libbb: use sendfile() to copy data between file descriptors

Busybox already uses sendfile in httpd. This patch proposes to use it
globally to copy data between file descriptors.

It speeds up the copying on slow systems a lot - below are the times needed
to copy a 450Mb file with and without this option enabled on a BeagleBone
Black:

sendfile:
user    0m0.000s
sys     0m8.170s

read/write 4k:
user    0m0.470s
sys     0m16.300s

function                                             old     new   delta
bb_full_fd_action                                    394     474     +80

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/Config.in b/Config.in
index b83beb5..285fe0a 100644
--- a/Config.in
+++ b/Config.in
@@ -264,6 +264,18 @@ config PAM
 	  Use PAM in some busybox applets (currently login and httpd) instead
 	  of direct access to password database.
 
+config FEATURE_USE_SENDFILE
+	bool "Use sendfile system call"
+	default y
+	help
+	  When enabled, busybox will use the kernel sendfile() function
+	  instead of read/write loops to copy data between file descriptors
+	  (for example, cp command does this a lot).
+	  If sendfile() doesn't work, copying code falls back to read/write
+	  loop. sendfile() was originally implemented for faster I/O
+	  from files to sockets, but since Linux 2.6.33 it was extended
+	  to work for many more file types.
+
 config LONG_OPTS
 	bool "Support for --long-options"
 	default y