Post #4149095

RE: https://hachyderm.io/@mitchellh/116993012549857824 If you're interested in "how" or "why": the major culprit is that each row in Alacritty has 32 bytes of metadata and each cell is 24 bytes. In Ghostty, every row and cell is represented by exactly 8 byte each. How? The first major culprit is styles. Alacritty stores the full cell style alongside each cell (foreground, background, underline, etc.). Ghostty stores a 16-bit style ID and de-dupes all styles into a look-aside custom reference-counted hash table. Next, codepoints. Alacritty stores multi-codepoint graphemes (like, Emoji) by having an 8-byte nullable pointer to a `Vec`. This hurts doubly: (1) its almost always null (because multi-codepoint is rare) yet you pay an 8 byte cost on every cell and (2) every multi-codepoint grapheme triggers a heap allocation to make that Vec. Ghostty stores single codepoints inline, but multiple codepoints in a look-aside table. The memory for this table uses a custom bitmap-tracked chunk-allocator (since grapheme frequency follows a measurable curve we calculated by scanning various online texts). The presence of graphemes is marked by a 2-bit content tag in our packed 64-bit cell. To keep the key small in the hash table, its limited to a 16-bit unsigned int that is an offset from a base pointer. Okay, the astute systems programmer will quickly notice there are a lot of 16-bit integers and ask: so this is all limited to a max of ~65K values? Nay. We maintain our grid using a linked list of contiguous ~400KB memory chunks (which themselves are in a memory pool using a custom allocator to speed up alloc/free). Each memory chunk is limited to 2^16. If/when we reach a limit, we move to the next page. In practice, this really doesn't happen except under pathological cases... the important point is we handle it. Lots, lots, lots more details, but thats a 10,000 foot view. These things alone account for ~95% of the difference of our uncompressed vs. Alacritty's uncompressed memory usage. (Theres also a reason why Alacritty's data structures aren't trivially compressable but thats a whole other topic)