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module: skip directories known not to exist

Rodrigo Muino Tomonari requested to merge github/fork/bnoordhuis/skip-enoent-module-dirs into master

There is no point in trying to search for files in a directory that we know does not exist, so stop doing that.

Reduces the total number of stat(2) calls and the number of stat(2) misses on a medium-sized application by about 21% and 29% respectively.

Reduces the total number of package.json open(2) calls and the number of open(2) misses by about 21% and 93% (!) respectively.

Before:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 50.93    0.178419          38      4702           lstat
 29.08    0.101875          36      2800      2010 stat
 11.36    0.039796          43       932       215 open
  5.39    0.018897          34       550           fstat
  3.24    0.011337          34       336           pread
------ ----------- ----------- --------- --------- ----------------
100.00    0.350324                  9320      2225 total

After:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 55.49    0.176638          38      4702           lstat
 24.76    0.078826          35      2225      1435 stat
 10.19    0.032434          44       733        16 open
  6.19    0.019719          36       550           fstat
  3.37    0.010723          32       336           pread
------ ----------- ----------- --------- --------- ----------------
100.00    0.318340                  8546      1451 total

CI: https://ci.nodejs.org/job/node-test-pull-request/4587/

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