miller/c

Data flow

Miller data flow is records produced by a record-reader in input/, followed by one or more mappers in mapping/, written by a record-writer in output/, controlled by logic in stream/. Argument parsing for initial stream setup is in cli/.

Memory management

Miller classes are in general highly modular, following a constructor/destructor model with minimal dependencies between classes. As a general rule, void-star payloads (sllv, lhmslv) must be freed by the callee (which has access to the data type) whereas non-void-star payloads (slls, hss) are freed by the container class.

One complication is for free-flags in lrec and slls: the idea is that an entire line is mallocked and presented by the record reader; then indivual fields are split out and populated into linked list or records. To reduce the amount of strduping there, free-flags are used to track which fields should be freed by the destructor and which are freed elsewhere.

The header_keeper object is an elaboration on this theme: suppose there is a CSV file with header line a,b,c and data lines 1,2,3, then 4,5,6, then 7,8,9. Then the keys a, b, and c are shared between all three records; they are retained in a single header_keeper object.

A bigger complication to the otherwise modular nature of Miller is its baton-passing memory-management model. Namely, one class may be responsible for freeing memory allocated by another class.

For example, using mlr cat: The record-reader produces records and returns pointers to them. The record-mapper is just a pass-through; it returns the record-pointers it receives. The record-writer formats the records to stdout and does not return them, so it is responsible for freeing them.

Similarly, mlr cut -x and any other mappers which modify record objects without creating new ones. By contrast,stats1 et al. produce their own records; they free what they do not pass on.

Performance optimizations

The initial implementation of Miller used hss (insertion-ordered string-to-string hash map) for record objects. Keys and values were strduped out of file-input lines. Each of the following produced from 5 to 30 percent performance gains:

• The lrec object is a hashless map suited to low access-to-creation ratio. See detailed comments in https://github.com/johnkerl/miller/blob/master/c/containers/lrec.h.
• Free-flags as discussed above removed additional occurrences of string copies.
• Using mmap to read files gets rid of double passes on record parsing (one to find end of line, and another to separate fields) as well as most use of malloc. Note however that standard input cannot be mmapped, so both record-reader options are retained.