odoc_driver is inefficient

[jonludlam]

I often see odoc_driver being very slow indeed at executing tasks. In addition, ps contains a lot of zombie odocs, so I'm wondering if our eio code is slow at doing waitpid or something?

e.g.

jjl25     846055 17.0  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846056 18.8  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846057 20.2  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846058 23.1  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846059 28.1  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846060 32.2  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846061 42.5  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846062 52.7  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846070 83.3  0.0      0     0 pts/6    Z+   11:49   0:00 [odoc] <defunct>
jjl25     846071  158  0.0 305928 39872 pts/6    R+   11:49   0:00 odoc html-generate-source --impl _odocl/csv/src/impl-csv_tool_lib__Pretty.odocl --sidebar _index/__pkg__/sidebar.odoc-sidebar ../mono/_build/default/csv/src/pretty.ml --search-uri ./sherlodoc_db.js --search-uri sherlodoc.js -o _html/