search

rust-lang / rust-analyzer

A Rust compiler front-end for IDEs
https://rust-analyzer.github.io/
Apache License 2.0
14.35k stars 1.62k forks source link

Investigate: `la_arena::Arena` size effect #18403

Open Veykril opened 1 month ago

Veykril commented 1 month ago

la_arena::Arena is currently a Vec, that is 3 words in size but usually stored in a frozen location. For Vecs we usually turn them into Box<[T]> when stored as those are only 2 words in size, we can't do that with Arena's though as we have no equivalent. It would be interesting to see if we can save a tiny bit of memory if we could. That means, we should check the places where we store Arena's in hot definitions (frequently used defs stored in queries), and also try to apply the vec -> box pattern in more places (a bunch still use Vecs as using this pattern requires splitting out a builder pattern that transform into the Box'd field form when finished)

ChayimFriedman2 commented 1 month ago

There's also ArenaMap that is Vec, but it is only in bodies source map and those aren't a big part of our memory usage (around 70mb for analysis-stats . for bodies & source maps).

Giga-Bowser commented 3 weeks ago

I'm looking into this but it doesn't look super promising. There are definitely memory savings to be had but they are in the range of single digit megabytes even when looking at big projects (like rust-analyzer itself). It's still something that should get done eventually but I don't think it's super high priority.

As for why this is the case, you can add an extra column of output to print_memory_usage that shows the per-entry size (literally total size divided by number of entries) and see that most of these structs are big enough that shaving off 8 bytes is a minuscule improvement. Here's a sample output from analysis-stats -v . (truncated to the 10 biggest contributors) showing that. 

Total:               116.80s, 658ginstr, 1874mb
   285mb  27149     10kb MirBodyQuery
   236mb 416977     594b TraitSolveQuery
   215mb  60670    3722b FileItemTreeWithSourceMapQuery
   168mb  30160      5kb InferQuery
   149mb  80975    1939b MacroArgQuery
    88mb 253301     366b ImplDatumQuery
    86mb  32201    2814b BodyQuery
    80mb   1583     51kb ExpandProcMacroQuery
    46mb    253    187kb CrateDefMapQuery
    41mb 186047     233b AttrsQuery