Parse base64-encoded data URIs more efficiently #10434
Open
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
(in some places) Very long data: URIs in source documents are causing outsized memory usage due to various parsing inefficiencies, for instance in Network.URI, TagSoup, and T.P.R.Markdown.source. See e.g. jgm#10075. This change improves the situation in a couple places we can control relatively easily by using an attoparsec text-specialized parser to consume base64-encoded strings. Attoparsec's takeWhile + inClass functions are designed to chew through long strings like this without doing unnecessary allocation, and the improvements in peak heap allocation are significant. One of the observations here is that if you parse something as a valid data: uri it shouldn't need any further escaping so we can short-circuit various processing steps that may unpack/iterate over the chars in the URI.
jgm
reviewed
Dec 4, 2024
Comment on lines
+25
to
+39
| parseBase64String = do | ||
| Sources ((pos, txt):rest) <- getInput | ||
| let r = A.parse pBase64 txt | ||
| case r of | ||
| Done remaining consumed -> do | ||
| let pos' = incSourceColumn pos (T.length consumed) | ||
| setInput $ Sources ((pos', remaining):rest) | ||
| return consumed | ||
| _ -> mzero | ||
|
|
||
| pBase64 :: A.Parser Text | ||
| pBase64 = do | ||
| most <- A.takeWhile1 (A.inClass "A-Za-z0-9+/") | ||
| rest <- A.takeWhile (== '=') | ||
| return $ most <> rest |
There was a problem hiding this comment.
Two thoughts on this:
- Is attoparsec really necessary? Why not just
Data.Text.takeWhile? - My experience is that parsers like this, which just manipulate the input directly using
getInputandsetInput, are problematic in parsec because parsec doesn't realize that input has been consumed. I've had to use a regular parsec parser somewhere in there to make it realize this. One option is just something likecount charsConsumed anyChar, and then you don't need to compute the end position manually...
There was a problem hiding this comment.
- In this spot I can possibly just borrow the fast
inClassfunction from attoparsec and use it with regular texttakeWhile, will have to fiddle with it. - will investigate, I took it for granted that I would make the parsec state happy by fiddling with the input as seen here but that was not based on deep understanding or rigorous analysis.
There was a problem hiding this comment.
It's probably fine to use attoparsec, but there might be a slight speedup if you can avoid it.
On 2: you could try putting this parser under many and see if parsec complains.
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.
(in some places)
Very long data: URIs in source documents are causing outsized memory usage due to various parsing inefficiencies, for instance in Network.URI, TagSoup, and T.P.R.Markdown.source. See e.g. #10075.
This change improves the situation in a couple places we can control relatively easily by using an attoparsec text-specialized parser to consume base64-encoded strings. Attoparsec's takeWhile + inClass functions are designed to chew through long strings like this without doing unnecessary allocation, and the improvements in peak heap allocation are significant.
One of the observations here is that if you parse something as a valid data: uri it shouldn't need any further escaping so we can short-circuit various processing steps that may unpack/iterate over the chars in the URI.
The code here is organized a little bit randomly and I expect it needs more work to go in.
Good improvements though. The peak heap allocation for html -> markdown for the example provided by @ebeigarts in #10075 goes down from ~2900MB to ~2500 MB on my computer, and markdown -> json goes from 1577 MB to 73 MB! As discussed in the issue comments HTML reading has, at least, a TagSoup issue.