The below patch changes sh to use vfork(2) instead of fork(2) in some
simple cases: a foreground simple command, not being a command
substitution, without redirections or assignments in a non-interactive
shell with job control disabled.
By restricting the use of vfork, different from what NetBSD has done, I
limit what code is executed in the vforked child process. For example,
there is no way opening general redirections in a vforked background
process can work, as opening a fifo may block, and any long-term
blocking of a vforked-but-not-execed process is bad.
Background simple commands are also excluded because our current
approach of expanding their arguments in the main shell environment is
incorrect (zsh is broken in the same way).
The restriction to non-interactive shells with job control disabled also
limits the amount of code I have to copy to the vfork code path.
Command substitutions containing one simple command could also use vfork
but this needs a little extra code.
The patch does not depend on the memory sharing semantics of vfork. As a
result, however, it calls strerror() (and therefore all sorts of NLS
code) and stdio in the child, in the case that exec fails. Even in the
normal case, malloc() may be called to allocate space for the full
pathname of each executable to be tried. This could be fixed by
reserving enough space for the pathname first and passing the errno
value back through memory and printing the message in the parent process
but this also introduces additional differences with the regular code
path.
The functions setjmp() and longjmp() are also used but only within a
process; no jump is made by the vforked process outside
vforkexecshell().
Simple tests on various microbenchmarks (i386, calling /bin/echo 10000
times, possibly from multiple processes at once) show a performance
improvement of 20-30%. The gain might be bigger on embedded
architectures where less time has been spent to optimize fork(2).
Unfortunately, it looks like rc.d may not be helped much by this. It
uses many subshells, most of which require a full fork() with the
current implementation of sh.
Index: bin/sh/jobs.h
===================================================================
--- bin/sh/jobs.h (revision 223060)
+++ bin/sh/jobs.h (working copy)
@@ -91,6 +91,7 @@
void showjobs(int, int);
struct job *makejob(union node *, int);
pid_t forkshell(struct job *, union node *, int);
+pid_t vforkexecshell(struct job *, char **, char **, const char *, int);
int waitforjob(struct job *, int *);
int stoppedjobs(void);
int backgndpidset(void);
Index: bin/sh/eval.c
===================================================================
--- bin/sh/eval.c (revision 223024)
+++ bin/sh/eval.c (working copy)
@@ -899,6 +899,15 @@
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 &&
+ !iflag && !mflag) {
+ vforkexecshell(jp, argv, environment(), path,
+ cmdentry.u.index);
+ goto parent;
+ }
if (forkshell(jp, cmd, mode) != 0)
goto parent; /* at end of routine */
if (flags & EV_BACKCMD) {
Index: bin/sh/jobs.c
===================================================================
--- bin/sh/jobs.c (revision 223060)
+++ bin/sh/jobs.c (working copy)
@@ -57,6 +57,7 @@
#undef CEOF /* syntax.h redefines this */
#endif
#include "redir.h"
+#include "exec.h"
#include "show.h"
#include "main.h"
#include "parser.h"
@@ -884,7 +885,48 @@
}
+pid_t
+vforkexecshell(struct job *jp, char **argv, char **envp, const char *path, int
idx)
+{
+ pid_t pid;
+ struct jmploc jmploc;
+ struct jmploc *savehandler;
+ TRACE(("vforkexecshell(%%%td, %p, %d) called\n", jp - jobtab, (void *)n,
+ mode));
+ INTOFF;
+ flushall();
+ savehandler = handler;
+ pid = vfork();
+ if (pid == -1) {
+ TRACE(("Vfork failed, errno=%d\n", errno));
+ INTON;
+ error("Cannot fork: %s", strerror(errno));
+ }
+ if (pid == 0) {
+ TRACE(("Child shell %d\n", (int)getpid()));
+ if (setjmp(jmploc.loc))
+ _exit(exception == EXEXEC ? exerrno : 2);
+ handler = &jmploc;
+ shellexec(argv, envp, path, idx);
+ }
+ handler = savehandler;
+ if (jp) {
+ struct procstat *ps = &jp->ps[jp->nprocs++];
+ ps->pid = pid;
+ ps->status = -1;
+ ps->cmd = nullstr;
+ jp->foreground = 1;
+#if JOBS
+ setcurjob(jp);
+#endif
+ }
+ INTON;
+ TRACE(("In parent shell: child = %d\n", (int)pid));
+ return pid;
+}
+
+
/*
* Wait for job to finish.
*
--
Jilles Tjoelker
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