gc/default/default.c: don't call malloc_usable_size when hint is present
#12490
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
…esent Depending on the allocator, `malloc_usable_size` may be very cheap or quite expensive. On `macOS` for instance, it's about as expensive as `malloc`. In many case we call `objspace_malloc_size` with as size we initially requested as `hint`. The real usable size may be a few bytes bigger, but since we only use that data to feed GC heuristics, I don't think it's very important to be perfectly accurate. It would make sense to call `malloc_usable_size` after growing a String or Array to use the extra capacity, but here we don't do that, so the call isn't worth its cost.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Depending on the allocator,
malloc_usable_sizemay be very cheap or quite expensive. OnmacOSfor instance, it's about as expensive asmalloc.In many case we call
objspace_malloc_sizewith as size we initially requested ashint. The real usable size may be a few bytes bigger, but since we only use that data to feed GC heuristics, I don't think it's very important to be perfectly accurate.It would make sense to call
malloc_usable_sizeafter growing a String or Array to use the extra capacity, but here we don't do that, so the call isn't worth its cost.References:
ruby/jsonexperimental branch profile showing a big overhead frommalloc_sizeon macOS: https://share.firefox.dev/40dqHXkmalloc_sizeperformance: https://lemire.me/blog/2017/09/15/how-fast-are-malloc_size-and-malloc_usable_size-in-c/