Tag Archives: benchmark

URL parser performance

URLs is a dear subject of mine on this blog, as readers might have noticed.

“URL” is this mythical concept of a string that identifies a resource online and yet there is no established standard for its syntax. There are instead multiple ones out of which one is on purpose “moving” so it never actually makes up its mind but instead keeps changing.

This then leads to there being basically no two URL parsers that treat URLs the same, to the extent that mixing parsers is considered a security risk.

The standards

The browsers have established their WHATWG URL Specification as a “living document”, saying how browsers should parse URLs, gradually taking steps away from the earlier established RFC 3986 and RFC 3987 attempts.

The WHATWG standard keeps changing and the world that tries to stick to RFC 3986 still needs to sometimes get and adapt to WHATWG influences in order to interoperate with the browser-centric part of the web. It leaves URL parsing and “URL syntax” everywhere in a sorry state.


In the curl project we decided in 2018 to help mitigate the mixed URL parser problem by adding a URL parser API so that applications that use libcurl can use the same parser for all its URL parser needs and thus avoid the dangerous mixing part.

The libcurl API for this purpose is designed to let users parse URLs, to extract individual components, to set/change individual components and finally to extract a normalized URL if wanted. Including some URL encoding/decoding and IDN support.


Thanks to the availability and functionality of the public libcurl URL API, we could build and ship the separate trurl tool earlier this year.


Some time ago I was made aware of an effort to (primarily) write a new URL parser for node js – although the parser is stand-alone and can be used by anyone else who wants to: The Ada URL Parser. The two primary developers behind this effort, Yagiz Nizipli and Daniel Lemire figured out that node does a large amount of URL parsing so by speeding up this parser alone it would apparently have a general performance impact.

Ada is C++ project designed to parse WHATWG URLs and the first time I was in contact with Yagiz he of course mentioned how much faster their parser is compared to curl’s.

You can also see them reproduce and talk about these numbers on this node js conference presentation.


Everyone who ever tried to write code faster than some other code has found themselves in a position where they need to compare. To benchmark one code set against the other. Benchmarking is an art that is close to statistics and marketing: very hard to do without letting your own biases or presumptions affect the outcome.

Speed vs the rest

After I first spoke with Yagiz, I did go back to the libcurl code to see what obvious mistakes I had done and what low hanging fruit there was to pick in order to speed things up a little. I found a few flaws that maybe did a minor difference, but in my view there are several other properties of the API that is actually more important than sheer speed:

  • non-breaking API and ABI
  • readable and maintainable code
  • sensible and consistent API
  • error codes that help users understand what the problem is

Of course, there is also the thing that if you first figure out how to parse a URL the fastest way, maybe you can work out a smoother API that works better with that parsing approach. That’s not how I went about when creating the libcurl API.

If we can maintain those properties mentioned above, I still want the parser to run as fast as possible. There is no point in being slower than necessary.

URLs vs URLs

Ada parses WHATWG URLs and libcurl parses RFC 3986 URLs. They parse URLs differently and provide different feature sets. They are not interchangeable.

In Ada’s benchmarks they have ignored the parser differences. Throw the parsers against each other, and according to all their public data since early 2023 their parser is 7-8 times faster.

700% faster really?

So how on earth can you make such a simple thing as URL parsing 700% faster? It never sat right with me when they claimed those numbers but since I had not compared them myself I trusted them. After all, they should be fairly easy to compare and they seemed clueful enough.

Until recently when I decided to reproduce their claims and see how much their numbers depends on their specific choices of URLs to parse. It taught me something.

Reproduce the numbers

In my tests, their parser is fast. It is clearly faster than the libcurl parser, and I too of course ignored the parser results since they would not be comparable anyway.

In my tests on my development machine, Ada is 1.25 – 1.8 times faster than libcurl. There is no doubt Ada is faster, just far away from the enormous difference they claim. How come?

  1. You use the input data that most favorably shows a difference
  2. You run the benchmark on a hardware for which your parser has magic hardware acceleration

I run a decently modern 13th gen Intel Core-I7 i7-13700K CPU in my development machine. It’s really fast, especially on single-thread stuff like this. On my machine, the Ada parser can parse more URLs/second than even the Ada people themselves claim, which just tells us they used slower machines to test on. Nothing wrong with that.

The Ada parser has code that is using platform specific instructions on some environments and the benchmark they decide to use when boasting about their parser was done on such a platform. An Apple m1 CPU to be specific. In most aspects except performance per watt, not a speed monster CPU.

In itself this is not wrong, but maybe a little misleading as this is far from clearly communicated.

I have a script, urlgen. that generates URLs in as many combinations as possible so that the parser’s every corner and angle are suitably exercised and verified. Many of those combination therefor illegal in subtle ways. This is the set of URLs I have thrown at the curl parser mostly, which then also might explain why this test data is the set that makes Ada least favorable (at 1.26 x the libcurl speed). Again: their parser is faster, no doubt. I have not found a test case that does not show it running faster than libcurl’s parser.

A small part of the explanation of how they are faster is of course that they do not provide the result, the individual components, in their own separately allocated strings.

Here’s a separate detailed document how I compared.

More mistakes

They also repeatably insist curl does not handle International Domain Names (IDN) correctly, which I simply cannot understand and I have not got any explanation for. curl has handled IDN since 2004. I’m guessing a mistake, an old bug or that they used a curl build without IDN support.


I would think a primary argument against using Ada vs libcurl’s parser is its size and code. Not that I believe that there are many situations where users are actually selecting between these two.

Ada header and source files are 22,774 lines of C++

libcurl URL API header and source files are 2,103 lines of C.

Comparing the code sizes like this is a little unfair since Ada has its own IDN management code included, which libcurl does not, and that part comes with several huge tables and more.

Improving libcurl?

I am sure there is more that can be done to speed up the libcurl URL parser, but there is also the case of diminishing returns. I think it is pretty fast already. On Ada’s test case using 100K URLs from Wikipedia, libcurl parses them at an average of 178 nanoseconds per URL on my machine. More than 5.6 million real world URLs parsed per second per core.

This, while also storing each URL component in a separate allocation after each parse, and also returning an error code that helps identifying the problem if the URL fails to parse. With an established and well-documented API that has been working since 2018 .

The hardware specific magic Ada uses can possibly be used by libcurl too. Maybe someone can try that out one day.

I think we have other areas in libcurl where work and effort are better spent right now.

curl performance

Benchmarks, speed, comparisons, performance. I get a lot of questions on how curl and libcurl compare against other tools or libraries, and I rarely have any specific answers as I personally basically never use or test any other tools or libraries!

This text will instead be more elaborate on how we work on libcurl, why I believe libcurl will remain the fastest alternative.


libcurl is low-level

libcurl is written in C and uses the native function calls of the operating system to perform its network operations. It features a lot of features, but when it comes to plain sending and receiving data the code paths are very short and the loops can’t be shortened or sped up by any significant amount. Based on these facts, I am confident that for simple single stream transfers you really cannot write a file transfer library that runs faster. (But yes, I believe other similarly low-level style libraries can reach the same speeds.)

When adding more complicated test cases, like doing SSL or perhaps many connections that need to be kept persistent between transfers, then of course libraries can start to differ. libcurl uses SSL libraries natively so if they are fast, so will libcurl’s SSL handling be and vice versa. Of course we also strive to provide features such as connection pooling, SSL session id reuse, DNS caching and more to make the normal and frequent use cases as fast as we possibly can. What takes time when using libcurl should be the underlying network operations, not the tricks libcurl adds to them.

event-based is the way to grow

If you plan on making an app that uses more than just a few connections, libcurl can of course still do the heavy lifting for you. You should consider taking precautions already when you do your design and make sure that you can use an event-based concept and avoid relying on select or poll for the socket handling. Using libcurl’s multi_socket API, you can go up and beyond tens of thousands of connections and still reach maximum performance. And this using basically all of the protocols libcurl supports, this is not limited to a small subset. (There are unfortunate exceptions, like for example “file://” URLs but there are completely technical reasons for this.)

Very few file transfer libraries have this direct support for event-based operations. I’ve read reports of apps that have gone up to and beyond 70,000 connections on the same host using libcurl like this. The fact that TCP only has a 16 bit field in the protocol header for “source port” of course forces users who want to try this stunt to use more than one interface as source address.

And before you ask: you cannot grow a client to that amount using any other technique than event-based using many connections in the same thread, as basically no other approach scales as well.

When handling very many connections, the mere “juggling” of the connections take time and can be done in good or bad ways. It would be interesting to one day measure exactly how good libcurl is at this.

Binding Benchmark and Comparisons

We are aware of something like 40 different bindings for libcurl to make it possible to use from just about any language you like. Lots, if not just every, language also tend to have its own native version of a transfer or at least HTTP library. For many languages, the native version is the one that is most preferred and most used in books, articles and promoted on the internet. Rarely can the native versions compete with the libcurl based ones in actual transfer performance. Because of what I mentioned above: libcurl does little extra when transferring stuff but the actual and raw transfer. Of course there still needs to be additional glue and logic to make libcurl work fine with the different languages’ own unique environments, but it is still often proven that it doesn’t make the speed gain get lost or become invisible. I’ll illustrate below with some sample environments.

Ruby comparisons

Paul Dix is a Ruby guy and he’s done a lot of work with HTTP libraries and Ruby, and he’s also done some benchmarks on libcurl-based Ruby libraries. They show that the tools built on top of libcurl run significantly faster than the native versions.

Perl comparisons

“Ivan” wrote up a benchmarking script that performs a number of transfers using three different mechanisms available to perl hackers. One of them being the official libcurl perl binding (WWW::Curl), one of them being the perl standard one called LWP. The results leaves no room for doubts: the libcurl-based version is significantly faster than the “native” alternatives.

PHP comparisons

The PHP binding for libcurl, PHP/CURL, is a popular one. In PHP the situation is possibly a bit different as they don’t have a native library that is nearly as feature complete as the libcurl binding, but they do have a native version for doing things like getting HTTP data etc. This function has been compared against PHP/CURL many times, for example Ricky’s comparisons, and Alix Axel’s comparisons. They all show that the libcurl-based alternative is faster. Exactly how much faster is of course depending on a lot of factors but I’m not going into such specific details here and now.

We miss more benchmarks!

I wish I knew about more benchmarks and comparisons of speed. If you know of others, or if you get inspired enough to write up and publish any after reading my rant here, please let me know! Not only is it fun and ego-boosting to see our project win, but I also want to learn from them and see where we’re lacking and if anyone beats us in a test, it’d be great to see what we could do to improve.