LINUX 进程内存分配基础知识：
Go 进程虚拟内存布局
Attention： Heap prof 没有跟踪CGO /系统调用（例如：malloc / mmap）内存，
所以用了cgo的程序( 包括开启-race选项 )，RSS可能远远大于 mheap的大小
>> 引入 cgosymbolizer 库来跟踪c go 的内存调用（ runtime #SetCgoTraceback）
对象分配流程
- 大于 32K 的大对象直接从 mheap（页堆） 分配。
- 小于 16B 的使用 mcache 的微型分配器分配
- 对象大小在 16B ~ 32K 之间的的，首先通过计算使用的大小规格，然后使用 mcache 中对应大小规格的块分配
- 如果对应的大小规格在 mcache 中没有可用的块，则向 mcentral 申请
- 如果 mcentral 中没有可用的块，则向 mheap 申请，并 根据 BestFit 算法找到最合适的 mspan 。如果申请到的 mspan 超出申请大小，将会根据需求进行切分，以返回用户所需的页数。剩余的页构成一个新的 mspan 放回 mheap 的空闲列表。
- 如果 mheap 中没有可用 span，则向操作系统申请一系列新的页（最小 1MB）。
所有在堆上的内存申请都来自 arena
暂时无法在文档外展示此内容
Go的内存分配思想源自TCMalloc， 具体实现可能每个版本不同
TCMalloc 的核心思想是将内存分为多个级别缩小锁的粒度。
在 TCMalloc 内存管理内部分为两个部分： 线程内存（thread memory) 和 页堆（page heap）
一些概念：
mheap
Go 使用 mheap 对象管理堆，只有一个 全局变量 。持有虚拟地址空间。
mcache
Go 像 TCMalloc 一样为每一个 逻辑处理器（P）（Logical Processors） 提供一个本地线程缓存（Local Thread Cache）称作 mcache ，所以如果 Goroutine 需要内存可以直接从 mcache 中获取，由于在同一时间只有一个 Goroutine 运行在 逻辑处理器（P）（Logical Processors） 上，所以中间不需要任何锁的参与
mcache 包含所有大小规格的 mspan 作为缓存
mcache 的作用是什么？
<=32K 字节的对象直接使用相应大小规格的 mspan 通过 mcache 分配
当 mcache 没有可用空间时会发生什么？
从 mcentral 的 mspans 列表获取一个新的所需大小规格的 mspan 。
mcentral
mcentral 对象收集所有给定规格大小的 span。每一个 mcentral 都包含两个 mspan 的列表：
- empty mspanList — 没有空闲对象或 span 已经被 mcache 缓存的 span 列表
- nonempty mspanList — 有空闲对象的 span 列表
Arena
事实证明 Go 的虚拟内存布局中包含一系列 arenas 。初始的堆映射是一个 arena ，如 64MB （基于 go 1.11.5）。
64位的Linux 一开始分配的Arena大小为64MB。
这些 arenas 就是我们所说的堆 。在 Go 中每一个 arena 都以 8KB 的粒度的页进行管理。
代码参考：
runtime/mheap.go (go1.17.7)
// Main malloc heap.
// The heap itself is the "free" and "scav" treaps,
// but all the other global data is here too.
//
// mheap must not be heap-allocated because it contains mSpanLists,
// which must not be heap-allocated.
//
//go:notinheap
type mheap struct {
// lock must only be acquired on the system stack, otherwise a g
// could self-deadlock if its stack grows with the lock held.
lock mutex
pages pageAlloc // page allocation data structure
sweepgen uint32 // sweep generation, see comment in mspan; written during STW
sweepDrained uint32 // all spans are swept or are being swept
sweepers uint32 // number of active sweepone calls
// allspans is a slice of all mspans ever created. Each mspan
// appears exactly once.
//
// The memory for allspans is manually managed and can be
// reallocated and move as the heap grows.
//
// In general, allspans is protected by mheap_.lock, which
// prevents concurrent access as well as freeing the backing
// store. Accesses during STW might not hold the lock, but
// must ensure that allocation cannot happen around the
// access (since that may free the backing store).
allspans []*mspan // all spans out there
_ uint32 // align uint64 fields on 32-bit for atomics
// Proportional sweep
//
// These parameters represent a linear function from gcController.heapLive
// to page sweep count. The proportional sweep system works to
// stay in the black by keeping the current page sweep count
// above this line at the current gcController.heapLive.
//
// The line has slope sweepPagesPerByte and passes through a
// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
// any given time, the system is at (gcController.heapLive,
// pagesSwept) in this space.
//
// It's important that the line pass through a point we
// control rather than simply starting at a (0,0) origin
// because that lets us adjust sweep pacing at any time while
// accounting for current progress. If we could only adjust
// the slope, it would create a discontinuity in debt if any
// progress has already been made.
pagesInUse uint64 // pages of spans in stats mSpanInUse; updated atomically
pagesSwept uint64 // pages swept this cycle; updated atomically
pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically
sweepHeapLiveBasis uint64 // value of gcController.heapLive to use as the origin of sweep ratio; written with lock, read without
sweepPagesPer byte float64 // proportional sweep ratio; written with lock, read without
// TODO(austin): pagesInUse should be a uintptr, but the 386
// compiler can't 8-byte align fields.
// scavengeGoal is the amount of total retained heap memory (measured by
// heapRetained) that the runtime will try to maintain by returning memory
// to the OS.
scavengeGoal uint64
// Page reclaimer state
// reclaimIndex is the page index in allArenas of next page to
// reclaim. Specifically, it refers to page (i %
// pagesPerArena) of arena allArenas[i / pagesPerArena].
//
// If this is >= 1<<63, the page reclaimer is done scanning
// the page marks.
//
// This is accessed atomically.
reclaimIndex uint64
// reclaimCredit is spare credit for extra pages swept. Since
// the page reclaimer works in large chunks, it may reclaim
// more than requested. Any spare pages released go to this
// credit pool.
//
// This is accessed atomically.
reclaimCredit uintptr
// arenas is the heap arena map. It points to the metadata for
// the heap for every arena frame of the entire usable virtual
// address space.
//
// Use arenaIndex to compute indexes into this array.
//
// For regions of the address space that are not backed by the
// Go heap, the arena map contains nil.
//
// Modifications are protected by mheap_.lock. Reads can be
// performed without locking; however, a given entry can
// transition from nil to non-nil at any time when the lock
// isn't held. (Entries never transitions back to nil.)
//
// In general, this is a two-level mapping consisting of an L1
// map and possibly many L2 maps. This saves space when there
// are a huge number of arena frames. However, on many
// platforms (even 64-bit), arenaL1Bits is 0, making this
// effectively a single-level map. In this case, arenas[0]
// will never be nil.
arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena
// heapArenaAlloc is pre-reserved space for allocating heapArena
// objects. This is only used on 32-bit, where we pre-reserve
// this space to avoid interleaving it with the heap itself.
heapArenaAlloc linearAlloc
// arenaHints is a list of addresses at which to attempt to
// add more heap arenas. This is initially populated with a
// set of general hint addresses, and grown with the bounds of
// actual heap arena ranges.
arenaHints *arenaHint
// arena is a pre-reserved space for allocating heap arenas
// (the actual arenas). This is only used on 32-bit.
arena linearAlloc
// allArenas is the arenaIndex of every mapped arena. This can
// be used to iterate through the address space.
//
// Access is protected by mheap_.lock. However, since this is
// append-only and old backing arrays are never freed, it is
// safe to acquire mheap_.lock, copy the slice header, and
// then release mheap_.lock.
allArenas []arenaIdx
// sweepArenas is a snapshot of allArenas taken at the
// beginning of the sweep cycle. This can be read safely by
// simply blocking GC (by disabling preemption).
sweepArenas []arenaIdx
// markArenas is a snapshot of allArenas taken at the beginning
// of the mark cycle. Because allArenas is append-only, neither
// this slice nor its contents will change during the mark, so
// it can be read safely.
markArenas []arenaIdx
// curArena is the arena that the heap is currently growing
// into. This should always be physPageSize-aligned.
curArena struct {
base, end uintptr
}
_ uint32 // ensure 64-bit alignment of central
// central free lists for small size classes.
// the padding makes sure that the mcentrals are
// spaced CacheLinePadSize bytes apart, so that each mcentral.lock
// gets its own cache line.
// central is indexed by spanClass.
central [numSpanClasses]struct {
mcentral mcentral
pad [cpu.CacheLinePadSize-unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
}
spanalloc fixalloc // allocator for span*
cachealloc fixalloc // allocator for mcache*
specialfinalizeralloc fixalloc // allocator for specialfinalizer*
specialprofilealloc fixalloc // allocator for specialprofile*
specialReachableAlloc fixalloc // allocator for specialReachable
speciallock mutex // lock for special record allocators.
arenaHintAlloc fixalloc // allocator for arenaHints
unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
}
var mheap_ mheap
// A heapArena stores metadata for a heap arena. heapArenas are stored
// outside of the Go heap and accessed via the mheap_.arenas index.
//
//go:notinheap
type heapArena struct {
// bitmap stores the pointer/scalar bitmap for the words in
// this arena. See mbitmap.go for a description. Use the
// heapBits type to access this.
bitmap [heapArenaBitmapBytes]byte
// spans maps from virtual address page ID within this arena to *mspan.
// For allocated spans, their pages map to the span itself.
// For free spans, only the lowest and highest pages map to the span itself.
// Internal pages map to an arbitrary span.
// For pages that have never been allocated, spans entries are nil.
//
// Modifications are protected by mheap.lock. Reads can be
// performed without locking, but ONLY from indexes that are
// known to contain in-use or stack spans. This means there
// must not be a safe-point between establishing that an
// address is live and looking it up in the spans array.
spans [pagesPerArena]*mspan
// pageInUse is a bitmap that indicates which spans are in
// state mSpanInUse. This bitmap is indexed by page number,
// but only the bit corresponding to the first page in each
// span is used.
//
// Reads and writes are atomic.
pageInUse [pagesPerArena / 8]uint8
// pageMarks is a bitmap that indicates which spans have any
// marked objects on them. Like pageInUse, only the bit
// corresponding to the first page in each span is used.
//
// Writes are done atomically during marking. Reads are
// non-atomic and lock-free since they only occur during
// sweeping (and hence never race with writes).
//
// This is used to quickly find whole spans that can be freed.
//
// TODO(austin): It would be nice if this was uint64 for
// faster scanning, but we don't have 64-bit atomic bit
// operations.
pageMarks [pagesPerArena / 8]uint8
// pageSpecials is a bitmap that indicates which spans have
// specials (finalizers or other). Like pageInUse, only the bit
// corresponding to the first page in each span is used.
//
// Writes are done atomically whenever a special is added to
// a span and whenever the last special is removed from a span.
// Reads are done atomically to find spans containing specials
// during marking.
pageSpecials [pagesPerArena / 8]uint8
// checkmarks stores the debug.gccheckmark state. It is only
// used if debug.gccheckmark > 0.
checkmarks *checkmarksMap
// zeroedBase marks the first byte of the first page in this
// arena which hasn't been used yet and is therefore already
// zero. zeroedBase is relative to the arena base.
// Increases monotonically until it hits heapArenaBytes.
//
// This field is sufficient to determine if an allocation
// needs to be zeroed because the page allocator follows an
// address-ordered first-fit policy.
//
// Read atomically and written with an atomic CAS.
zeroedBase uintptr
}
参考文献：
a-visual-guide-to-golang-memory-allocator-from-ground-up
a-visual-guide-to-golang-memory-allocator-from-ground-up中文版
