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rustdoc-search: shard the search result descriptions ## Preview This makes no visual changes to rustdoc search. It's a pure perf improvement. <details><summary>old</summary> Preview: <http://notriddle.com/rustdoc-html-demo-10/doc/std/index.html?search=vec> WebPageTest Comparison with before branch on a sort of worst case (searching `vec`, winds up downloading most of the shards anyway): <https://www.webpagetest.org/video/compare.php?tests=240317_AiDc61_2EM,240317_AiDcM0_2EN> Waterfall diagram:  </details> Preview: <http://notriddle.com/rustdoc-html-demo-10/doc2/std/index.html?search=vec> WebPageTest Comparison with before branch on a sort of worst case (searching `vec`, winds up downloading most of the shards anyway): <https://www.webpagetest.org/video/compare.php?tests=240322_BiDcCH_13R,240322_AiDcJY_104>  ## Description r? `@GuillaumeGomez` The descriptions are, on almost all crates[^1], the majority of the size of the search index, even though they aren't really used for searching. This makes it relatively easy to separate them into their own files. Additionally, this PR pulls out information about whether there's a description into a bitmap. This allows us to sort, truncate, *then* download. This PR also bumps us to ES8. Out of the browsers we support, all of them support async functions according to caniuse. https://caniuse.com/async-functions [^1]: <https://microsoft.github.io/windows-docs-rs/>, a crate with 44MiB of pure names and no descriptions for them, is an outlier and should not be counted. But this PR should improve it, by replacing a long line of empty strings with a compressed bitmap with a single Run section. Just not very much. ## Detailed sizes ```console $ cat test.sh set -ex cp ../search-index*.js search-index.js awk 'FNR==NR {a++;next} FNR<a-3' search-index.js{,} | awk 'NR>1 {gsub(/\],\\$/,""); gsub(/^\["[^"]+",/,""); print} {next}' | sed -E "s:\\\\':':g" > search-index.json jq -c '.t' search-index.json > t.json jq -c '.n' search-index.json > n.json jq -c '.q' search-index.json > q.json jq -c '.D' search-index.json > D.json jq -c '.e' search-index.json > e.json jq -c '.i' search-index.json > i.json jq -c '.f' search-index.json > f.json jq -c '.c' search-index.json > c.json jq -c '.p' search-index.json > p.json jq -c '.a' search-index.json > a.json du -hs t.json n.json q.json D.json e.json i.json f.json c.json p.json a.json $ bash test.sh + cp ../search-index1.78.0.js search-index.js + awk 'FNR==NR {a++;next} FNR<a-3' search-index.js search-index.js + awk 'NR>1 {gsub(/\],\\$/,""); gsub(/^\["[^"]+",/,""); print} {next}' + sed -E 's:\\'\'':'\'':g' + jq -c .t search-index.json + jq -c .n search-index.json + jq -c .q search-index.json + jq -c .D search-index.json + jq -c .e search-index.json + jq -c .i search-index.json + jq -c .f search-index.json + jq -c .c search-index.json + jq -c .p search-index.json + jq -c .a search-index.json + du -hs t.json n.json q.json D.json e.json i.json f.json c.json p.json a.json 64K t.json 800K n.json 8.0K q.json 4.0K D.json 16K e.json 192K i.json 544K f.json 4.0K c.json 36K p.json 20K a.json ``` These are, roughly, the size of each section in the standard library (this tool actually excludes libtest, for parsing-json-with-awk reasons, but libtest is tiny so it's probably not important). t = item type, like "struct", "free fn", or "type alias". Since one byte is used for every item, this implies that there are approximately 64 thousand items in the standard library. n = name, and that's now the largest section of the search index with the descriptions removed from it q = parent *module* path, stored parallel to the items within D = the size of each description shard, stored as vlq hex numbers e = empty description bit flags, stored as a roaring bitmap i = parent *type* index as a link into `p`, stored as decimal json numbers; used only for associated types; might want to switch to vlq hex, since that's shorter, but that would be a separate pr f = function signature, stored as lists of lists that index into `p` c = deprecation flag, stored as a roaring bitmap p = parent *type*, stored separately and linked into from `i` and `f` a = alias, as [[key, value]] pairs ## Search performance http://notriddle.com/rustdoc-html-demo-11/perf-shard/index.html For example, in stm32f4: <table><thead><tr><th>before<th>after</tr></thead> <tbody><tr><td> ``` Testing T -> U ... in_args = 0, returned = 0, others = 200 wall time = 617 Testing T, U ... in_args = 0, returned = 0, others = 200 wall time = 198 Testing T -> T ... in_args = 0, returned = 0, others = 200 wall time = 282 Testing crc32 ... in_args = 0, returned = 0, others = 0 wall time = 426 Testing spi::pac ... in_args = 0, returned = 0, others = 0 wall time = 673 ``` </td><td> ``` Testing T -> U ... in_args = 0, returned = 0, others = 200 wall time = 716 Testing T, U ... in_args = 0, returned = 0, others = 200 wall time = 207 Testing T -> T ... in_args = 0, returned = 0, others = 200 wall time = 289 Testing crc32 ... in_args = 0, returned = 0, others = 0 wall time = 418 Testing spi::pac ... in_args = 0, returned = 0, others = 0 wall time = 687 ``` </td></tr><tr><td> ``` user: 005.345 s sys: 002.955 s wall: 006.899 s child_RSS_high: 583664 KiB group_mem_high: 557876 KiB ``` </td><td> ``` user: 004.652 s sys: 000.565 s wall: 003.865 s child_RSS_high: 538696 KiB group_mem_high: 511724 KiB ``` </td></tr> </table> This perf tester is janky and unscientific enough that the apparent differences might just be noise. If it's not an order of magnitude, it's probably not real. ## Future possibilities * Currently, results are not shown until the descriptions are downloaded. Theoretically, the description-less results could be shown. But actually doing that, and making sure it works properly, would require extra work (we have to be careful to avoid layout jumps). * More than just descriptions can be sharded this way. But we have to be careful to make sure the size wins are worth the round trips. Ideally, data that’s needed only for display should be sharded while data needed for search isn’t. * [Full text search](https://internals.rust-lang.org/t/full-text-search-for-rustdoc-and-doc-rs/20427) also needs this kind of infrastructure. A good implementation might store a compressed bloom filter in the search index, then download the full keyword in shards. But, we have to be careful not just of the amount readers have to download, but also of the amount that [publishers](https://gist.github.com/notriddle/c289e77f3ed469d1c0238d1d135d49e1) have to store. |
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The Rust Programming Language
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