__txn_logrec_init and __wt_txn_log_op functions write packed integers. They use the packing API in src/include/packing_inline.h to do this. The packing API is a very powerful, general serialization API that allows callers to serialize a set of types as a compact binary formation. For instance, "IIIuu" means serialize three unsigned integers and two WT_ITEMs.

Unfortunately, this general API has a lot of overhead that can mitigated by special casing the code instead. Rather then code calling __wt_struct_size/__wt_struct_pack, WT logging code should call the __wt_vsize_uint/__wt_vpackuint functions directly. In micro benchmarks, this results in a ~20x perf improvement. See PERF-4608.

• A definition of done,
  Change dist/log.py to generate code that bypasses __wt_struct_size/__wt_struct_pack and calls __wt_vsize_uint/__wt_vpackuint directly.

• The reproducing steps, and
  https://github.com/mongodb/mongo/compare/master...markbenvenuto:mongo:sep_bm_wt_pack?expand=1

• Links to other relevant tickets/resources.
  PERF-4608