Re: Why does not rgrep use "grep -r"?

[Stefan Monnier]

>> If grep can do recursive searches then why not use that in rgrep?
> I don't know the reason of the rgrep authors (though I suspect it was
> portability concerns), but I have noticed something odd about "grep -r":
> -- sometimes it seems _much_ slower than "find ... -type f | xargs grep"
> on very large trees (I think I noticed with trees in NFS, where speed is
> a perennial issue).  Dunno why this...

I don't know why that is, but the difference may come from diferences in
the way the tree is traversed.  There are many tricks that one can try
to use to speed up tree traversal and unless both commands share their
tree-traversal code, it's likely that find's tree traversal has been
better optimized.

Some of the tricks that can be used:
- A dir without subdirs has a refcount of 2 (one ref from the parent,
  one from its "." entry).  If you know a dir has no subdirs you can
  avoid calling `stat' on each file to determine whether it's a subdir
  or not.
- some filesystems can provide the "dir/notdir" info without going
  through `stat', but that requires the use of a separate syscall.
- assuming the inodes numbers are related to the position of the inode
  on the drive (which is not the case for log-structured filesystems but
  is the case for many others) you can reorder your traversal to try and
  produce a more sequential scan of your disk.
- if the OS&filesystem gives you access to disk-block-numbers instead of only
  inode numbers, you can use that to do the above.
- ...


        Stefan