Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> Also, if people really want to optimize the code that executes an
> external program (whether in shell or directly), I think it might be
> worth it to look at replacing the "fork()" with a "vfork()".
>
> Something like this
>
> - cmd->pid = fork();
> + cmd->pid = (cmd->git_cmd || cmd->env) ? fork() : vfork();
>
> might work (the native git_cmd case needs a real fork, and if we
> change the environment variables we need it too, but the other cases
> look like they might work with vfork()).
>
> Using vfork() can be hugely more efficient, because you don't have the
> extra page table copies and teardown, but also avoid a lot of possible
> copy-on-write faults.
Fwiw, most of the vfork preparation was already done by Brandon
and myself a few weeks ago, and cooking in pu.
I think only the patch below would be needed to enable vfork
(along with any build-time detection)
However, I haven't noticed enough forking in git to make a
difference (but maybe others do). I think it would make a
bigger difference if such changes were made to bash, dash,
make, and perl5.
--------8<------------
Subject: [PATCH] run-command: use vfork instead of fork
To enable vfork, we merely have to avoid modifying memory we
share with the parent, so the guard functions
`child_(error|warn|die)_fn` can now be disabled.
FIXME: still missing autoconf + Makefile portability tweaks.
Signed-off-by: Eric Wong <e@xxxxxxxxx>
---
run-command.c | 28 +---------------------------
1 file changed, 1 insertion(+), 27 deletions(-)
diff --git a/run-command.c b/run-command.c
index 9e36151bf9..0292dd94b6 100644
--- a/run-command.c
+++ b/run-command.c
@@ -324,25 +324,6 @@ static void fake_fatal(const char *err, va_list params)
vreportf("fatal: ", err, params);
}
-static void child_error_fn(const char *err, va_list params)
-{
- const char msg[] = "error() should not be called in child\n";
- xwrite(2, msg, sizeof(msg) - 1);
-}
-
-static void child_warn_fn(const char *err, va_list params)
-{
- const char msg[] = "warn() should not be called in child\n";
- xwrite(2, msg, sizeof(msg) - 1);
-}
-
-static void NORETURN child_die_fn(const char *err, va_list params)
-{
- const char msg[] = "die() should not be called in child\n";
- xwrite(2, msg, sizeof(msg) - 1);
- _exit(2);
-}
-
/* this runs in the parent process */
static void child_err_spew(struct child_process *cmd, struct child_err *cerr)
{
@@ -658,17 +639,10 @@ int start_command(struct child_process *cmd)
* never be released in the child process. This means only
* Async-Signal-Safe functions are permitted in the child.
*/
- cmd->pid = fork();
+ cmd->pid = vfork();
failed_errno = errno;
if (!cmd->pid) {
int sig;
- /*
- * Ensure the default die/error/warn routines do not get
- * called, they can take stdio locks and malloc.
- */
- set_die_routine(child_die_fn);
- set_error_routine(child_error_fn);
- set_warn_routine(child_warn_fn);
close(notify_pipe[0]);
set_cloexec(notify_pipe[1]);
--
Also fetchable: git://bogomips.org/git-svn vfork-test
commit 5f88d79182aaabc5ea467d1d29e13e45bd2b99bf