go / gofrontend / af683486b4de5503b2b6d9ae974a2ab1eeb92290 / . / libgo / go / runtime / mpagealloc.go

// Copyright 2019 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. | |

// Page allocator. | |

// | |

// The page allocator manages mapped pages (defined by pageSize, NOT | |

// physPageSize) for allocation and re-use. It is embedded into mheap. | |

// | |

// Pages are managed using a bitmap that is sharded into chunks. | |

// In the bitmap, 1 means in-use, and 0 means free. The bitmap spans the | |

// process's address space. Chunks are managed in a sparse-array-style structure | |

// similar to mheap.arenas, since the bitmap may be large on some systems. | |

// | |

// The bitmap is efficiently searched by using a radix tree in combination | |

// with fast bit-wise intrinsics. Allocation is performed using an address-ordered | |

// first-fit approach. | |

// | |

// Each entry in the radix tree is a summary that describes three properties of | |

// a particular region of the address space: the number of contiguous free pages | |

// at the start and end of the region it represents, and the maximum number of | |

// contiguous free pages found anywhere in that region. | |

// | |

// Each level of the radix tree is stored as one contiguous array, which represents | |

// a different granularity of subdivision of the processes' address space. Thus, this | |

// radix tree is actually implicit in these large arrays, as opposed to having explicit | |

// dynamically-allocated pointer-based node structures. Naturally, these arrays may be | |

// quite large for system with large address spaces, so in these cases they are mapped | |

// into memory as needed. The leaf summaries of the tree correspond to a bitmap chunk. | |

// | |

// The root level (referred to as L0 and index 0 in pageAlloc.summary) has each | |

// summary represent the largest section of address space (16 GiB on 64-bit systems), | |

// with each subsequent level representing successively smaller subsections until we | |

// reach the finest granularity at the leaves, a chunk. | |

// | |

// More specifically, each summary in each level (except for leaf summaries) | |

// represents some number of entries in the following level. For example, each | |

// summary in the root level may represent a 16 GiB region of address space, | |

// and in the next level there could be 8 corresponding entries which represent 2 | |

// GiB subsections of that 16 GiB region, each of which could correspond to 8 | |

// entries in the next level which each represent 256 MiB regions, and so on. | |

// | |

// Thus, this design only scales to heaps so large, but can always be extended to | |

// larger heaps by simply adding levels to the radix tree, which mostly costs | |

// additional virtual address space. The choice of managing large arrays also means | |

// that a large amount of virtual address space may be reserved by the runtime. | |

package runtime | |

import ( | |

"runtime/internal/atomic" | |

"unsafe" | |

) | |

const ( | |

// The size of a bitmap chunk, i.e. the amount of bits (that is, pages) to consider | |

// in the bitmap at once. | |

pallocChunkPages = 1 << logPallocChunkPages | |

pallocChunkBytes = pallocChunkPages * pageSize | |

logPallocChunkPages = 9 | |

logPallocChunkBytes = logPallocChunkPages + pageShift | |

// The number of radix bits for each level. | |

// | |

// The value of 3 is chosen such that the block of summaries we need to scan at | |

// each level fits in 64 bytes (2^3 summaries * 8 bytes per summary), which is | |

// close to the L1 cache line width on many systems. Also, a value of 3 fits 4 tree | |

// levels perfectly into the 21-bit pallocBits summary field at the root level. | |

// | |

// The following equation explains how each of the constants relate: | |

// summaryL0Bits + (summaryLevels-1)*summaryLevelBits + logPallocChunkBytes = heapAddrBits | |

// | |

// summaryLevels is an architecture-dependent value defined in mpagealloc_*.go. | |

summaryLevelBits = 3 | |

summaryL0Bits = heapAddrBits - logPallocChunkBytes - (summaryLevels-1)*summaryLevelBits | |

// pallocChunksL2Bits is the number of bits of the chunk index number | |

// covered by the second level of the chunks map. | |

// | |

// See (*pageAlloc).chunks for more details. Update the documentation | |

// there should this change. | |

pallocChunksL2Bits = heapAddrBits - logPallocChunkBytes - pallocChunksL1Bits | |

pallocChunksL1Shift = pallocChunksL2Bits | |

) | |

// Maximum searchAddr value, which indicates that the heap has no free space. | |

// | |

// We alias maxOffAddr just to make it clear that this is the maximum address | |

// for the page allocator's search space. See maxOffAddr for details. | |

var maxSearchAddr = maxOffAddr | |

// Global chunk index. | |

// | |

// Represents an index into the leaf level of the radix tree. | |

// Similar to arenaIndex, except instead of arenas, it divides the address | |

// space into chunks. | |

type chunkIdx uint | |

// chunkIndex returns the global index of the palloc chunk containing the | |

// pointer p. | |

func chunkIndex(p uintptr) chunkIdx { | |

return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes) | |

} | |

// chunkIndex returns the base address of the palloc chunk at index ci. | |

func chunkBase(ci chunkIdx) uintptr { | |

return uintptr(ci)*pallocChunkBytes + arenaBaseOffset | |

} | |

// chunkPageIndex computes the index of the page that contains p, | |

// relative to the chunk which contains p. | |

func chunkPageIndex(p uintptr) uint { | |

return uint(p % pallocChunkBytes / pageSize) | |

} | |

// l1 returns the index into the first level of (*pageAlloc).chunks. | |

func (i chunkIdx) l1() uint { | |

if pallocChunksL1Bits == 0 { | |

// Let the compiler optimize this away if there's no | |

// L1 map. | |

return 0 | |

} else { | |

return uint(i) >> pallocChunksL1Shift | |

} | |

} | |

// l2 returns the index into the second level of (*pageAlloc).chunks. | |

func (i chunkIdx) l2() uint { | |

if pallocChunksL1Bits == 0 { | |

return uint(i) | |

} else { | |

return uint(i) & (1<<pallocChunksL2Bits - 1) | |

} | |

} | |

// offAddrToLevelIndex converts an address in the offset address space | |

// to the index into summary[level] containing addr. | |

func offAddrToLevelIndex(level int, addr offAddr) int { | |

return int((addr.a - arenaBaseOffset) >> levelShift[level]) | |

} | |

// levelIndexToOffAddr converts an index into summary[level] into | |

// the corresponding address in the offset address space. | |

func levelIndexToOffAddr(level, idx int) offAddr { | |

return offAddr{(uintptr(idx) << levelShift[level]) + arenaBaseOffset} | |

} | |

// addrsToSummaryRange converts base and limit pointers into a range | |

// of entries for the given summary level. | |

// | |

// The returned range is inclusive on the lower bound and exclusive on | |

// the upper bound. | |

func addrsToSummaryRange(level int, base, limit uintptr) (lo int, hi int) { | |

// This is slightly more nuanced than just a shift for the exclusive | |

// upper-bound. Note that the exclusive upper bound may be within a | |

// summary at this level, meaning if we just do the obvious computation | |

// hi will end up being an inclusive upper bound. Unfortunately, just | |

// adding 1 to that is too broad since we might be on the very edge of | |

// of a summary's max page count boundary for this level | |

// (1 << levelLogPages[level]). So, make limit an inclusive upper bound | |

// then shift, then add 1, so we get an exclusive upper bound at the end. | |

lo = int((base - arenaBaseOffset) >> levelShift[level]) | |

hi = int(((limit-1)-arenaBaseOffset)>>levelShift[level]) + 1 | |

return | |

} | |

// blockAlignSummaryRange aligns indices into the given level to that | |

// level's block width (1 << levelBits[level]). It assumes lo is inclusive | |

// and hi is exclusive, and so aligns them down and up respectively. | |

func blockAlignSummaryRange(level int, lo, hi int) (int, int) { | |

e := uintptr(1) << levelBits[level] | |

return int(alignDown(uintptr(lo), e)), int(alignUp(uintptr(hi), e)) | |

} | |

type pageAlloc struct { | |

// Radix tree of summaries. | |

// | |

// Each slice's cap represents the whole memory reservation. | |

// Each slice's len reflects the allocator's maximum known | |

// mapped heap address for that level. | |

// | |

// The backing store of each summary level is reserved in init | |

// and may or may not be committed in grow (small address spaces | |

// may commit all the memory in init). | |

// | |

// The purpose of keeping len <= cap is to enforce bounds checks | |

// on the top end of the slice so that instead of an unknown | |

// runtime segmentation fault, we get a much friendlier out-of-bounds | |

// error. | |

// | |

// To iterate over a summary level, use inUse to determine which ranges | |

// are currently available. Otherwise one might try to access | |

// memory which is only Reserved which may result in a hard fault. | |

// | |

// We may still get segmentation faults < len since some of that | |

// memory may not be committed yet. | |

summary [summaryLevels][]pallocSum | |

// chunks is a slice of bitmap chunks. | |

// | |

// The total size of chunks is quite large on most 64-bit platforms | |

// (O(GiB) or more) if flattened, so rather than making one large mapping | |

// (which has problems on some platforms, even when PROT_NONE) we use a | |

// two-level sparse array approach similar to the arena index in mheap. | |

// | |

// To find the chunk containing a memory address `a`, do: | |

// chunkOf(chunkIndex(a)) | |

// | |

// Below is a table describing the configuration for chunks for various | |

// heapAddrBits supported by the runtime. | |

// | |

// heapAddrBits | L1 Bits | L2 Bits | L2 Entry Size | |

// ------------------------------------------------ | |

// 32 | 0 | 10 | 128 KiB | |

// 33 (iOS) | 0 | 11 | 256 KiB | |

// 48 | 13 | 13 | 1 MiB | |

// | |

// There's no reason to use the L1 part of chunks on 32-bit, the | |

// address space is small so the L2 is small. For platforms with a | |

// 48-bit address space, we pick the L1 such that the L2 is 1 MiB | |

// in size, which is a good balance between low granularity without | |

// making the impact on BSS too high (note the L1 is stored directly | |

// in pageAlloc). | |

// | |

// To iterate over the bitmap, use inUse to determine which ranges | |

// are currently available. Otherwise one might iterate over unused | |

// ranges. | |

// | |

// TODO(mknyszek): Consider changing the definition of the bitmap | |

// such that 1 means free and 0 means in-use so that summaries and | |

// the bitmaps align better on zero-values. | |

chunks [1 << pallocChunksL1Bits]*[1 << pallocChunksL2Bits]pallocData | |

// The address to start an allocation search with. It must never | |

// point to any memory that is not contained in inUse, i.e. | |

// inUse.contains(searchAddr.addr()) must always be true. The one | |

// exception to this rule is that it may take on the value of | |

// maxOffAddr to indicate that the heap is exhausted. | |

// | |

// We guarantee that all valid heap addresses below this value | |

// are allocated and not worth searching. | |

searchAddr offAddr | |

// start and end represent the chunk indices | |

// which pageAlloc knows about. It assumes | |

// chunks in the range [start, end) are | |

// currently ready to use. | |

start, end chunkIdx | |

// inUse is a slice of ranges of address space which are | |

// known by the page allocator to be currently in-use (passed | |

// to grow). | |

// | |

// This field is currently unused on 32-bit architectures but | |

// is harmless to track. We care much more about having a | |

// contiguous heap in these cases and take additional measures | |

// to ensure that, so in nearly all cases this should have just | |

// 1 element. | |

// | |

// All access is protected by the mheapLock. | |

inUse addrRanges | |

// scav stores the scavenger state. | |

// | |

// All fields are protected by mheapLock. | |

scav struct { | |

// inUse is a slice of ranges of address space which have not | |

// yet been looked at by the scavenger. | |

inUse addrRanges | |

// gen is the scavenge generation number. | |

gen uint32 | |

// reservationBytes is how large of a reservation should be made | |

// in bytes of address space for each scavenge iteration. | |

reservationBytes uintptr | |

// released is the amount of memory released this generation. | |

released uintptr | |

// scavLWM is the lowest (offset) address that the scavenger reached this | |

// scavenge generation. | |

scavLWM offAddr | |

// freeHWM is the highest (offset) address of a page that was freed to | |

// the page allocator this scavenge generation. | |

freeHWM offAddr | |

} | |

// mheap_.lock. This level of indirection makes it possible | |

// to test pageAlloc indepedently of the runtime allocator. | |

mheapLock *mutex | |

// sysStat is the runtime memstat to update when new system | |

// memory is committed by the pageAlloc for allocation metadata. | |

sysStat *uint64 | |

// Whether or not this struct is being used in tests. | |

test bool | |

} | |

func (s *pageAlloc) init(mheapLock *mutex, sysStat *uint64) { | |

if levelLogPages[0] > logMaxPackedValue { | |

// We can't represent 1<<levelLogPages[0] pages, the maximum number | |

// of pages we need to represent at the root level, in a summary, which | |

// is a big problem. Throw. | |

print("runtime: root level max pages = ", 1<<levelLogPages[0], "\n") | |

print("runtime: summary max pages = ", maxPackedValue, "\n") | |

throw("root level max pages doesn't fit in summary") | |

} | |

s.sysStat = sysStat | |

// Initialize s.inUse. | |

s.inUse.init(sysStat) | |

// System-dependent initialization. | |

s.sysInit() | |

// Start with the searchAddr in a state indicating there's no free memory. | |

s.searchAddr = maxSearchAddr | |

// Set the mheapLock. | |

s.mheapLock = mheapLock | |

// Initialize scavenge tracking state. | |

s.scav.scavLWM = maxSearchAddr | |

} | |

// tryChunkOf returns the bitmap data for the given chunk. | |

// | |

// Returns nil if the chunk data has not been mapped. | |

func (s *pageAlloc) tryChunkOf(ci chunkIdx) *pallocData { | |

l2 := s.chunks[ci.l1()] | |

if l2 == nil { | |

return nil | |

} | |

return &l2[ci.l2()] | |

} | |

// chunkOf returns the chunk at the given chunk index. | |

// | |

// The chunk index must be valid or this method may throw. | |

func (s *pageAlloc) chunkOf(ci chunkIdx) *pallocData { | |

return &s.chunks[ci.l1()][ci.l2()] | |

} | |

// grow sets up the metadata for the address range [base, base+size). | |

// It may allocate metadata, in which case *s.sysStat will be updated. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) grow(base, size uintptr) { | |

// Round up to chunks, since we can't deal with increments smaller | |

// than chunks. Also, sysGrow expects aligned values. | |

limit := alignUp(base+size, pallocChunkBytes) | |

base = alignDown(base, pallocChunkBytes) | |

// Grow the summary levels in a system-dependent manner. | |

// We just update a bunch of additional metadata here. | |

s.sysGrow(base, limit) | |

// Update s.start and s.end. | |

// If no growth happened yet, start == 0. This is generally | |

// safe since the zero page is unmapped. | |

firstGrowth := s.start == 0 | |

start, end := chunkIndex(base), chunkIndex(limit) | |

if firstGrowth || start < s.start { | |

s.start = start | |

} | |

if end > s.end { | |

s.end = end | |

} | |

// Note that [base, limit) will never overlap with any existing | |

// range inUse because grow only ever adds never-used memory | |

// regions to the page allocator. | |

s.inUse.add(makeAddrRange(base, limit)) | |

// A grow operation is a lot like a free operation, so if our | |

// chunk ends up below s.searchAddr, update s.searchAddr to the | |

// new address, just like in free. | |

if b := (offAddr{base}); b.lessThan(s.searchAddr) { | |

s.searchAddr = b | |

} | |

// Add entries into chunks, which is sparse, if needed. Then, | |

// initialize the bitmap. | |

// | |

// Newly-grown memory is always considered scavenged. | |

// Set all the bits in the scavenged bitmaps high. | |

for c := chunkIndex(base); c < chunkIndex(limit); c++ { | |

if s.chunks[c.l1()] == nil { | |

// Create the necessary l2 entry. | |

// | |

// Store it atomically to avoid races with readers which | |

// don't acquire the heap lock. | |

r := sysAlloc(unsafe.Sizeof(*s.chunks[0]), s.sysStat) | |

atomic.StorepNoWB(unsafe.Pointer(&s.chunks[c.l1()]), r) | |

} | |

s.chunkOf(c).scavenged.setRange(0, pallocChunkPages) | |

} | |

// Update summaries accordingly. The grow acts like a free, so | |

// we need to ensure this newly-free memory is visible in the | |

// summaries. | |

s.update(base, size/pageSize, true, false) | |

} | |

// update updates heap metadata. It must be called each time the bitmap | |

// is updated. | |

// | |

// If contig is true, update does some optimizations assuming that there was | |

// a contiguous allocation or free between addr and addr+npages. alloc indicates | |

// whether the operation performed was an allocation or a free. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) update(base, npages uintptr, contig, alloc bool) { | |

// base, limit, start, and end are inclusive. | |

limit := base + npages*pageSize - 1 | |

sc, ec := chunkIndex(base), chunkIndex(limit) | |

// Handle updating the lowest level first. | |

if sc == ec { | |

// Fast path: the allocation doesn't span more than one chunk, | |

// so update this one and if the summary didn't change, return. | |

x := s.summary[len(s.summary)-1][sc] | |

y := s.chunkOf(sc).summarize() | |

if x == y { | |

return | |

} | |

s.summary[len(s.summary)-1][sc] = y | |

} else if contig { | |

// Slow contiguous path: the allocation spans more than one chunk | |

// and at least one summary is guaranteed to change. | |

summary := s.summary[len(s.summary)-1] | |

// Update the summary for chunk sc. | |

summary[sc] = s.chunkOf(sc).summarize() | |

// Update the summaries for chunks in between, which are | |

// either totally allocated or freed. | |

whole := s.summary[len(s.summary)-1][sc+1 : ec] | |

if alloc { | |

// Should optimize into a memclr. | |

for i := range whole { | |

whole[i] = 0 | |

} | |

} else { | |

for i := range whole { | |

whole[i] = freeChunkSum | |

} | |

} | |

// Update the summary for chunk ec. | |

summary[ec] = s.chunkOf(ec).summarize() | |

} else { | |

// Slow general path: the allocation spans more than one chunk | |

// and at least one summary is guaranteed to change. | |

// | |

// We can't assume a contiguous allocation happened, so walk over | |

// every chunk in the range and manually recompute the summary. | |

summary := s.summary[len(s.summary)-1] | |

for c := sc; c <= ec; c++ { | |

summary[c] = s.chunkOf(c).summarize() | |

} | |

} | |

// Walk up the radix tree and update the summaries appropriately. | |

changed := true | |

for l := len(s.summary) - 2; l >= 0 && changed; l-- { | |

// Update summaries at level l from summaries at level l+1. | |

changed = false | |

// "Constants" for the previous level which we | |

// need to compute the summary from that level. | |

logEntriesPerBlock := levelBits[l+1] | |

logMaxPages := levelLogPages[l+1] | |

// lo and hi describe all the parts of the level we need to look at. | |

lo, hi := addrsToSummaryRange(l, base, limit+1) | |

// Iterate over each block, updating the corresponding summary in the less-granular level. | |

for i := lo; i < hi; i++ { | |

children := s.summary[l+1][i<<logEntriesPerBlock : (i+1)<<logEntriesPerBlock] | |

sum := mergeSummaries(children, logMaxPages) | |

old := s.summary[l][i] | |

if old != sum { | |

changed = true | |

s.summary[l][i] = sum | |

} | |

} | |

} | |

} | |

// allocRange marks the range of memory [base, base+npages*pageSize) as | |

// allocated. It also updates the summaries to reflect the newly-updated | |

// bitmap. | |

// | |

// Returns the amount of scavenged memory in bytes present in the | |

// allocated range. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) allocRange(base, npages uintptr) uintptr { | |

limit := base + npages*pageSize - 1 | |

sc, ec := chunkIndex(base), chunkIndex(limit) | |

si, ei := chunkPageIndex(base), chunkPageIndex(limit) | |

scav := uint(0) | |

if sc == ec { | |

// The range doesn't cross any chunk boundaries. | |

chunk := s.chunkOf(sc) | |

scav += chunk.scavenged.popcntRange(si, ei+1-si) | |

chunk.allocRange(si, ei+1-si) | |

} else { | |

// The range crosses at least one chunk boundary. | |

chunk := s.chunkOf(sc) | |

scav += chunk.scavenged.popcntRange(si, pallocChunkPages-si) | |

chunk.allocRange(si, pallocChunkPages-si) | |

for c := sc + 1; c < ec; c++ { | |

chunk := s.chunkOf(c) | |

scav += chunk.scavenged.popcntRange(0, pallocChunkPages) | |

chunk.allocAll() | |

} | |

chunk = s.chunkOf(ec) | |

scav += chunk.scavenged.popcntRange(0, ei+1) | |

chunk.allocRange(0, ei+1) | |

} | |

s.update(base, npages, true, true) | |

return uintptr(scav) * pageSize | |

} | |

// findMappedAddr returns the smallest mapped offAddr that is | |

// >= addr. That is, if addr refers to mapped memory, then it is | |

// returned. If addr is higher than any mapped region, then | |

// it returns maxOffAddr. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) findMappedAddr(addr offAddr) offAddr { | |

// If we're not in a test, validate first by checking mheap_.arenas. | |

// This is a fast path which is only safe to use outside of testing. | |

ai := arenaIndex(addr.addr()) | |

if s.test || mheap_.arenas[ai.l1()] == nil || mheap_.arenas[ai.l1()][ai.l2()] == nil { | |

vAddr, ok := s.inUse.findAddrGreaterEqual(addr.addr()) | |

if ok { | |

return offAddr{vAddr} | |

} else { | |

// The candidate search address is greater than any | |

// known address, which means we definitely have no | |

// free memory left. | |

return maxOffAddr | |

} | |

} | |

return addr | |

} | |

// find searches for the first (address-ordered) contiguous free region of | |

// npages in size and returns a base address for that region. | |

// | |

// It uses s.searchAddr to prune its search and assumes that no palloc chunks | |

// below chunkIndex(s.searchAddr) contain any free memory at all. | |

// | |

// find also computes and returns a candidate s.searchAddr, which may or | |

// may not prune more of the address space than s.searchAddr already does. | |

// This candidate is always a valid s.searchAddr. | |

// | |

// find represents the slow path and the full radix tree search. | |

// | |

// Returns a base address of 0 on failure, in which case the candidate | |

// searchAddr returned is invalid and must be ignored. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) find(npages uintptr) (uintptr, offAddr) { | |

// Search algorithm. | |

// | |

// This algorithm walks each level l of the radix tree from the root level | |

// to the leaf level. It iterates over at most 1 << levelBits[l] of entries | |

// in a given level in the radix tree, and uses the summary information to | |

// find either: | |

// 1) That a given subtree contains a large enough contiguous region, at | |

// which point it continues iterating on the next level, or | |

// 2) That there are enough contiguous boundary-crossing bits to satisfy | |

// the allocation, at which point it knows exactly where to start | |

// allocating from. | |

// | |

// i tracks the index into the current level l's structure for the | |

// contiguous 1 << levelBits[l] entries we're actually interested in. | |

// | |

// NOTE: Technically this search could allocate a region which crosses | |

// the arenaBaseOffset boundary, which when arenaBaseOffset != 0, is | |

// a discontinuity. However, the only way this could happen is if the | |

// page at the zero address is mapped, and this is impossible on | |

// every system we support where arenaBaseOffset != 0. So, the | |

// discontinuity is already encoded in the fact that the OS will never | |

// map the zero page for us, and this function doesn't try to handle | |

// this case in any way. | |

// i is the beginning of the block of entries we're searching at the | |

// current level. | |

i := 0 | |

// firstFree is the region of address space that we are certain to | |

// find the first free page in the heap. base and bound are the inclusive | |

// bounds of this window, and both are addresses in the linearized, contiguous | |

// view of the address space (with arenaBaseOffset pre-added). At each level, | |

// this window is narrowed as we find the memory region containing the | |

// first free page of memory. To begin with, the range reflects the | |

// full process address space. | |

// | |

// firstFree is updated by calling foundFree each time free space in the | |

// heap is discovered. | |

// | |

// At the end of the search, base.addr() is the best new | |

// searchAddr we could deduce in this search. | |

firstFree := struct { | |

base, bound offAddr | |

}{ | |

base: minOffAddr, | |

bound: maxOffAddr, | |

} | |

// foundFree takes the given address range [addr, addr+size) and | |

// updates firstFree if it is a narrower range. The input range must | |

// either be fully contained within firstFree or not overlap with it | |

// at all. | |

// | |

// This way, we'll record the first summary we find with any free | |

// pages on the root level and narrow that down if we descend into | |

// that summary. But as soon as we need to iterate beyond that summary | |

// in a level to find a large enough range, we'll stop narrowing. | |

foundFree := func(addr offAddr, size uintptr) { | |

if firstFree.base.lessEqual(addr) && addr.add(size-1).lessEqual(firstFree.bound) { | |

// This range fits within the current firstFree window, so narrow | |

// down the firstFree window to the base and bound of this range. | |

firstFree.base = addr | |

firstFree.bound = addr.add(size - 1) | |

} else if !(addr.add(size-1).lessThan(firstFree.base) || firstFree.bound.lessThan(addr)) { | |

// This range only partially overlaps with the firstFree range, | |

// so throw. | |

print("runtime: addr = ", hex(addr.addr()), ", size = ", size, "\n") | |

print("runtime: base = ", hex(firstFree.base.addr()), ", bound = ", hex(firstFree.bound.addr()), "\n") | |

throw("range partially overlaps") | |

} | |

} | |

// lastSum is the summary which we saw on the previous level that made us | |

// move on to the next level. Used to print additional information in the | |

// case of a catastrophic failure. | |

// lastSumIdx is that summary's index in the previous level. | |

lastSum := packPallocSum(0, 0, 0) | |

lastSumIdx := -1 | |

nextLevel: | |

for l := 0; l < len(s.summary); l++ { | |

// For the root level, entriesPerBlock is the whole level. | |

entriesPerBlock := 1 << levelBits[l] | |

logMaxPages := levelLogPages[l] | |

// We've moved into a new level, so let's update i to our new | |

// starting index. This is a no-op for level 0. | |

i <<= levelBits[l] | |

// Slice out the block of entries we care about. | |

entries := s.summary[l][i : i+entriesPerBlock] | |

// Determine j0, the first index we should start iterating from. | |

// The searchAddr may help us eliminate iterations if we followed the | |

// searchAddr on the previous level or we're on the root leve, in which | |

// case the searchAddr should be the same as i after levelShift. | |

j0 := 0 | |

if searchIdx := offAddrToLevelIndex(l, s.searchAddr); searchIdx&^(entriesPerBlock-1) == i { | |

j0 = searchIdx & (entriesPerBlock - 1) | |

} | |

// Run over the level entries looking for | |

// a contiguous run of at least npages either | |

// within an entry or across entries. | |

// | |

// base contains the page index (relative to | |

// the first entry's first page) of the currently | |

// considered run of consecutive pages. | |

// | |

// size contains the size of the currently considered | |

// run of consecutive pages. | |

var base, size uint | |

for j := j0; j < len(entries); j++ { | |

sum := entries[j] | |

if sum == 0 { | |

// A full entry means we broke any streak and | |

// that we should skip it altogether. | |

size = 0 | |

continue | |

} | |

// We've encountered a non-zero summary which means | |

// free memory, so update firstFree. | |

foundFree(levelIndexToOffAddr(l, i+j), (uintptr(1)<<logMaxPages)*pageSize) | |

s := sum.start() | |

if size+s >= uint(npages) { | |

// If size == 0 we don't have a run yet, | |

// which means base isn't valid. So, set | |

// base to the first page in this block. | |

if size == 0 { | |

base = uint(j) << logMaxPages | |

} | |

// We hit npages; we're done! | |

size += s | |

break | |

} | |

if sum.max() >= uint(npages) { | |

// The entry itself contains npages contiguous | |

// free pages, so continue on the next level | |

// to find that run. | |

i += j | |

lastSumIdx = i | |

lastSum = sum | |

continue nextLevel | |

} | |

if size == 0 || s < 1<<logMaxPages { | |

// We either don't have a current run started, or this entry | |

// isn't totally free (meaning we can't continue the current | |

// one), so try to begin a new run by setting size and base | |

// based on sum.end. | |

size = sum.end() | |

base = uint(j+1)<<logMaxPages - size | |

continue | |

} | |

// The entry is completely free, so continue the run. | |

size += 1 << logMaxPages | |

} | |

if size >= uint(npages) { | |

// We found a sufficiently large run of free pages straddling | |

// some boundary, so compute the address and return it. | |

addr := levelIndexToOffAddr(l, i).add(uintptr(base) * pageSize).addr() | |

return addr, s.findMappedAddr(firstFree.base) | |

} | |

if l == 0 { | |

// We're at level zero, so that means we've exhausted our search. | |

return 0, maxSearchAddr | |

} | |

// We're not at level zero, and we exhausted the level we were looking in. | |

// This means that either our calculations were wrong or the level above | |

// lied to us. In either case, dump some useful state and throw. | |

print("runtime: summary[", l-1, "][", lastSumIdx, "] = ", lastSum.start(), ", ", lastSum.max(), ", ", lastSum.end(), "\n") | |

print("runtime: level = ", l, ", npages = ", npages, ", j0 = ", j0, "\n") | |

print("runtime: s.searchAddr = ", hex(s.searchAddr.addr()), ", i = ", i, "\n") | |

print("runtime: levelShift[level] = ", levelShift[l], ", levelBits[level] = ", levelBits[l], "\n") | |

for j := 0; j < len(entries); j++ { | |

sum := entries[j] | |

print("runtime: summary[", l, "][", i+j, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") | |

} | |

throw("bad summary data") | |

} | |

// Since we've gotten to this point, that means we haven't found a | |

// sufficiently-sized free region straddling some boundary (chunk or larger). | |

// This means the last summary we inspected must have had a large enough "max" | |

// value, so look inside the chunk to find a suitable run. | |

// | |

// After iterating over all levels, i must contain a chunk index which | |

// is what the final level represents. | |

ci := chunkIdx(i) | |

j, searchIdx := s.chunkOf(ci).find(npages, 0) | |

if j == ^uint(0) { | |

// We couldn't find any space in this chunk despite the summaries telling | |

// us it should be there. There's likely a bug, so dump some state and throw. | |

sum := s.summary[len(s.summary)-1][i] | |

print("runtime: summary[", len(s.summary)-1, "][", i, "] = (", sum.start(), ", ", sum.max(), ", ", sum.end(), ")\n") | |

print("runtime: npages = ", npages, "\n") | |

throw("bad summary data") | |

} | |

// Compute the address at which the free space starts. | |

addr := chunkBase(ci) + uintptr(j)*pageSize | |

// Since we actually searched the chunk, we may have | |

// found an even narrower free window. | |

searchAddr := chunkBase(ci) + uintptr(searchIdx)*pageSize | |

foundFree(offAddr{searchAddr}, chunkBase(ci+1)-searchAddr) | |

return addr, s.findMappedAddr(firstFree.base) | |

} | |

// alloc allocates npages worth of memory from the page heap, returning the base | |

// address for the allocation and the amount of scavenged memory in bytes | |

// contained in the region [base address, base address + npages*pageSize). | |

// | |

// Returns a 0 base address on failure, in which case other returned values | |

// should be ignored. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) alloc(npages uintptr) (addr uintptr, scav uintptr) { | |

// If the searchAddr refers to a region which has a higher address than | |

// any known chunk, then we know we're out of memory. | |

if chunkIndex(s.searchAddr.addr()) >= s.end { | |

return 0, 0 | |

} | |

// If npages has a chance of fitting in the chunk where the searchAddr is, | |

// search it directly. | |

searchAddr := minOffAddr | |

if pallocChunkPages-chunkPageIndex(s.searchAddr.addr()) >= uint(npages) { | |

// npages is guaranteed to be no greater than pallocChunkPages here. | |

i := chunkIndex(s.searchAddr.addr()) | |

if max := s.summary[len(s.summary)-1][i].max(); max >= uint(npages) { | |

j, searchIdx := s.chunkOf(i).find(npages, chunkPageIndex(s.searchAddr.addr())) | |

if j == ^uint(0) { | |

print("runtime: max = ", max, ", npages = ", npages, "\n") | |

print("runtime: searchIdx = ", chunkPageIndex(s.searchAddr.addr()), ", s.searchAddr = ", hex(s.searchAddr.addr()), "\n") | |

throw("bad summary data") | |

} | |

addr = chunkBase(i) + uintptr(j)*pageSize | |

searchAddr = offAddr{chunkBase(i) + uintptr(searchIdx)*pageSize} | |

goto Found | |

} | |

} | |

// We failed to use a searchAddr for one reason or another, so try | |

// the slow path. | |

addr, searchAddr = s.find(npages) | |

if addr == 0 { | |

if npages == 1 { | |

// We failed to find a single free page, the smallest unit | |

// of allocation. This means we know the heap is completely | |

// exhausted. Otherwise, the heap still might have free | |

// space in it, just not enough contiguous space to | |

// accommodate npages. | |

s.searchAddr = maxSearchAddr | |

} | |

return 0, 0 | |

} | |

Found: | |

// Go ahead and actually mark the bits now that we have an address. | |

scav = s.allocRange(addr, npages) | |

// If we found a higher searchAddr, we know that all the | |

// heap memory before that searchAddr in an offset address space is | |

// allocated, so bump s.searchAddr up to the new one. | |

if s.searchAddr.lessThan(searchAddr) { | |

s.searchAddr = searchAddr | |

} | |

return addr, scav | |

} | |

// free returns npages worth of memory starting at base back to the page heap. | |

// | |

// s.mheapLock must be held. | |

func (s *pageAlloc) free(base, npages uintptr) { | |

// If we're freeing pages below the s.searchAddr, update searchAddr. | |

if b := (offAddr{base}); b.lessThan(s.searchAddr) { | |

s.searchAddr = b | |

} | |

// Update the free high watermark for the scavenger. | |

limit := base + npages*pageSize - 1 | |

if offLimit := (offAddr{limit}); s.scav.freeHWM.lessThan(offLimit) { | |

s.scav.freeHWM = offLimit | |

} | |

if npages == 1 { | |

// Fast path: we're clearing a single bit, and we know exactly | |

// where it is, so mark it directly. | |

i := chunkIndex(base) | |

s.chunkOf(i).free1(chunkPageIndex(base)) | |

} else { | |

// Slow path: we're clearing more bits so we may need to iterate. | |

sc, ec := chunkIndex(base), chunkIndex(limit) | |

si, ei := chunkPageIndex(base), chunkPageIndex(limit) | |

if sc == ec { | |

// The range doesn't cross any chunk boundaries. | |

s.chunkOf(sc).free(si, ei+1-si) | |

} else { | |

// The range crosses at least one chunk boundary. | |

s.chunkOf(sc).free(si, pallocChunkPages-si) | |

for c := sc + 1; c < ec; c++ { | |

s.chunkOf(c).freeAll() | |

} | |

s.chunkOf(ec).free(0, ei+1) | |

} | |

} | |

s.update(base, npages, true, false) | |

} | |

const ( | |

pallocSumBytes = unsafe.Sizeof(pallocSum(0)) | |

// maxPackedValue is the maximum value that any of the three fields in | |

// the pallocSum may take on. | |

maxPackedValue = 1 << logMaxPackedValue | |

logMaxPackedValue = logPallocChunkPages + (summaryLevels-1)*summaryLevelBits | |

freeChunkSum = pallocSum(uint64(pallocChunkPages) | | |

uint64(pallocChunkPages<<logMaxPackedValue) | | |

uint64(pallocChunkPages<<(2*logMaxPackedValue))) | |

) | |

// pallocSum is a packed summary type which packs three numbers: start, max, | |

// and end into a single 8-byte value. Each of these values are a summary of | |

// a bitmap and are thus counts, each of which may have a maximum value of | |

// 2^21 - 1, or all three may be equal to 2^21. The latter case is represented | |

// by just setting the 64th bit. | |

type pallocSum uint64 | |

// packPallocSum takes a start, max, and end value and produces a pallocSum. | |

func packPallocSum(start, max, end uint) pallocSum { | |

if max == maxPackedValue { | |

return pallocSum(uint64(1 << 63)) | |

} | |

return pallocSum((uint64(start) & (maxPackedValue - 1)) | | |

((uint64(max) & (maxPackedValue - 1)) << logMaxPackedValue) | | |

((uint64(end) & (maxPackedValue - 1)) << (2 * logMaxPackedValue))) | |

} | |

// start extracts the start value from a packed sum. | |

func (p pallocSum) start() uint { | |

if uint64(p)&uint64(1<<63) != 0 { | |

return maxPackedValue | |

} | |

return uint(uint64(p) & (maxPackedValue - 1)) | |

} | |

// max extracts the max value from a packed sum. | |

func (p pallocSum) max() uint { | |

if uint64(p)&uint64(1<<63) != 0 { | |

return maxPackedValue | |

} | |

return uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)) | |

} | |

// end extracts the end value from a packed sum. | |

func (p pallocSum) end() uint { | |

if uint64(p)&uint64(1<<63) != 0 { | |

return maxPackedValue | |

} | |

return uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) | |

} | |

// unpack unpacks all three values from the summary. | |

func (p pallocSum) unpack() (uint, uint, uint) { | |

if uint64(p)&uint64(1<<63) != 0 { | |

return maxPackedValue, maxPackedValue, maxPackedValue | |

} | |

return uint(uint64(p) & (maxPackedValue - 1)), | |

uint((uint64(p) >> logMaxPackedValue) & (maxPackedValue - 1)), | |

uint((uint64(p) >> (2 * logMaxPackedValue)) & (maxPackedValue - 1)) | |

} | |

// mergeSummaries merges consecutive summaries which may each represent at | |

// most 1 << logMaxPagesPerSum pages each together into one. | |

func mergeSummaries(sums []pallocSum, logMaxPagesPerSum uint) pallocSum { | |

// Merge the summaries in sums into one. | |

// | |

// We do this by keeping a running summary representing the merged | |

// summaries of sums[:i] in start, max, and end. | |

start, max, end := sums[0].unpack() | |

for i := 1; i < len(sums); i++ { | |

// Merge in sums[i]. | |

si, mi, ei := sums[i].unpack() | |

// Merge in sums[i].start only if the running summary is | |

// completely free, otherwise this summary's start | |

// plays no role in the combined sum. | |

if start == uint(i)<<logMaxPagesPerSum { | |

start += si | |

} | |

// Recompute the max value of the running sum by looking | |

// across the boundary between the running sum and sums[i] | |

// and at the max sums[i], taking the greatest of those two | |

// and the max of the running sum. | |

if end+si > max { | |

max = end + si | |

} | |

if mi > max { | |

max = mi | |

} | |

// Merge in end by checking if this new summary is totally | |

// free. If it is, then we want to extend the running sum's | |

// end by the new summary. If not, then we have some alloc'd | |

// pages in there and we just want to take the end value in | |

// sums[i]. | |

if ei == 1<<logMaxPagesPerSum { | |

end += 1 << logMaxPagesPerSum | |

} else { | |

end = ei | |

} | |

} | |

return packPallocSum(start, max, end) | |

} |