svn commit: r366697 - head/usr.bin/xinstall

[Alex Richardson]

Author: arichardson
Date: Wed Oct 14 12:28:41 2020
New Revision: 366697
URL: https://svnweb.freebsd.org/changeset/base/366697

Log:
  install(1): Avoid unncessary fstatfs() calls and use mmap() based on size
  
  According to git blame the trymmap() function was added in 1996 to skip
  mmap() calls for NFS file systems. However, nowadays mmap() should be
  perfectly safe even on NFS. Importantly, onl ufs and cd9660 file systems
  were whitelisted so we don't use mmap() on ZFS. It also prevents the use
  of mmap() when bootstrapping from macOS/Linux since on those systems the
  trymmap() function was always returning zero due to the missing MFSNAMELEN
  define.
  
  This change keeps the trymmap() function but changes it to check whether
  using mmap() can reduce the number of system calls that are required.
  Using mmap() only reduces the number of system calls if we need multiple read()
  syscalls, i.e. if the file size is > MAXBSIZE. However, mmap() is more expensive
  than read() so this sets the threshold at 4 fewer syscalls. Additionally, for
  larger file size mmap() can significantly increase the number of page faults,
  so avoid it in that case.
  
  It's unclear whether using mmap() is ever faster than a read with an appropriate
  buffer size, but this change at least removes two unnecessary system calls
  for every file that is installed.
  
  Reviewed By:	markj
  Differential Revision: https://reviews.freebsd.org/D26041

Modified:
  head/usr.bin/xinstall/xinstall.c

Modified: head/usr.bin/xinstall/xinstall.c
==============================================================================
--- head/usr.bin/xinstall/xinstall.c	Wed Oct 14 10:12:39 2020	(r366696)
+++ head/usr.bin/xinstall/xinstall.c	Wed Oct 14 12:28:41 2020	(r366697)
@@ -148,7 +148,7 @@ static void	metadata_log(const char *, const char *, s
 		    const char *, const char *, off_t);
 static int	parseid(const char *, id_t *);
 static int	strip(const char *, int, const char *, char **);
-static int	trymmap(int);
+static int	trymmap(size_t);
 static void	usage(void);
 
 int
@@ -1087,7 +1087,7 @@ compare(int from_fd, const char *from_name __unused, s
 		if (do_digest)
 			digest_init(&ctx);
 		done_compare = 0;
-		if (trymmap(from_fd) && trymmap(to_fd)) {
+		if (trymmap(from_len) && trymmap(to_len)) {
 			p = mmap(NULL, from_len, PROT_READ, MAP_SHARED,
 			    from_fd, (off_t)0);
 			if (p == MAP_FAILED)
@@ -1248,13 +1248,8 @@ copy(int from_fd, const char *from_name, int to_fd, co
 
 	digest_init(&ctx);
 
-	/*
-	 * Mmap and write if less than 8M (the limit is so we don't totally
-	 * trash memory on big files.  This is really a minor hack, but it
-	 * wins some CPU back.
-	 */
 	done_copy = 0;
-	if (size <= 8 * 1048576 && trymmap(from_fd) &&
+	if (trymmap((size_t)size) &&
 	    (p = mmap(NULL, (size_t)size, PROT_READ, MAP_SHARED,
 		    from_fd, (off_t)0)) != MAP_FAILED) {
 		nw = write(to_fd, p, size);
@@ -1523,20 +1518,23 @@ usage(void)
  *	return true (1) if mmap should be tried, false (0) if not.
  */
 static int
-trymmap(int fd)
+trymmap(size_t filesize)
 {
-/*
- * The ifdef is for bootstrapping - f_fstypename doesn't exist in
- * pre-Lite2-merge systems.
- */
-#ifdef MFSNAMELEN
-	struct statfs stfs;
-
-	if (fstatfs(fd, &stfs) != 0)
-		return (0);
-	if (strcmp(stfs.f_fstypename, "ufs") == 0 ||
-	    strcmp(stfs.f_fstypename, "cd9660") == 0)
-		return (1);
-#endif
-	return (0);
+	/*
+	 * This function existed to skip mmap() for NFS file systems whereas
+	 * nowadays mmap() should be perfectly safe. Nevertheless, using mmap()
+	 * only reduces the number of system calls if we need multiple read()
+	 * syscalls, i.e. if the file size is > MAXBSIZE. However, mmap() is
+	 * more expensive than read() so set the threshold at 4 fewer syscalls.
+	 * Additionally, for larger file size mmap() can significantly increase
+	 * the number of page faults, so avoid it in that case.
+	 *
+	 * Note: the 8MB limit is not based on any meaningful benchmarking
+	 * results, it is simply reusing the same value that was used before
+	 * and also matches bin/cp.
+	 *
+	 * XXX: Maybe we shouldn't bother with mmap() at all, since we use
+	 * MAXBSIZE the syscall overhead of read() shouldn't be too high?
+	 */
+	return (filesize > 4 * MAXBSIZE && filesize < 8 * 1024 * 1024);
 }