• Create a collection with 240 M records containing nothing but default _id (i.e. db.c.insert({})).
• run mongoexport with a projection stage, e.g.

  mongoexport -d test -c c --type=csv -f _id -o /dev/null
  

Memory behavior as follows:

• (collection is being created A-B; during this period we see WT cache usage grow, and at A we see a difference of about 500 MB between allocated memory and WT cache; this is probably a small (~3%) WT accounting error related to unaccounted for allocator overhead. This is unrelated to this ticket.)
• starting at B the mongoexport query is running, and we see apparently linear memory growth outside WT cache ("tcmalloc allocated minus wt cache") rising to several GB
• the growth pattern appears to correspond to a growing buffer that is doubled in size every time more space is needed; at C and D we see temporary spikes, presumably corresponding to both the old buffer and the new expanded buffer existing simultaneously while the old data is copied to the new buffer.