Dear Café, We have a stream of ~900byte JSON documents (lazy ByteStrings) that we would like to validate (NB not parse) as quickly as possible. We just need to check that the bytes are a syntactically valid JSON document and then pass them elsewhere; we do not care what the content is. The documents are encoded in UTF-8 and have no extraneous whitespace. We have made a number of attempts: 1. Use Aeson to parse the documents and then discard the result. This was actually pretty good: Criterion clocked it at ~30us per document. 2. A hand-written Attoparsec parser. This did quite a bit worse: ~70us per document. 3. Use Get (from the binary package), which was a bit messier as it was factored to avoid all backtracking, but managed to get down to around ~20us per document. 4. Build a table-based, Word8-labelled DFA (with a minor hack to deal with values-within-values) and run it with ByteString.Lazy.foldl'. Also around ~20us per document. We didn't pursue the Attoparsec idea any further, but the other three all allocate ~85kB per document, and this now seems to be the limiting factor for performance. In more detail, looking at the threadscope output, we see it parse 5 or 6 documents in ~150us (allocating ~500kB) and then run the GC, which takes about another 150us. The GC stats indicate that it's only copying a couple of kB of live data each time, so most of that 500kB is junk. Now 85kB per document is ~96B per byte, which is only a pointer-and-a-half. It's not entirely surprising that there's this much allocation going on each time round the loop, but we'd like to try and get rid of it. I've tried the usual strictness tricks and have peered in a mystified fashion at the output of -ddump-simpl but am none the wiser. There's a copy of the relevant code for option 4 at https://github.com/DaveCTurner/json-validator. I've hacked around with it a bit since producing the numbers above, so it's now apparently a bit slower than Aeson but allocates less too (~65kB). Could anyone help, e.g. by pointing me at the bit in the Core that is allocating within the main loop? Many thanks, David