// 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. package runtime import ( "runtime/internal/sys" ) // pageBits is a bitmap representing one bit per page in a palloc chunk. type pageBits [pallocChunkPages / 64]uint64 // get returns the value of the i'th bit in the bitmap. func (b *pageBits) get(i uint) uint { return uint((b[i/64] >> (i % 64)) & 1) } // block64 returns the 64-bit aligned block of bits containing the i'th bit. func (b *pageBits) block64(i uint) uint64 { return b[i/64] } // set sets bit i of pageBits. func (b *pageBits) set(i uint) { b[i/64] |= 1 << (i % 64) } // setRange sets bits in the range [i, i+n). func (b *pageBits) setRange(i, n uint) { _ = b[i/64] if n == 1 { // Fast path for the n == 1 case. b.set(i) return } // Set bits [i, j]. j := i + n - 1 if i/64 == j/64 { b[i/64] |= ((uint64(1) << n) - 1) << (i % 64) return } _ = b[j/64] // Set leading bits. b[i/64] |= ^uint64(0) << (i % 64) for k := i/64 + 1; k < j/64; k++ { b[k] = ^uint64(0) } // Set trailing bits. b[j/64] |= (uint64(1) << (j%64 + 1)) - 1 } // setAll sets all the bits of b. func (b *pageBits) setAll() { for i := range b { b[i] = ^uint64(0) } } // setBlock64 sets the 64-bit aligned block of bits containing the i'th bit that // are set in v. func (b *pageBits) setBlock64(i uint, v uint64) { b[i/64] |= v } // clear clears bit i of pageBits. func (b *pageBits) clear(i uint) { b[i/64] &^= 1 << (i % 64) } // clearRange clears bits in the range [i, i+n). func (b *pageBits) clearRange(i, n uint) { _ = b[i/64] if n == 1 { // Fast path for the n == 1 case. b.clear(i) return } // Clear bits [i, j]. j := i + n - 1 if i/64 == j/64 { b[i/64] &^= ((uint64(1) << n) - 1) << (i % 64) return } _ = b[j/64] // Clear leading bits. b[i/64] &^= ^uint64(0) << (i % 64) for k := i/64 + 1; k < j/64; k++ { b[k] = 0 } // Clear trailing bits. b[j/64] &^= (uint64(1) << (j%64 + 1)) - 1 } // clearAll frees all the bits of b. func (b *pageBits) clearAll() { for i := range b { b[i] = 0 } } // clearBlock64 clears the 64-bit aligned block of bits containing the i'th bit that // are set in v. func (b *pageBits) clearBlock64(i uint, v uint64) { b[i/64] &^= v } // popcntRange counts the number of set bits in the // range [i, i+n). func (b *pageBits) popcntRange(i, n uint) (s uint) { if n == 1 { return uint((b[i/64] >> (i % 64)) & 1) } _ = b[i/64] j := i + n - 1 if i/64 == j/64 { return uint(sys.OnesCount64((b[i/64] >> (i % 64)) & ((1 << n) - 1))) } _ = b[j/64] s += uint(sys.OnesCount64(b[i/64] >> (i % 64))) for k := i/64 + 1; k < j/64; k++ { s += uint(sys.OnesCount64(b[k])) } s += uint(sys.OnesCount64(b[j/64] & ((1 << (j%64 + 1)) - 1))) return } // pallocBits is a bitmap that tracks page allocations for at most one // palloc chunk. // // The precise representation is an implementation detail, but for the // sake of documentation, 0s are free pages and 1s are allocated pages. type pallocBits pageBits // summarize returns a packed summary of the bitmap in pallocBits. func (b *pallocBits) summarize() pallocSum { var start, most, cur uint const notSetYet = ^uint(0) // sentinel for start value start = notSetYet for i := 0; i < len(b); i++ { x := b[i] if x == 0 { cur += 64 continue } t := uint(sys.TrailingZeros64(x)) l := uint(sys.LeadingZeros64(x)) // Finish any region spanning the uint64s cur += t if start == notSetYet { start = cur } most = max(most, cur) // Final region that might span to next uint64 cur = l } if start == notSetYet { // Made it all the way through without finding a single 1 bit. const n = uint(64 * len(b)) return packPallocSum(n, n, n) } most = max(most, cur) if most >= 64-2 { // There is no way an internal run of zeros could beat max. return packPallocSum(start, most, cur) } // Now look inside each uint64 for runs of zeros. // All uint64s must be nonzero, or we would have aborted above. outer: for i := 0; i < len(b); i++ { x := b[i] // Look inside this uint64. We have a pattern like // 000000 1xxxxx1 000000 // We need to look inside the 1xxxxx1 for any contiguous // region of zeros. // We already know the trailing zeros are no larger than max. Remove them. x >>= sys.TrailingZeros64(x) & 63 if x&(x+1) == 0 { // no more zeros (except at the top). continue } // Strategy: shrink all runs of zeros by max. If any runs of zero // remain, then we've identified a larger maximum zero run. p := most // number of zeros we still need to shrink by. k := uint(1) // current minimum length of runs of ones in x. for { // Shrink all runs of zeros by p places (except the top zeros). for p > 0 { if p <= k { // Shift p ones down into the top of each run of zeros. x |= x >> (p & 63) if x&(x+1) == 0 { // no more zeros (except at the top). continue outer } break } // Shift k ones down into the top of each run of zeros. x |= x >> (k & 63) if x&(x+1) == 0 { // no more zeros (except at the top). continue outer } p -= k // We've just doubled the minimum length of 1-runs. // This allows us to shift farther in the next iteration. k *= 2 } // The length of the lowest-order zero run is an increment to our maximum. j := uint(sys.TrailingZeros64(^x)) // count contiguous trailing ones x >>= j & 63 // remove trailing ones j = uint(sys.TrailingZeros64(x)) // count contiguous trailing zeros x >>= j & 63 // remove zeros most += j // we have a new maximum! if x&(x+1) == 0 { // no more zeros (except at the top). continue outer } p = j // remove j more zeros from each zero run. } } return packPallocSum(start, most, cur) } // find searches for npages contiguous free pages in pallocBits and returns // the index where that run starts, as well as the index of the first free page // it found in the search. searchIdx represents the first known free page and // where to begin the next search from. // // If find fails to find any free space, it returns an index of ^uint(0) and // the new searchIdx should be ignored. // // Note that if npages == 1, the two returned values will always be identical. func (b *pallocBits) find(npages uintptr, searchIdx uint) (uint, uint) { if npages == 1 { addr := b.find1(searchIdx) return addr, addr } else if npages <= 64 { return b.findSmallN(npages, searchIdx) } return b.findLargeN(npages, searchIdx) } // find1 is a helper for find which searches for a single free page // in the pallocBits and returns the index. // // See find for an explanation of the searchIdx parameter. func (b *pallocBits) find1(searchIdx uint) uint { _ = b[0] // lift nil check out of loop for i := searchIdx / 64; i < uint(len(b)); i++ { x := b[i] if ^x == 0 { continue } return i*64 + uint(sys.TrailingZeros64(^x)) } return ^uint(0) } // findSmallN is a helper for find which searches for npages contiguous free pages // in this pallocBits and returns the index where that run of contiguous pages // starts as well as the index of the first free page it finds in its search. // // See find for an explanation of the searchIdx parameter. // // Returns a ^uint(0) index on failure and the new searchIdx should be ignored. // // findSmallN assumes npages <= 64, where any such contiguous run of pages // crosses at most one aligned 64-bit boundary in the bits. func (b *pallocBits) findSmallN(npages uintptr, searchIdx uint) (uint, uint) { end, newSearchIdx := uint(0), ^uint(0) for i := searchIdx / 64; i < uint(len(b)); i++ { bi := b[i] if ^bi == 0 { end = 0 continue } // First see if we can pack our allocation in the trailing // zeros plus the end of the last 64 bits. if newSearchIdx == ^uint(0) { // The new searchIdx is going to be at these 64 bits after any // 1s we file, so count trailing 1s. newSearchIdx = i*64 + uint(sys.TrailingZeros64(^bi)) } start := uint(sys.TrailingZeros64(bi)) if end+start >= uint(npages) { return i*64 - end, newSearchIdx } // Next, check the interior of the 64-bit chunk. j := findBitRange64(^bi, uint(npages)) if j < 64 { return i*64 + j, newSearchIdx } end = uint(sys.LeadingZeros64(bi)) } return ^uint(0), newSearchIdx } // findLargeN is a helper for find which searches for npages contiguous free pages // in this pallocBits and returns the index where that run starts, as well as the // index of the first free page it found it its search. // // See alloc for an explanation of the searchIdx parameter. // // Returns a ^uint(0) index on failure and the new searchIdx should be ignored. // // findLargeN assumes npages > 64, where any such run of free pages // crosses at least one aligned 64-bit boundary in the bits. func (b *pallocBits) findLargeN(npages uintptr, searchIdx uint) (uint, uint) { start, size, newSearchIdx := ^uint(0), uint(0), ^uint(0) for i := searchIdx / 64; i < uint(len(b)); i++ { x := b[i] if x == ^uint64(0) { size = 0 continue } if newSearchIdx == ^uint(0) { // The new searchIdx is going to be at these 64 bits after any // 1s we file, so count trailing 1s. newSearchIdx = i*64 + uint(sys.TrailingZeros64(^x)) } if size == 0 { size = uint(sys.LeadingZeros64(x)) start = i*64 + 64 - size continue } s := uint(sys.TrailingZeros64(x)) if s+size >= uint(npages) { size += s return start, newSearchIdx } if s < 64 { size = uint(sys.LeadingZeros64(x)) start = i*64 + 64 - size continue } size += 64 } if size < uint(npages) { return ^uint(0), newSearchIdx } return start, newSearchIdx } // allocRange allocates the range [i, i+n). func (b *pallocBits) allocRange(i, n uint) { (*pageBits)(b).setRange(i, n) } // allocAll allocates all the bits of b. func (b *pallocBits) allocAll() { (*pageBits)(b).setAll() } // free1 frees a single page in the pallocBits at i. func (b *pallocBits) free1(i uint) { (*pageBits)(b).clear(i) } // free frees the range [i, i+n) of pages in the pallocBits. func (b *pallocBits) free(i, n uint) { (*pageBits)(b).clearRange(i, n) } // freeAll frees all the bits of b. func (b *pallocBits) freeAll() { (*pageBits)(b).clearAll() } // pages64 returns a 64-bit bitmap representing a block of 64 pages aligned // to 64 pages. The returned block of pages is the one containing the i'th // page in this pallocBits. Each bit represents whether the page is in-use. func (b *pallocBits) pages64(i uint) uint64 { return (*pageBits)(b).block64(i) } // allocPages64 allocates a 64-bit block of 64 pages aligned to 64 pages according // to the bits set in alloc. The block set is the one containing the i'th page. func (b *pallocBits) allocPages64(i uint, alloc uint64) { (*pageBits)(b).setBlock64(i, alloc) } // findBitRange64 returns the bit index of the first set of // n consecutive 1 bits. If no consecutive set of 1 bits of // size n may be found in c, then it returns an integer >= 64. // n must be > 0. func findBitRange64(c uint64, n uint) uint { // This implementation is based on shrinking the length of // runs of contiguous 1 bits. We remove the top n-1 1 bits // from each run of 1s, then look for the first remaining 1 bit. p := n - 1 // number of 1s we want to remove. k := uint(1) // current minimum width of runs of 0 in c. for p > 0 { if p <= k { // Shift p 0s down into the top of each run of 1s. c &= c >> (p & 63) break } // Shift k 0s down into the top of each run of 1s. c &= c >> (k & 63) if c == 0 { return 64 } p -= k // We've just doubled the minimum length of 0-runs. // This allows us to shift farther in the next iteration. k *= 2 } // Find first remaining 1. // Since we shrunk from the top down, the first 1 is in // its correct original position. return uint(sys.TrailingZeros64(c)) } // pallocData encapsulates pallocBits and a bitmap for // whether or not a given page is scavenged in a single // structure. It's effectively a pallocBits with // additional functionality. // // Update the comment on (*pageAlloc).chunks should this // structure change. type pallocData struct { pallocBits scavenged pageBits } // allocRange sets bits [i, i+n) in the bitmap to 1 and // updates the scavenged bits appropriately. func (m *pallocData) allocRange(i, n uint) { // Clear the scavenged bits when we alloc the range. m.pallocBits.allocRange(i, n) m.scavenged.clearRange(i, n) } // allocAll sets every bit in the bitmap to 1 and updates // the scavenged bits appropriately. func (m *pallocData) allocAll() { // Clear the scavenged bits when we alloc the range. m.pallocBits.allocAll() m.scavenged.clearAll() }