Re: [boost] Reminder: Review for Zach's parsing library starts on Feb 19th

Zach Laine wrote:

On Fri, Mar 1, 2024 at 2:15 AM Peter Dimov wrote:

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Zach Laine wrote:

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I'm not sure what the tests reported earlier were doing, but I definitely don't see orders of magnitude difference.

I used this file:

https://github.com/boostorg/json/blob/develop/bench/data/twitter.j son

Using that file, I get similar results to what I already posted (the numbers are different, but the ratios between them are the same).

I managed to make a change that was much smaller -- no template parameter required. I consistenty see 1.5x slowdown for Parser vs. Boost.JSON for files around this size, and 2x or so for much larger files (~25MB).

That's probably because you are measuring I/O in addition to parsing.

https://github.com/cmazakas/parser-review/blob/main/test/json.cpp

only measures the parsing part.

Quite right! After building it locally, I do see an advantage to the template parameter approach -- that is, controlling the contents of scoped_trace_t with an NTTP that is true/false when trace is enabled/disabled. scoped_trace_t is specialized to be empty when the NTTP is false. I'll be committing the template param-based approach soon.

For those keeping track, with Christian's test framework, I get 35X worse performance of the JSON parser example vs. Boost.JSON. This seems like a reasonable factor to me. I wrote that example in a single, and made no attempt to optimize it in any way; it is an example. Boost.JSON is a dedicated JSON parser emphasizing efficiency.

There's probably some room for Boost.Parser automatically applying certain optimizations to "naively" written parsers. For example, a lot of the time here is spent in skipping whitespace, using this rule: auto const ws_def = '\x09'_l | '\x0a' | '\x0d' | '\x20'; This generates an or_parser that contains four separate omit_parsers, each with a char_ inside. If I change it by hand to auto const ws_def = bp::omit[ bp::char_( "\x09\x0a\x0d\x20" ) ]; which I assume is equivalent, time improves from 365ms to 316ms (*). And that's still far from optimal, because (1) skipping whitespace can in principle be done with the equivalent of `find_first_not_of` and (2) the characters are a runtime value in the above, but something like `cxchar<'\0x09', '\0x0a', '\0x0d', '\x20'>` could be even faster. It should be possible for the library to automatically turn the "naive" ws_def into something more optimal, and then special-case `skip` for this common case. In the general case this is complicated by the existence of attributes and semantic actions; transforming parsers should take care to preserve those. But in this specific case none are present. (*) Under clang-cl. GCC -O3 has different views of the world and times there are 165ms and 181ms, respectively.