grow the heap without requesting more
    // memory from the OS. If this difference is significantly
    // larger than the heap size, it indicates there was a recent
    // transient spike in live heap size.
    //申请但是为分配的堆内存，（或者回收了的堆内存）
    HeapIdle uint64

    // HeapInuse is bytes in in-use spans.
    //
    // In-use spans have at least one object in them. These spans
    // can only be used for other objects of roughly the same
    // size.
    //
    // HeapInuse minus HeapAlloc esimates the amount of memory
    // that has been dedicated to particular size classes, but is
    // not currently being used. This is an upper bound on
    // fragmentation, but in general this memory can be reused
    // efficiently.
    //正在使用的堆内存
    HeapInuse uint64

    // HeapReleased is bytes of physical memory returned to the OS.
    //
    // This counts heap memory from idle spans that was returned
    // to the OS and has not yet been reacquired for the heap.
    //返回给OS的堆内存，类似C/C++中的free。
    HeapReleased uint64

    // HeapObjects is the number of allocated heap objects.
    //
    // Like HeapAlloc, this increases as objects are allocated and
    // decreases as the heap is swept and unreachable objects are
    // freed.
    //堆内存块申请的量
    HeapObjects uint64

    // Stack memory statistics.
    //
    // Stacks are not considered part of the heap, but the runtime
    // can reuse a span of heap memory for stack memory, and
    // vice-versa.

    // StackInuse is bytes in stack spans.
    //
    // In-us

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