Smarty will create subdirectories under the
compiled templates and
cache
directories if $use_sub_dirs is set to TRUE,
default is FALSE.
In an environment where there are potentially tens of thousands of files created,
this may help the filesystem speed.
On the other hand, some environments do not allow PHP processes to
create directories, so this must be disabled which is the default.
Sub directories are more efficient, so use them if you can. Theoretically you get much better perfomance on a filesystem with 10 directories each having 100 files, than with 1 directory having 1000 files. This was certainly the case with Solaris 7 (UFS)... with newer filesystems such as ext3 and especially reiserfs, the difference is almost nothing.
$use_sub_dirs=true doesn't work with
safe_mode=On,
that's why it's switchable and why it's off by default.
$use_sub_dirs=true on Windows can cause problems.
Safe_mode is being deprecated in PHP6.
See also
$compile_id,
$cache_dir,
and
$compile_dir.