sh: Use vfork in a few common cases.

This uses vfork() for simple commands and command substitutions containing a
single simple command, invoking an external program under certain conditions
(no redirections or variable assignments, non-interactive shell, no job
control). These restrictions limit the amount of code executed in a vforked
child.

There is a large speedup (for example 35%) in microbenchmarks. The
difference in buildkernel is smaller (for example 0.5%) but still
statistically significant. See
http://lists.freebsd.org/pipermail/freebsd-hackers/2012-January/037581.html
for some numbers.

The use of vfork() can be disabled by setting a variable named
SH_DISABLE_VFORK.

1 files changed, 9 insertions, 0 deletions

diff --git a/bin/sh/eval.c b/bin/sh/eval.c
index a5f0aff..2d90921 100644
--- a/bin/sh/eval.c
+++ b/bin/sh/eval.c
@@ -921,6 +921,15 @@ evalcommand(union node *cmd, int flags, struct backcmd *backcmd)
 			if (pipe(pip) < 0)
 				error("Pipe call failed: %s", strerror(errno));
 		}
+		if (cmdentry.cmdtype == CMDNORMAL &&
+		    cmd->ncmd.redirect == NULL &&
+		    varlist.list == NULL &&
+		    (mode == FORK_FG || mode == FORK_NOJOB) &&
+		    !disvforkset() && !iflag && !mflag) {
+			vforkexecshell(jp, argv, environment(), path,
+			    cmdentry.u.index, flags & EV_BACKCMD ? pip : NULL);
+			goto parent;
+		}
 		if (forkshell(jp, cmd, mode) != 0)
 			goto parent;	/* at end of routine */
 		if (flags & EV_BACKCMD) {