// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Memory allocator.
//
// This was originally based on tcmalloc, but has diverged quite a bit.
// http://goog-perftools.sourceforge.net/doc/tcmalloc.html
// The main allocator works in runs of pages.
// Small allocation sizes (up to and including 32 kB) are
// rounded to one of about 70 size classes, each of which
// has its own free set of objects of exactly that size.
// Any free page of memory can be split into a set of objects
// of one size class, which are then managed using a free bitmap.
//
// The allocator's data structures are:
//
// fixalloc: a free-list allocator for fixed-size off-heap objects,
// used to manage storage used by the allocator.
// mheap: the malloc heap, managed at page (8192-byte) granularity.
// mspan: a run of pages managed by the mheap.
// mcentral: collects all spans of a given size class.
// mcache: a per-P cache of mspans with free space.
// mstats: allocation statistics.
//
// Allocating a small object proceeds up a hierarchy of caches:
//
// 1. Round the size up to one of the small size classes
// and look in the corresponding mspan in this P's mcache.
// Scan the mspan's free bitmap to find a free slot.
// If there is a free slot, allocate it.
// This can all be done without acquiring a lock.
//
// 2. If the mspan has no free slots, obtain a new mspan
// from the mcentral's list of mspans of the required size
// class that have free space.
// Obtaining a whole span amortizes the cost of locking
// the mcentral.
//
// 3. If the mcentral's mspan list is empty, obtain a run
// of pages from the mheap to use for the mspan.
//
// 4. If the mheap is empty or has no page runs large enough,
// allocate a new group of pages (at least 1MB) from the
// operating system. Allocating a large run of pages
// amortizes the cost of talking to the operating system.
//
// Sweeping an mspan and freeing objects on it proceeds up a similar
// hierarchy:
//
// 1. If the mspan is being swept in response to allocation, it
// is returned to the mcache to satisfy the allocation.
//
// 2. Otherwise, if the mspan still has allocated objects in it,
// it is placed on the mcentral free list for the mspan's size
// class.
//
// 3. Otherwise, if all objects in the mspan are free, the mspan
// is now "idle", so it is returned to the mheap and no longer
// has a size class.
// This may coalesce it with adjacent idle mspans.
//
// 4. If an mspan remains idle for long enough, return its pages
// to the operating system.
//
// Allocating and freeing a large object uses the mheap
// directly, bypassing the mcache and mcentral.
//
// Free object slots in an mspan are zeroed only if mspan.needzero is
// false. If needzero is true, objects are zeroed as they are
// allocated. There are various benefits to delaying zeroing this way:
//
// 1. Stack frame allocation can avoid zeroing altogether.
//
// 2. It exhibits better temporal locality, since the program is
// probably about to write to the memory.
//
// 3. We don't zero pages that never get reused.
// Virtual memory layout
//
// The heap consists of a set of arenas, which are 64MB on 64-bit and
// 4MB on 32-bit (heapArenaBytes). Each arena's start address is also
// aligned to the arena size.
//
// Each arena has an associated heapArena object that stores the
// metadata for that arena: the heap bitmap for all words in the arena
// and the span map for all pages in the arena. heapArena objects are
// themselves allocated off-heap.
//
// Since arenas are aligned, the address space can be viewed as a
// series of arena frames. The arena map (mheap_.arenas) maps from
// arena frame number to *heapArena, or nil for parts of the address
// space not backed by the Go heap. The arena map is structured as a
// two-level array consisting of a "L1" arena map and many "L2" arena
// maps; however, since arenas are large, on many architectures, the
// arena map consists of a single, large L2 map.
//
// The arena map covers the entire possible address space, allowing
// the Go heap to use any part of the address space. The allocator
// attempts to keep arenas contiguous so that large spans (and hence
// large objects) can cross arenas.
package runtime
import (
"runtime/internal/atomic"
"runtime/internal/math"
"runtime/internal/sys"
"unsafe"
)
const (
debugMalloc = false
maxTinySize = _TinySize
tinySizeClass = _TinySizeClass
maxSmallSize = _MaxSmallSize
pageShift = _PageShift
pageSize = _PageSize
pageMask = _PageMask
// By construction, single page spans of the smallest object class
// have the most objects per span.
maxObjsPerSpan = pageSize / 8
concurrentSweep = _ConcurrentSweep
_PageSize = 1 << _PageShift
_PageMask = _PageSize - 1
// _64bit = 1 on 64-bit systems, 0 on 32-bit systems
_64bit = 1 << (^uintptr(0) >> 63) / 2
// Tiny allocator parameters, see "Tiny allocator" comment in malloc.go.
_TinySize = 16
_TinySizeClass = int8(2)
_FixAllocChunk = 16 << 10 // Chunk size for FixAlloc
// Per-P, per order stack segment cache size.
_StackCacheSize = 32 * 1024
// Number of orders that get caching. Order 0 is FixedStack
// and each successive order is twice as large.
// We want to cache 2KB, 4KB, 8KB, and 16KB stacks. Larger stacks
// will be allocated directly.
// Since FixedStack is different on different systems, we
// must vary NumStackOrders to keep the same maximum cached size.
// OS | FixedStack | NumStackOrders
// -----------------+------------+---------------
// linux/darwin/bsd | 2KB | 4
// windows/32 | 4KB | 3
// windows/64 | 8KB | 2
// plan9 | 4KB | 3
_NumStackOrders = 4 - sys.PtrSize/4*sys.GoosWindows - 1*sys.GoosPlan9
// heapAddrBits is the number of bits in a heap address. On
// amd64, addresses are sign-extended beyond heapAddrBits. On
// other arches, they are zero-extended.
//
// On most 64-bit platforms, we limit this to 48 bits based on a
// combination of hardware and OS limitations.
//
// amd64 hardware limits addresses to 48 bits, sign-extended
// to 64 bits. Addresses where the top 16 bits are not either
// all 0 or all 1 are "non-canonical" and invalid. Because of
// these "negative" addresses, we offset addresses by 1<<47
// (arenaBaseOffset) on amd64 before computing indexes into
// the heap arenas index. In 2017, amd64 hardware added
// support for 57 bit addresses; however, currently only Linux
// supports this extension and the kernel will never choose an
// address above 1<<47 unless mmap is called with a hint
// address above 1<<47 (which we never do).
//
// arm64 hardware (as of ARMv8) limits user addresses to 48
// bits, in the range [0, 1<<48).
//
// ppc64, mips64, and s390x support arbitrary 64 bit addresses
// in hardware. On Linux, Go leans on stricter OS limits. Based
// on Linux's processor.h, the user address space is limited as
// follows on 64-bit architectures:
//
// Architecture Name Maximum Value (exclusive)
// ---------------------------------------------------------------------
// amd64 TASK_SIZE_MAX 0x007ffffffff000 (47 bit addresses)
// arm64 TASK_SIZE_64 0x01000000000000 (48 bit addresses)
// ppc64{,le} TASK_SIZE_USER64 0x00400000000000 (46 bit addresses)
// mips64{,le} TASK_SIZE64 0x00010000000000 (40 bit addresses)
// s390x TASK_SIZE 1<<64 (64 bit addresses)
//
// These limits may increase over time, but are currently at
// most 48 bits except on s390x. On all architectures, Linux
// starts placing mmap'd regions at addresses that are
// significantly below 48 bits, so even if it's possible to
// exceed Go's 48 bit limit, it's extremely unlikely in
// practice.
//
// On aix/ppc64, the limits is increased to 1<<60 to accept addresses
// returned by mmap syscall. These are in range:
// 0x0a00000000000000 - 0x0afffffffffffff
//
// On 32-bit platforms, we accept the full 32-bit address
// space because doing so is cheap.
// mips32 only has access to the low 2GB of virtual memory, so
// we further limit it to 31 bits.
//
// WebAssembly currently has a limit of 4GB linear memory.
heapAddrBits = (_64bit*(1-sys.GoarchWasm)*(1-sys.GoosAix))*48 + (1-_64bit+sys.GoarchWasm)*(32-(sys.GoarchMips+sys.GoarchMipsle)) + 60*sys.GoosAix
// maxAlloc is the maximum size of an allocation. On 64-bit,
// it's theoretically possible to allocate 1<<heapAddrBits bytes. On
// 32-bit, however, this is one less than 1<<32 because the
// number of bytes in the address space doesn't actually fit
// in a uintptr.
maxAlloc = (1 << heapAddrBits) - (1-_64bit)*1
// The number of bits in a heap address, the size of heap
// arenas, and the L1 and L2 arena map sizes are related by
//
// (1 << addr bits) = arena size * L1 entries * L2 entries
//
// Currently, we balance these as follows:
//
// Platform Addr bits Arena size L1 entries L2 entries
// -------------- --------- ---------- ---------- -----------
// */64-bit 48 64MB 1 4M (32MB)
// aix/64-bit 60 256MB 4096 4M (32MB)
// windows/64-bit 48 4MB 64 1M (8MB)
// */32-bit 32 4MB 1 1024 (4KB)
// */mips(le) 31 4MB 1 512 (2KB)
// heapArenaBytes is the size of a heap arena. The heap
// consists of mappings of size heapArenaBytes, aligned to
// heapArenaBytes. The initial heap mapping is one arena.
//
// This is currently 64MB on 64-bit non-Windows and 4MB on
// 32-bit and on Windows. We use smaller arenas on Windows
// because all committed memory is charged to the process,
// even if it's not touched. Hence, for processes with small
// heaps, the mapped arena space needs to be commensurate.
// This is particularly important with the race detector,
// since it significantly amplifies the cost of committed
// memory.
heapArenaBytes = 1 << logHeapArenaBytes
// logHeapArenaBytes is log_2 of heapArenaBytes. For clarity,
// prefer using heapArenaBytes where possible (we need the
// constant to compute some other constants).
logHeapArenaBytes = (6+20)*(_64bit*(1-sys.GoosWindows)*(1-sys.GoosAix)*(1-sys.GoarchWasm)) + (2+20)*(_64bit*sys.GoosWindows) + (2+20)*(1-_64bit) + (8+20)*sys.GoosAix + (2+20)*sys.GoarchWasm
// heapArenaBitmapBytes is the size of each heap arena's bitmap.
heapArenaBitmapBytes = heapArenaBytes / (sys.PtrSize * 8 / 2)
pagesPerArena = heapArenaBytes / pageSize
// arenaL1Bits is the number of bits of the arena number
// covered by the first level arena map.
//
// This number should be small, since the first level arena
// map requires PtrSize*(1<<arenaL1Bits) of space in the
// binary's BSS. It can be zero, in which case the first level
// index is effectively unused. There is a performance benefit
// to this, since the generated code can be more efficient,
// but comes at the cost of having a large L2 mapping.
//
// We use the L1 map on 64-bit Windows because the arena size
// is small, but the address space is still 48 bits, and
// there's a high cost to having a large L2.
//
// We use the L1 map on aix/ppc64 to keep the same L2 value
// as on Linux.
arenaL1Bits = 6*(_64bit*sys.GoosWindows) + 12*sys.GoosAix
// arenaL2Bits is the number of bits of the arena number
// covered by the second level arena index.
//
// The size of each arena map allocation is proportional to
// 1<<arenaL2Bits, so it's important that this not be too
// large. 48 bits leads to 32MB arena index allocations, which
// is about the practical threshold.
arenaL2Bits = heapAddrBits - logHeapArenaBytes - arenaL1Bits
// arenaL1Shift is the number of bits to shift an arena frame
// number by to compute an index into the first level arena map.
arenaL1Shift = arenaL2Bits
// arenaBits is the total bits in a combined arena map index.
// This is split between the index into the L1 arena map and
// the L2 arena map.
arenaBits = arenaL1Bits + arenaL2Bits
// arenaBaseOffset is the pointer value that corresponds to
// index 0 in the heap arena map.
//
// On amd64, the address space is 48 bits, sign extended to 64
// bits. This offset lets us handle "negative" addresses (or
// high addresses if viewed as unsigned).
//
// On other platforms, the user address space is contiguous
// and starts at 0, so no offset is necessary.
arenaBaseOffset uintptr = sys.GoarchAmd64 * (1 << 47)
// Max number of threads to run garbage collection.
// 2, 3, and 4 are all plausible maximums depending
// on the hardware details of the machine. The garbage
// collector scales well to 32 cpus.
_MaxGcproc = 32
// minLegalPointer is the smallest possible legal pointer.
// This is the smallest possible architectural page size,
// since we assume that the first page is never mapped.
//
// This should agree with minZeroPage in the compiler.
minLegalPointer uintptr = 4096
)
// physPageSize is the size in bytes of the OS's physical pages.
// Mapping and unmapping operations must be done at multiples of
// physPageSize.
//
// This must be set by the OS init code (typically in osinit) before
// mallocinit.
var physPageSize uintptr
// physHugePageSize is the size in bytes of the OS's default physical huge
// page size whose allocation is opaque to the application. It is assumed
// and verified to be a power of two.
//
// If set, this must be set by the OS init code (typically in osinit) before
// mallocinit. However, setting it at all is optional, and leaving the default
// value is always safe (though potentially less efficient).
//
// Since physHugePageSize is always assumed to be a power of two,
// physHugePageShift is defined as physHugePageSize == 1 << physHugePageShift.
// The purpose of physHugePageShift is to avoid doing divisions in
// performance critical functions.
var (
physHugePageSize uintptr
physHugePageShift uint
)
// OS memory management abstraction layer
//
// Regions of the address space managed by the runtime may be in one of four
// states at any given time:
// 1) None - Unreserved and unmapped, the default state of any region.
// 2) Reserved - Owned by the runtime, but accessing it would cause a fault.
// Does not count against the process' memory footprint.
// 3) Prepared - Reserved, intended not to be backed by physical memory (though
// an OS may implement this lazily). Can transition efficiently to
// Ready. Accessing memory in such a region is undefined (may
// fault, may give back unexpected zeroes, etc.).
// 4) Ready - may be accessed safely.
//
// This set of states is more than is strictly necessary to support all the
// currently supported platforms. One could get by with just None, Reserved, and
// Ready. However, the Prepared state gives us flexibility for performance
// purposes. For example, on POSIX-y operating systems, Reserved is usually a
// private anonymous mmap'd region with PROT_NONE set, and to transition
// to Ready would require setting PROT_READ|PROT_WRITE. However the
// underspecification of Prepared lets us use just MADV_FREE to transition from
// Ready to Prepared. Thus with the Prepared state we can set the permission
// bits just once early on, we can efficiently tell the OS that it's free to
// take pages away from us when we don't strictly need them.
//
// For each OS there is a common set of helpers defined that transition
// memory regions between these states. The helpers are as follows:
//
// sysAlloc transitions an OS-chosen region of memory from None to Ready.
// More specifically, it obtains a large chunk of zeroed memory from the
// operating system, typically on the order of a hundred kilobytes
// or a megabyte. This memory is always immediately available for use.
//
// sysFree transitions a memory region from any state to None. Therefore, it
// returns memory unconditionally. It is used if an out-of-memory error has been
// detected midway through an allocation or to carve out an aligned section of
// the address space. It is okay if sysFree is a no-op only if sysReserve always
// returns a memory region aligned to the heap allocator's alignment
// restrictions.
//
// sysReserve transitions a memory region from None to Reserved. It reserves
// address space in such a way that it would cause a fatal fault upon access
// (either via permissions or not committing the memory). Such a reservation is
// thus never backed by physical memory.
// If the pointer passed to it is non-nil, the caller wants the
// reservation there, but sysReserve can still choose another
// location if that one is unavailable.
// NOTE: sysReserve returns OS-aligned memory, but the heap allocator
// may use larger alignment, so the caller must be careful to realign the
// memory obtained by sysReserve.
//
// sysMap transitions a memory region from Reserved to Prepared. It ensures the
// memory region can be efficiently transitioned to Ready.
//
// sysUsed transitions a memory region from Prepared to Ready. It notifies the
// operating system that the memory region is needed and ensures that the region
// may be safely accessed. This is typically a no-op on systems that don't have
// an explicit commit step and hard over-commit limits, but is critical on
// Windows, for example.
//
// sysUnused transitions a memory region from Ready to Prepared. It notifies the
// operating system that the physical pages backing this memory region are no
// longer needed and can be reused for other purposes. The contents of a
// sysUnused memory region are considered forfeit and the region must not be
// accessed again until sysUsed is called.
//
// sysFault transitions a memory region from Ready or Prepared to Reserved. It
// marks a region such that it will always fault if accessed. Used only for
// debugging the runtime.
func mallocinit() {
if class_to_size[_TinySizeClass] != _TinySize {
throw("bad TinySizeClass")
}
testdefersizes()
if heapArenaBitmapBytes&(heapArenaBitmapBytes-1) != 0 {
// heapBits expects modular arithmetic on bitmap
// addresses to work.
throw("heapArenaBitmapBytes not a power of 2")
}
// Copy class sizes out for statistics table.
for i := range class_to_size {
memstats.by_size[i].size = uint32(class_to_size[i])
}
// Check physPageSize.
if physPageSize == 0 {
// The OS init code failed to fetch the physical page size.
throw("failed to get system page size")
}
if physPageSize < minPhysPageSize {
print("system page size (", physPageSize, ") is smaller than minimum page size (", minPhysPageSize, ")\n")
throw("bad system page size")
}
if physPageSize&(physPageSize-1) != 0 {
print("system page size (", physPageSize, ") must be a power of 2\n")
throw("bad system page size")
}
if physHugePageSize&(physHugePageSize-1) != 0 {
print("system huge page size (", physHugePageSize, ") must be a power of 2\n")
throw("bad system huge page size")
}
if physHugePageSize != 0 {
// Since physHugePageSize is a power of 2, it suffices to increase
// physHugePageShift until 1<<physHugePageShift == physHugePageSize.
for 1<<physHugePageShift != physHugePageSize {
physHugePageShift++
}
}
// Initialize the heap.
mheap_.init()
_g_ := getg()
_g_.m.mcache = allocmcache()
// Create initial arena growth hints.
if sys.PtrSize == 8 {
// On a 64-bit machine, we pick the following hints
// because:
//
// 1. Starting from the middle of the address space
// makes it easier to grow out a contiguous range
// without running in to some other mapping.
//
// 2. This makes Go heap addresses more easily
// recognizable when debugging.
//
// 3. Stack scanning in gccgo is still conservative,
// so it's important that addresses be distinguishable
// from other data.
//
// Starting at 0x00c0 means that the valid memory addresses
// will begin 0x00c0, 0x00c1, ...
// In little-endian, that's c0 00, c1 00, ... None of those are valid
// UTF-8 sequences, and they are otherwise as far away from
// ff (likely a common byte) as possible. If that fails, we try other 0xXXc0
// addresses. An earlier attempt to use 0x11f8 caused out of memory errors
// on OS X during thread allocations. 0x00c0 causes conflicts with
// AddressSanitizer which reserves all memory up to 0x0100.
// These choices reduce the odds of a conservative garbage collector
// not collecting memory because some non-pointer block of memory
// had a bit pattern that matched a memory address.
//
// However, on arm64, we ignore all this advice above and slam the
// allocation at 0x40 << 32 because when using 4k pages with 3-level
// translation buffers, the user address space is limited to 39 bits
// On darwin/arm64, the address space is even smaller.
// On AIX, mmaps starts at 0x0A00000000000000 for 64-bit.
// processes.
for i := 0x7f; i >= 0; i-- {
var p uintptr
switch {
case GOARCH == "arm64" && GOOS == "darwin":
p = uintptr(i)<<40 | uintptrMask&(0x0013<<28)
case GOARCH == "arm64":
p = uintptr(i)<<40 | uintptrMask&(0x0040<<32)
case GOOS == "aix":
if i == 0 {
// We don't use addresses directly after 0x0A00000000000000
// to avoid collisions with others mmaps done by non-go programs.
continue
}
p = uintptr(i)<<40 | uintptrMask&(0xa0<<52)
case raceenabled:
// The TSAN runtime requires the heap
// to be in the range [0x00c000000000,
// 0x00e000000000).
p = uintptr(i)<<32 | uintptrMask&(0x00c0<<32)
if p >= uintptrMask&0x00e000000000 {
continue
}
default:
p = uintptr(i)<<40 | uintptrMask&(0x00c0<<32)
}
hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
hint.addr = p
hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
}
} else {
// On a 32-bit machine, we're much more concerned
// about keeping the usable heap contiguous.
// Hence:
//
// 1. We reserve space for all heapArenas up front so
// they don't get interleaved with the heap. They're
// ~258MB, so this isn't too bad. (We could reserve a
// smaller amount of space up front if this is a
// problem.)
//
// 2. We hint the heap to start right above the end of
// the binary so we have the best chance of keeping it
// contiguous.
//
// 3. We try to stake out a reasonably large initial
// heap reservation.
const arenaMetaSize = (1 << arenaBits) * unsafe.Sizeof(heapArena{})
meta := uintptr(sysReserve(nil, arenaMetaSize))
if meta != 0 {
mheap_.heapArenaAlloc.init(meta, arenaMetaSize)
}
// We want to start the arena low, but if we're linked
// against C code, it's possible global constructors
// have called malloc and adjusted the process' brk.
// Query the brk so we can avoid trying to map the
// region over it (which will cause the kernel to put
// the region somewhere else, likely at a high
// address).
procBrk := sbrk0()
// If we ask for the end of the data segment but the
// operating system requires a little more space
// before we can start allocating, it will give out a
// slightly higher pointer. Except QEMU, which is
// buggy, as usual: it won't adjust the pointer
// upward. So adjust it upward a little bit ourselves:
// 1/4 MB to get away from the running binary image.
p := firstmoduledata.end
if p < procBrk {
p = procBrk
}
if mheap_.heapArenaAlloc.next <= p && p < mheap_.heapArenaAlloc.end {
p = mheap_.heapArenaAlloc.end
}
p = round(p+(256<<10), heapArenaBytes)
// Because we're worried about fragmentation on
// 32-bit, we try to make a large initial reservation.
arenaSizes := []uintptr{
512 << 20,
256 << 20,
128 << 20,
}
for _, arenaSize := range arenaSizes {
a, size := sysReserveAligned(unsafe.Pointer(p), arenaSize, heapArenaBytes)
if a != nil {
mheap_.arena.init(uintptr(a), size)
p = uintptr(a) + size // For hint below
break
}
}
hint := (*arenaHint)(mheap_.arenaHintAlloc.alloc())
hint.addr = p
hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
}
}
// sysAlloc allocates heap arena space for at least n bytes. The
// returned pointer is always heapArenaBytes-aligned and backed by
// h.arenas metadata. The returned size is always a multiple of
// heapArenaBytes. sysAlloc returns nil on failure.
// There is no corresponding free function.
//
// sysAlloc returns a memory region in the Prepared state. This region must
// be transitioned to Ready before use.
//
// h must be locked.
func (h *mheap) sysAlloc(n uintptr) (v unsafe.Pointer, size uintptr) {
n = round(n, heapArenaBytes)
// First, try the arena pre-reservation.
v = h.arena.alloc(n, heapArenaBytes, &memstats.heap_sys)
if v != nil {
size = n
goto mapped
}
// Try to grow the heap at a hint address.
for h.arenaHints != nil {
hint := h.arenaHints
p := hint.addr
if hint.down {
p -= n
}
if p+n < p {
// We can't use this, so don't ask.
v = nil
} else if arenaIndex(p+n-1) >= 1<<arenaBits {
// Outside addressable heap. Can't use.
v = nil
} else {
v = sysReserve(unsafe.Pointer(p), n)
}
if p == uintptr(v) {
// Success. Update the hint.
if !hint.down {
p += n
}
hint.addr = p
size = n
break
}
// Failed. Discard this hint and try the next.
//
// TODO: This would be cleaner if sysReserve could be
// told to only return the requested address. In
// particular, this is already how Windows behaves, so
// it would simplify things there.
if v != nil {
sysFree(v, n, nil)
}
h.arenaHints = hint.next
h.arenaHintAlloc.free(unsafe.Pointer(hint))
}
if size == 0 {
if raceenabled {
// The race detector assumes the heap lives in
// [0x00c000000000, 0x00e000000000), but we
// just ran out of hints in this region. Give
// a nice failure.
throw("too many address space collisions for -race mode")
}
// All of the hints failed, so we'll take any
// (sufficiently aligned) address the kernel will give
// us.
v, size = sysReserveAligned(nil, n, heapArenaBytes)
if v == nil {
return nil, 0
}
// Create new hints for extending this region.
hint := (*arenaHint)(h.arenaHintAlloc.alloc())
hint.addr, hint.down = uintptr(v), true
hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
hint = (*arenaHint)(h.arenaHintAlloc.alloc())
hint.addr = uintptr(v) + size
hint.next, mheap_.arenaHints = mheap_.arenaHints, hint
}
// Check for bad pointers or pointers we can't use.
{
var bad string
p := uintptr(v)
if p+size < p {
bad = "region exceeds uintptr range"
} else if arenaIndex(p) >= 1<<arenaBits {
bad = "base outside usable address space"
} else if arenaIndex(p+size-1) >= 1<<arenaBits {
bad = "end outside usable address space"
}
if bad != "" {
// This should be impossible on most architectures,
// but it would be really confusing to debug.
print("runtime: memory allocated by OS [", hex(p), ", ", hex(p+size), ") not in usable address space: ", bad, "\n")
throw("memory reservation exceeds address space limit")
}
}
if uintptr(v)&(heapArenaBytes-1) != 0 {
throw("misrounded allocation in sysAlloc")
}
// Transition from Reserved to Prepared.
sysMap(v, size, &memstats.heap_sys)
mapped:
// Create arena metadata.
for ri := arenaIndex(uintptr(v)); ri <= arenaIndex(uintptr(v)+size-1); ri++ {
l2 := h.arenas[ri.l1()]
if l2 == nil {
// Allocate an L2 arena map.
l2 = (*[1 << arenaL2Bits]*heapArena)(persistentalloc(unsafe.Sizeof(*l2), sys.PtrSize, nil))
if l2 == nil {
throw("out of memory allocating heap arena map")
}
atomic.StorepNoWB(unsafe.Pointer(&h.arenas[ri.l1()]), unsafe.Pointer(l2))
}
if l2[ri.l2()] != nil {
throw("arena already initialized")
}
var r *heapArena
r = (*heapArena)(h.heapArenaAlloc.alloc(unsafe.Sizeof(*r), sys.PtrSize, &memstats.gc_sys))
if r == nil {
r = (*heapArena)(persistentalloc(unsafe.Sizeof(*r), sys.PtrSize, &memstats.gc_sys))
if r == nil {
throw("out of memory allocating heap arena metadata")
}
}
// Add the arena to the arenas list.
if len(h.allArenas) == cap(h.allArenas) {
size := 2 * uintptr(cap(h.allArenas)) * sys.PtrSize
if size == 0 {
size = physPageSize
}
newArray := (*notInHeap)(persistentalloc(size, sys.PtrSize, &memstats.gc_sys))
if newArray == nil {
throw("out of memory allocating allArenas")
}
oldSlice := h.allArenas
*(*notInHeapSlice)(unsafe.Pointer(&h.allArenas)) = notInHeapSlice{newArray, len(h.allArenas), int(size / sys.PtrSize)}
copy(h.allArenas, oldSlice)
// Do not free the old backing array because
// there may be concurrent readers. Since we
// double the array each time, this can lead
// to at most 2x waste.
}
h.allArenas = h.allArenas[:len(h.allArenas)+1]
h.allArenas[len(h.allArenas)-1] = ri
// Store atomically just in case an object from the
// new heap arena becomes visible before the heap lock
// is released (which shouldn't happen, but there's
// little downside to this).
atomic.StorepNoWB(unsafe.Pointer(&l2[ri.l2()]), unsafe.Pointer(r))
}
// Tell the race detector about the new heap memory.
if raceenabled {
racemapshadow(v, size)
}
return
}
// sysReserveAligned is like sysReserve, but the returned pointer is
// aligned to align bytes. It may reserve either n or n+align bytes,
// so it returns the size that was reserved.
func sysReserveAligned(v unsafe.Pointer, size, align uintptr) (unsafe.Pointer, uintptr) {
// Since the alignment is rather large in uses of this
// function, we're not likely to get it by chance, so we ask
// for a larger region and remove the parts we don't need.
retries := 0
retry:
p := uintptr(sysReserve(v, size+align))
switch {
case p == 0:
return nil, 0
case p&(align-1) == 0:
// We got lucky and got an aligned region, so we can
// use the whole thing.
return unsafe.Pointer(p), size + align
case GOOS == "windows":
// On Windows we can't release pieces of a
// reservation, so we release the whole thing and
// re-reserve the aligned sub-region. This may race,
// so we may have to try again.
sysFree(unsafe.Pointer(p), size+align, nil)
p = round(p, align)
p2 := sysReserve(unsafe.Pointer(p), size)
if p != uintptr(p2) {
// Must have raced. Try again.
sysFree(p2, size, nil)
if retries++; retries == 100 {
throw("failed to allocate aligned heap memory; too many retries")
}
goto retry
}
// Success.
return p2, size
default:
// Trim off the unaligned parts.
pAligned := round(p, align)
sysFree(unsafe.Pointer(p), pAligned-p, nil)
end := pAligned + size
endLen := (p + size + align) - end
if endLen > 0 {
sysFree(unsafe.Pointer(end), endLen, nil)
}
return unsafe.Pointer(pAligned), size
}
}
// base address for all 0-byte allocations
var zerobase uintptr
// nextFreeFast returns the next free object if one is quickly available.
// Otherwise it returns 0.
func nextFreeFast(s *mspan) gclinkptr {
theBit := sys.Ctz64(s.allocCache) // Is there a free object in the allocCache?
if theBit < 64 {
result := s.freeindex + uintptr(theBit)
if result < s.nelems {
freeidx := result + 1
if freeidx%64 == 0 && freeidx != s.nelems {
return 0
}
s.allocCache >>= uint(theBit + 1)
s.freeindex = freeidx
s.allocCount++
return gclinkptr(result*s.elemsize + s.base())
}
}
return 0
}
// nextFree returns the next free object from the cached span if one is available.
// Otherwise it refills the cache with a span with an available object and
// returns that object along with a flag indicating that this was a heavy
// weight allocation. If it is a heavy weight allocation the caller must
// determine whether a new GC cycle needs to be started or if the GC is active
// whether this goroutine needs to assist the GC.
//
// Must run in a non-preemptible context since otherwise the owner of
// c could change.
func (c *mcache) nextFree(spc spanClass) (v gclinkptr, s *mspan, shouldhelpgc bool) {
s = c.alloc[spc]
shouldhelpgc = false
freeIndex := s.nextFreeIndex()
if freeIndex == s.nelems {
// The span is full.
if uintptr(s.allocCount) != s.nelems {
println("runtime: s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
throw("s.allocCount != s.nelems && freeIndex == s.nelems")
}
c.refill(spc)
shouldhelpgc = true
s = c.alloc[spc]
freeIndex = s.nextFreeIndex()
}
if freeIndex >= s.nelems {
throw("freeIndex is not valid")
}
v = gclinkptr(freeIndex*s.elemsize + s.base())
s.allocCount++
if uintptr(s.allocCount) > s.nelems {
println("s.allocCount=", s.allocCount, "s.nelems=", s.nelems)
throw("s.allocCount > s.nelems")
}
return
}
// Allocate an object of size bytes.
// Small objects are allocated from the per-P cache's free lists.
// Large objects (> 32 kB) are allocated straight from the heap.
func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
if gcphase == _GCmarktermination {
throw("mallocgc called with gcphase == _GCmarktermination")
}
if size == 0 {
return unsafe.Pointer(&zerobase)
}
if debug.sbrk != 0 {
align := uintptr(16)
if typ != nil {
// TODO(austin): This should be just
// align = uintptr(typ.align)
// but that's only 4 on 32-bit platforms,
// even if there's a uint64 field in typ (see #599).
// This causes 64-bit atomic accesses to panic.
// Hence, we use stricter alignment that matches
// the normal allocator better.
if size&7 == 0 {
align = 8
} else if size&3 == 0 {
align = 4
} else if size&1 == 0 {
align = 2
} else {
align = 1
}
}
return persistentalloc(size, align, &memstats.other_sys)
}
// assistG is the G to charge for this allocation, or nil if
// GC is not currently active.
var assistG *g
if gcBlackenEnabled != 0 {
// Charge the current user G for this allocation.
assistG = getg()
if assistG.m.curg != nil {
assistG = assistG.m.curg
}
// Charge the allocation against the G. We'll account
// for internal fragmentation at the end of mallocgc.
assistG.gcAssistBytes -= int64(size)
if assistG.gcAssistBytes < 0 {
// This G is in debt. Assist the GC to correct
// this before allocating. This must happen
// before disabling preemption.
gcAssistAlloc(assistG)
}
}
// Set mp.mallocing to keep from being preempted by GC.
mp := acquirem()
if mp.mallocing != 0 {
throw("malloc deadlock")
}
if mp.gsignal == getg() {
throw("malloc during signal")
}
mp.mallocing = 1
shouldhelpgc := false
dataSize := size
c := gomcache()
var x unsafe.Pointer
noscan := typ == nil || typ.ptrdata == 0
if size <= maxSmallSize {
if noscan && size < maxTinySize {
// Tiny allocator.
//
// Tiny allocator combines several tiny allocation requests
// into a single memory block. The resulting memory block
// is freed when all subobjects are unreachable. The subobjects
// must be noscan (don't have pointers), this ensures that
// the amount of potentially wasted memory is bounded.
//
// Size of the memory block used for combining (maxTinySize) is tunable.
// Current setting is 16 bytes, which relates to 2x worst case memory
// wastage (when all but one subobjects are unreachable).
// 8 bytes would result in no wastage at all, but provides less
// opportunities for combining.
// 32 bytes provides more opportunities for combining,
// but can lead to 4x worst case wastage.
// The best case winning is 8x regardless of block size.
//
// Objects obtained from tiny allocator must not be freed explicitly.
// So when an object will be freed explicitly, we ensure that
// its size >= maxTinySize.
//
// SetFinalizer has a special case for objects potentially coming
// from tiny allocator, it such case it allows to set finalizers
// for an inner byte of a memory block.
//
// The main targets of tiny allocator are small strings and
// standalone escaping variables. On a json benchmark
// the allocator reduces number of allocations by ~12% and
// reduces heap size by ~20%.
off := c.tinyoffset
// Align tiny pointer for required (conservative) alignment.
if size&7 == 0 {
off = round(off, 8)
} else if size&3 == 0 {
off = round(off, 4)
} else if size&1 == 0 {
off = round(off, 2)
}
if off+size <= maxTinySize && c.tiny != 0 {
// The object fits into existing tiny block.
x = unsafe.Pointer(c.tiny + off)
c.tinyoffset = off + size
c.local_tinyallocs++
mp.mallocing = 0
releasem(mp)
return x
}
// Allocate a new maxTinySize block.
span := c.alloc[tinySpanClass]
v := nextFreeFast(span)
if v == 0 {
v, _, shouldhelpgc = c.nextFree(tinySpanClass)
}
x = unsafe.Pointer(v)
(*[2]uint64)(x)[0] = 0
(*[2]uint64)(x)[1] = 0
// See if we need to replace the existing tiny block with the new one
// based on amount of remaining free space.
if size < c.tinyoffset || c.tiny == 0 {
c.tiny = uintptr(x)
c.tinyoffset = size
}
size = maxTinySize
} else {
var sizeclass uint8
if size <= smallSizeMax-8 {
sizeclass = size_to_class8[(size+smallSizeDiv-1)/smallSizeDiv]
} else {
sizeclass = size_to_class128[(size-smallSizeMax+largeSizeDiv-1)/largeSizeDiv]
}
size = uintptr(class_to_size[sizeclass])
spc := makeSpanClass(sizeclass, noscan)
span := c.alloc[spc]
v := nextFreeFast(span)
if v == 0 {
v, span, shouldhelpgc = c.nextFree(spc)
}
x = unsafe.Pointer(v)
if needzero && span.needzero != 0 {
memclrNoHeapPointers(unsafe.Pointer(v), size)
}
}
} else {
var s *mspan
shouldhelpgc = true
systemstack(func() {
s = largeAlloc(size, needzero, noscan)
})
s.freeindex = 1
s.allocCount = 1
x = unsafe.Pointer(s.base())
size = s.elemsize
}
var scanSize uintptr
if !noscan {
// If allocating a defer+arg block, now that we've picked a malloc size
// large enough to hold everything, cut the "asked for" size down to
// just the defer header, so that the GC bitmap will record the arg block
// as containing nothing at all (as if it were unused space at the end of
// a malloc block caused by size rounding).
// The defer arg areas are scanned as part of scanstack.
if typ == deferType {
dataSize = unsafe.Sizeof(_defer{})
}
heapBitsSetType(uintptr(x), size, dataSize, typ)
if dataSize > typ.size {
// Array allocation. If there are any
// pointers, GC has to scan to the last
// element.
if typ.ptrdata != 0 {
scanSize = dataSize - typ.size + typ.ptrdata
}
} else {
scanSize = typ.ptrdata
}
c.local_scan += scanSize
}
// Ensure that the stores above that initialize x to
// type-safe memory and set the heap bits occur before
// the caller can make x observable to the garbage
// collector. Otherwise, on weakly ordered machines,
// the garbage collector could follow a pointer to x,
// but see uninitialized memory or stale heap bits.
publicationBarrier()
// Allocate black during GC.
// All slots hold nil so no scanning is needed.
// This may be racing with GC so do it atomically if there can be
// a race marking the bit.
if gcphase != _GCoff {
gcmarknewobject(uintptr(x), size, scanSize)
}
if raceenabled {
racemalloc(x, size)
}
if msanenabled {
msanmalloc(x, size)
}
mp.mallocing = 0
releasem(mp)
if debug.allocfreetrace != 0 {
tracealloc(x, size, typ)
}
if rate := MemProfileRate; rate > 0 {
if rate != 1 && size < c.next_sample {
c.next_sample -= size
} else {
mp := acquirem()
profilealloc(mp, x, size)
releasem(mp)
}
}
if assistG != nil {
// Account for internal fragmentation in the assist
// debt now that we know it.
assistG.gcAssistBytes -= int64(size - dataSize)
}
if shouldhelpgc {
if t := (gcTrigger{kind: gcTriggerHeap}); t.test() {
gcStart(t)
}
}
return x
}
func largeAlloc(size uintptr, needzero bool, noscan bool) *mspan {
// print("largeAlloc size=", size, "\n")
if size+_PageSize < size {
throw("out of memory")
}
npages := size >> _PageShift
if size&_PageMask != 0 {
npages++
}
// Deduct credit for this span allocation and sweep if
// necessary. mHeap_Alloc will also sweep npages, so this only
// pays the debt down to npage pages.
deductSweepCredit(npages*_PageSize, npages)
s := mheap_.alloc(npages, makeSpanClass(0, noscan), true, needzero)
if s == nil {
throw("out of memory")
}
s.limit = s.base() + size
heapBitsForAddr(s.base()).initSpan(s)
return s
}
// implementation of new builtin
// compiler (both frontend and SSA backend) knows the signature
// of this function
func newobject(typ *_type) unsafe.Pointer {
return mallocgc(typ.size, typ, true)
}
//go:linkname reflect_unsafe_New reflect.unsafe_New
func reflect_unsafe_New(typ *_type) unsafe.Pointer {
return mallocgc(typ.size, typ, true)
}
//go:linkname reflectlite_unsafe_New internal/reflectlite.unsafe_New
func reflectlite_unsafe_New(typ *_type) unsafe.Pointer {
return mallocgc(typ.size, typ, true)
}
// newarray allocates an array of n elements of type typ.
func newarray(typ *_type, n int) unsafe.Pointer {
if n == 1 {
return mallocgc(typ.size, typ, true)
}
mem, overflow := math.MulUintptr(typ.size, uintptr(n))
if overflow || mem > maxAlloc || n < 0 {
panic(plainError("runtime: allocation size out of range"))
}
return mallocgc(mem, typ, true)
}
//go:linkname reflect_unsafe_NewArray reflect.unsafe_NewArray
func reflect_unsafe_NewArray(typ *_type, n int) unsafe.Pointer {
return newarray(typ, n)
}
func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
mp.mcache.next_sample = nextSample()
mProf_Malloc(x, size)
}
// nextSample returns the next sampling point for heap profiling. The goal is
// to sample allocations on average every MemProfileRate bytes, but with a
// completely random distribution over the allocation timeline; this
// corresponds to a Poisson process with parameter MemProfileRate. In Poisson
// processes, the distance between two samples follows the exponential
// distribution (exp(MemProfileRate)), so the best return value is a random
// number taken from an exponential distribution whose mean is MemProfileRate.
func nextSample() uintptr {
if GOOS == "plan9" {
// Plan 9 doesn't support floating point in note handler.
if g := getg(); g == g.m.gsignal {
return nextSampleNoFP()
}
}
return uintptr(fastexprand(MemProfileRate))
}
// fastexprand returns a random number from an exponential distribution with
// the specified mean.
func fastexprand(mean int) int32 {
// Avoid overflow. Maximum possible step is
// -ln(1/(1<<randomBitCount)) * mean, approximately 20 * mean.
switch {
case mean > 0x7000000:
mean = 0x7000000
case mean == 0:
return 0
}
// Take a random sample of the exponential distribution exp(-mean*x).
// The probability distribution function is mean*exp(-mean*x), so the CDF is
// p = 1 - exp(-mean*x), so
// q = 1 - p == exp(-mean*x)
// log_e(q) = -mean*x
// -log_e(q)/mean = x
// x = -log_e(q) * mean
// x = log_2(q) * (-log_e(2)) * mean ; Using log_2 for efficiency
const randomBitCount = 26
q := fastrand()%(1<<randomBitCount) + 1
qlog := fastlog2(float64(q)) - randomBitCount
if qlog > 0 {
qlog = 0
}
const minusLog2 = -0.6931471805599453 // -ln(2)
return int32(qlog*(minusLog2*float64(mean))) + 1
}
// nextSampleNoFP is similar to nextSample, but uses older,
// simpler code to avoid floating point.
func nextSampleNoFP() uintptr {
// Set first allocation sample size.
rate := MemProfileRate
if rate > 0x3fffffff { // make 2*rate not overflow
rate = 0x3fffffff
}
if rate != 0 {
return uintptr(fastrand() % uint32(2*rate))
}
return 0
}
type persistentAlloc struct {
base *notInHeap
off uintptr
}
var globalAlloc struct {
mutex
persistentAlloc
}
// persistentChunkSize is the number of bytes we allocate when we grow
// a persistentAlloc.
const persistentChunkSize = 256 << 10
// persistentChunks is a list of all the persistent chunks we have
// allocated. The list is maintained through the first word in the
// persistent chunk. This is updated atomically.
var persistentChunks *notInHeap
// Wrapper around sysAlloc that can allocate small chunks.
// There is no associated free operation.
// Intended for things like function/type/debug-related persistent data.
// If align is 0, uses default align (currently 8).
// The returned memory will be zeroed.
//
// Consider marking persistentalloc'd types go:notinheap.
func persistentalloc(size, align uintptr, sysStat *uint64) unsafe.Pointer {
var p *notInHeap
systemstack(func() {
p = persistentalloc1(size, align, sysStat)
})
return unsafe.Pointer(p)
}
// Must run on system stack because stack growth can (re)invoke it.
// See issue 9174.
//go:systemstack
func persistentalloc1(size, align uintptr, sysStat *uint64) *notInHeap {
const (
maxBlock = 64 << 10 // VM reservation granularity is 64K on windows
)
if size == 0 {
throw("persistentalloc: size == 0")
}
if align != 0 {
if align&(align-1) != 0 {
throw("persistentalloc: align is not a power of 2")
}
if align > _PageSize {
throw("persistentalloc: align is too large")
}
} else {
align = 8
}
if size >= maxBlock {
return (*notInHeap)(sysAlloc(size, sysStat))
}
mp := acquirem()
var persistent *persistentAlloc
if mp != nil && mp.p != 0 {
persistent = &mp.p.ptr().palloc
} else {
lock(&globalAlloc.mutex)
persistent = &globalAlloc.persistentAlloc
}
persistent.off = round(persistent.off, align)
if persistent.off+size > persistentChunkSize || persistent.base == nil {
persistent.base = (*notInHeap)(sysAlloc(persistentChunkSize, &memstats.other_sys))
if persistent.base == nil {
if persistent == &globalAlloc.persistentAlloc {
unlock(&globalAlloc.mutex)
}
throw("runtime: cannot allocate memory")
}
// Add the new chunk to the persistentChunks list.
for {
chunks := uintptr(unsafe.Pointer(persistentChunks))
*(*uintptr)(unsafe.Pointer(persistent.base)) = chunks
if atomic.Casuintptr((*uintptr)(unsafe.Pointer(&persistentChunks)), chunks, uintptr(unsafe.Pointer(persistent.base))) {
break
}
}
persistent.off = round(sys.PtrSize, align)
}
p := persistent.base.add(persistent.off)
persistent.off += size
releasem(mp)
if persistent == &globalAlloc.persistentAlloc {
unlock(&globalAlloc.mutex)
}
if sysStat != &memstats.other_sys {
mSysStatInc(sysStat, size)
mSysStatDec(&memstats.other_sys, size)
}
return p
}
// inPersistentAlloc reports whether p points to memory allocated by
// persistentalloc. This must be nosplit because it is called by the
// cgo checker code, which is called by the write barrier code.
//go:nosplit
func inPersistentAlloc(p uintptr) bool {
chunk := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&persistentChunks)))
for chunk != 0 {
if p >= chunk && p < chunk+persistentChunkSize {
return true
}
chunk = *(*uintptr)(unsafe.Pointer(chunk))
}
return false
}
// linearAlloc is a simple linear allocator that pre-reserves a region
// of memory and then maps that region into the Ready state as needed. The
// caller is responsible for locking.
type linearAlloc struct {
next uintptr // next free byte
mapped uintptr // one byte past end of mapped space
end uintptr // end of reserved space
}
func (l *linearAlloc) init(base, size uintptr) {
l.next, l.mapped = base, base
l.end = base + size
}
func (l *linearAlloc) alloc(size, align uintptr, sysStat *uint64) unsafe.Pointer {
p := round(l.next, align)
if p+size > l.end {
return nil
}
l.next = p + size
if pEnd := round(l.next-1, physPageSize); pEnd > l.mapped {
// Transition from Reserved to Prepared to Ready.
sysMap(unsafe.Pointer(l.mapped), pEnd-l.mapped, sysStat)
sysUsed(unsafe.Pointer(l.mapped), pEnd-l.mapped)
l.mapped = pEnd
}
return unsafe.Pointer(p)
}
// notInHeap is off-heap memory allocated by a lower-level allocator
// like sysAlloc or persistentAlloc.
//
// In general, it's better to use real types marked as go:notinheap,
// but this serves as a generic type for situations where that isn't
// possible (like in the allocators).
//
// TODO: Use this as the return type of sysAlloc, persistentAlloc, etc?
//
//go:notinheap
type notInHeap struct{}
func (p *notInHeap) add(bytes uintptr) *notInHeap {
return (*notInHeap)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + bytes))
}
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