vole
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| 1 | package pages |
| 2 | |
| 3 | // The blob cache is required, not a nicety: cold git reads are ~1.2 ms p50, |
| 4 | // and the same reads served from memory are 0.075 ms — the difference between a |
| 5 | // docs site that feels like a file server and one that feels like a database. |
| 6 | |
| 7 | import ( |
| 8 | "container/list" |
| 9 | "strings" |
| 10 | "sync" |
| 11 | ) |
| 12 | |
| 13 | type cacheEntry struct { |
| 14 | key string |
| 15 | commit string |
| 16 | sha string |
| 17 | ctype string |
| 18 | body []byte |
| 19 | modUnix int64 |
| 20 | } |
| 21 | |
| 22 | type lruCache struct { |
| 23 | mu sync.Mutex |
| 24 | max int64 |
| 25 | used int64 |
| 26 | ll *list.List // front = most recently used |
| 27 | items map[string]*list.Element |
| 28 | // maxEntry keeps one large file from evicting the whole site. Bodies bigger |
| 29 | // than this are served from git every time. |
| 30 | maxEntry int64 |
| 31 | // lookups caches path resolution: (commit, path) → tree entry. Without it a |
| 32 | // warm request still walks the commit and its trees on every hit, which is |
| 33 | // most of the cold cost the cache exists to remove. |
| 34 | lookups map[string]treeEntry |
| 35 | } |
| 36 | |
| 37 | func newLRU(max int64) *lruCache { |
| 38 | if max <= 0 { |
| 39 | max = 32 << 20 |
| 40 | } |
| 41 | return &lruCache{ |
| 42 | max: max, |
| 43 | maxEntry: min(max/8, 1<<20), |
| 44 | ll: list.New(), |
| 45 | items: map[string]*list.Element{}, |
| 46 | lookups: map[string]treeEntry{}, |
| 47 | } |
| 48 | } |
| 49 | |
| 50 | // lookupGet returns a cached resolution. |
| 51 | func (c *lruCache) lookup(key string) (treeEntry, bool) { |
| 52 | c.mu.Lock() |
| 53 | defer c.mu.Unlock() |
| 54 | e, ok := c.lookups[key] |
| 55 | return e, ok |
| 56 | } |
| 57 | |
| 58 | func (c *lruCache) setLookup(key string, e treeEntry) { |
| 59 | c.mu.Lock() |
| 60 | defer c.mu.Unlock() |
| 61 | // Path resolutions are tiny; a plain cap keeps the map from growing with |
| 62 | // history. Clearing is cheap and only costs a walk on the next request. |
| 63 | if len(c.lookups) > 8192 { |
| 64 | c.lookups = map[string]treeEntry{} |
| 65 | } |
| 66 | c.lookups[key] = e |
| 67 | } |
| 68 | |
| 69 | func (c *lruCache) get(key string) (*cacheEntry, bool) { |
| 70 | c.mu.Lock() |
| 71 | defer c.mu.Unlock() |
| 72 | el, ok := c.items[key] |
| 73 | if !ok { |
| 74 | return nil, false |
| 75 | } |
| 76 | c.ll.MoveToFront(el) |
| 77 | return el.Value.(*cacheEntry), true |
| 78 | } |
| 79 | |
| 80 | func (c *lruCache) put(e *cacheEntry) { |
| 81 | size := int64(len(e.body)) |
| 82 | if size > c.maxEntry || size > c.max { |
| 83 | return // not cacheable; served straight from git |
| 84 | } |
| 85 | c.mu.Lock() |
| 86 | defer c.mu.Unlock() |
| 87 | if old, ok := c.items[e.key]; ok { |
| 88 | c.used -= int64(len(old.Value.(*cacheEntry).body)) |
| 89 | c.ll.Remove(old) |
| 90 | delete(c.items, e.key) |
| 91 | } |
| 92 | el := c.ll.PushFront(e) |
| 93 | c.items[e.key] = el |
| 94 | c.used += size |
| 95 | for c.used > c.max { |
| 96 | back := c.ll.Back() |
| 97 | if back == nil { |
| 98 | break |
| 99 | } |
| 100 | victim := back.Value.(*cacheEntry) |
| 101 | c.ll.Remove(back) |
| 102 | delete(c.items, victim.key) |
| 103 | c.used -= int64(len(victim.body)) |
| 104 | } |
| 105 | } |
| 106 | |
| 107 | // dropCommit forgets every path of one commit. A push moves the ref, so the old |
| 108 | // commit's pages are unreachable and their memory may as well be freed now |
| 109 | // rather than when the LRU happens to reach them. |
| 110 | func (c *lruCache) dropCommit(commit string) { |
| 111 | if commit == "" { |
| 112 | return |
| 113 | } |
| 114 | c.mu.Lock() |
| 115 | defer c.mu.Unlock() |
| 116 | for key, el := range c.items { |
| 117 | e := el.Value.(*cacheEntry) |
| 118 | if e.commit != commit { |
| 119 | continue |
| 120 | } |
| 121 | c.ll.Remove(el) |
| 122 | delete(c.items, key) |
| 123 | c.used -= int64(len(e.body)) |
| 124 | } |
| 125 | for key := range c.lookups { |
| 126 | if strings.HasPrefix(key, commit+"\x00") { |
| 127 | delete(c.lookups, key) |
| 128 | } |
| 129 | } |
| 130 | } |
| 131 | |
| 132 | func (c *lruCache) stats() (entries int, bytes int64) { |
| 133 | c.mu.Lock() |
| 134 | defer c.mu.Unlock() |
| 135 | return len(c.items), c.used |
| 136 | } |
| 137 | |
| 138 | func min(a, b int64) int64 { |
| 139 | if a < b { |
| 140 | return a |
| 141 | } |
| 142 | return b |
| 143 | } |
| 144 |