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Comment by klysm | Hacker News Reader

In what context is json slower than protobuf?

Did you mean to ask the reverse question (in what context is protobuf slower than json)? Because that's definitely the question on my mind, since GP's assertion runs counter to my expectations and experience.

JSON is a heavy-weight format that requires significantly more memory for both serialization and deserialization, and the representations of values in JSON are optimized for human consumption rather than machine consumption.

Just listing a few examples:

- Strings in JSON require you to scan for the terminating quotation mark that isn't escaped. Meanwhile, in protobuf, the length of the string is given to you; you can just grab the bytes directly.

- Parsing an integer in JSON requires multiplication by 10 and addition / subtraction for each digit. Meanwhile, in protobuf, fixed64 and related types are either in host order or are just a ntohl away; int64 and other varint types only require bit twiddling (masks, shifts, etc). Do you think it's easier to parse "4294967296" from a string, or 5 bytes along the lines of {0x88, 0x80, 0x80, 0x80, 0x00}?

- Having a format agreed-upon ahead of time (protobuf) means that your keys don't require more string parsing (JSON).

The benchmarks available for protobuf generally have it parsing like 5x slower than json (and I suspect the payloads are smaller, but not 5x smaller). I don't think that the code generators shipped with protobuf generate parsers of comparable quality to simdjson, so it's a bit unfair in that sense.

Can you point to some of these benchmarks? https://news.ycombinator.com/item?id=26934854 suggests that in at least one synthetic benchmark (with a 7.5KB protobuf message which expands to a 17KB JSON payload), protobuf parsing at 2GB/s would be comparable to JSON parsing at 5GB/s.

Meanwhile, simdjson's numbers (https://github.com/simdjson/simdjson/blob/master/doc/gbps.pn...) show a peak parsing speed of about 3GB/s depending on the workload. Of course, it's not clear you can compare these directly, since they were probably not run on systems with comparable specs. But it's not clear to me that there's a 5x difference.

Perhaps my experience differs because I'm used to seeing very large messages being passed around, but I'd be happy to reconsider. (Or maybe I should go all-in on Cap'n Proto.)

> - Parsing an integer in JSON requires multiplication by 10 and addition / subtraction for each digit. Meanwhile, in protobuf, fixed64 and related types are either in host order or are just a ntohl away; int64 and other varint types only require bit twiddling (masks, shifts, etc). Do you think it's easier to parse "4294967296" from a string, or 5 bytes along the lines of {0x88, 0x80, 0x80, 0x80, 0x00}?

For this one, actually I think the varint may be harder because you have to parse it before you know which byte the next value starts on, but recently there has been some progress in the area of fast varint parsers. For parsing decimal numbers, a good start is here http://0x80.pl/articles/simd-parsing-int-sequences.html. Some users at https://highload.fun/tasks/1/leaderboard are calculating the sum of parsed base-10 integers at about the speed of reading the string from RAM, but this task is subtly easier than parsing each number individually, which may only be doable at half or a quarter of the speed of reading the string from RAM, and then you'd have to pay a bit more to also write out the parsed values to another buffer.

From the intro of your first link:

> While conversion from a string into an integer value is feasible with SIMD instructions, this application is unpractical. For typical cases, when a single value is parsed, scalar procedures — like the standard atoi or strtol — are faster than any fancy SSE code.

> However, SIMD procedures can be really fast and convert in parallel several numbers. There is only one "but": the input data has to be regular and valid, i.e. the input string must contain only ASCII digits.

There definitely are some benefits and speedups available with SIMD, but that intro doesn't inspire a whole lot of confidence in its relevance to JSON parsing, where the only case where you might have this regularity is if you definitely have an array of integers. (JSON strings are not restricted to ASCII, as they can and do include Unicode.)

I think you'd have to pay some additional copies to perform batch processing of integers in json documents in the general case. Last I checked simdjson included the typical scalar code for parsing base-10 integers and a fairly expensive procedure for parsing base-10 doubles (where most of the runtime is paid in exchange for getting the final bit of the mantissa right, which was not reasonable for our use case but is reasonable for a general-purpose library).

That said, it's not clear to me that the scalar integer parsing code should win even if you're only parsing integers individually. For inputs that have the length of the number vary unpredictably, it pays a significant amount of time for branch misses, while the vector code can replace this with a data dependency.

Edit: After writing the above, I thought that probably most documents have a regular pattern of number lengths. I don't know if this works well with branch predictors if number of branches in the pattern is pretty long (in terms of the sum of the lengths), but probably the branches cost ~nothing for a lot of real-world inputs.

Run benchmarks. It doesn’t matter what I think is easier to parse.

Protobuff is slower than JSON.parse in node by 4-8x for my data sets: large reference data needed.

I was only measuring decode time, since that’s I can recompute my data well in advance.

In browser Javascript, because there you have to load the decoder which is already slower to parse than JSON, then run it in the JS vm to do the actual decoding whereas JSON is built in and has a native highly-optimized parser.

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