svn commit: r358152 - head/bin/sh

[Hiroki Sato]

Author: hrs
Date: Thu Feb 20 03:01:27 2020
New Revision: 358152
URL: https://svnweb.freebsd.org/changeset/base/358152

Log:
  Improve performance of "read" built-in command when using a seekable
  fd.
  
  The read built-in command calls read(2) with a 1-byte buffer because
  newline characters need to be detected even on a byte stream which
  comes from a non-seekable file descriptor.  Because of this, the
  following script calls >6,000 read(2) to show a 6KiB file:
  
   while read IN; do echo "$IN"; done < /COPYRIGHT
  
  When the input byte stream is seekable, it is possible to read a data
  block and then reposition the file pointer to where a newline
  character found.  This change adds a small buffer to do this and
  reduces the number of read(2) calls.
  
  Theoretically, multiple built-in commands reading the same seekable
  byte stream in a single pipe chain can share the buffer.  However,
  this change just makes a single invocation of the read built-in
  allocate a buffer and deallocate it every time for simplicity.
  Although this causes read(2) to read the same regions multiple times,
  the performance penalty should be small compared to the reduction of
  read(2) calls.
  
  Reviewed by:		jilles
  MFC after:		1 week
  Differential Revision:	https://reviews.freebsd.org/D23747

Modified:
  head/bin/sh/miscbltin.c

Modified: head/bin/sh/miscbltin.c
==============================================================================
--- head/bin/sh/miscbltin.c	Thu Feb 20 01:45:55 2020	(r358151)
+++ head/bin/sh/miscbltin.c	Thu Feb 20 03:01:27 2020	(r358152)
@@ -66,10 +66,79 @@ __FBSDID("$FreeBSD$");
 
 #undef eflag
 
+#define	READ_BUFLEN	1024
+struct fdctx {
+	int	fd;
+	size_t	off;	/* offset in buf */
+	size_t	buflen;
+	char	*ep;	/* tail pointer */
+	char	buf[READ_BUFLEN];
+};
+
+static void fdctx_init(int, struct fdctx *);
+static void fdctx_destroy(struct fdctx *);
+static ssize_t fdgetc(struct fdctx *, char *);
 int readcmd(int, char **);
 int umaskcmd(int, char **);
 int ulimitcmd(int, char **);
 
+static void
+fdctx_init(int fd, struct fdctx *fdc)
+{
+	off_t cur;
+
+	/* Check if fd is seekable. */
+	cur = lseek(fd, 0, SEEK_CUR);
+	*fdc = (struct fdctx){
+		.fd = fd,
+		.buflen = (cur != -1) ? READ_BUFLEN : 1,
+		.ep = &fdc->buf[0],	/* No data */
+	};
+}
+
+static ssize_t
+fdgetc(struct fdctx *fdc, char *c)
+{
+	ssize_t nread;
+
+	if (&fdc->buf[fdc->off] == fdc->ep) {
+		nread = read(fdc->fd, fdc->buf, fdc->buflen);
+		if (nread > 0) {
+			fdc->off = 0;
+			fdc->ep = fdc->buf + nread;
+		} else
+			return (nread);
+	}
+	*c = fdc->buf[fdc->off++];
+
+	return (1);
+}
+
+static void
+fdctx_destroy(struct fdctx *fdc)
+{
+	size_t residue;
+
+	if (fdc->buflen > 1) {
+	/*
+	 * Reposition the file offset.  Here is the layout of buf:
+	 *
+	 *     | off
+	 *     v 
+	 * |*****************|-------|
+	 * buf               ep   buf+buflen
+	 *     |<- residue ->|
+	 *
+	 * off: current character
+	 * ep:  offset just after read(2)
+	 * residue: length for reposition
+	 */
+		residue = (fdc->ep - fdc->buf) - fdc->off;
+		if (residue > 0)
+			(void) lseek(fdc->fd, -residue, SEEK_CUR);
+	}
+}
+
 /*
  * The read builtin.  The -r option causes backslashes to be treated like
  * ordinary characters.
@@ -108,6 +177,7 @@ readcmd(int argc __unused, char **argv __unused)
 	fd_set ifds;
 	ssize_t nread;
 	int sig;
+	struct fdctx fdctx;
 
 	rflag = 0;
 	prompt = NULL;
@@ -173,8 +243,9 @@ readcmd(int argc __unused, char **argv __unused)
 	backslash = 0;
 	STARTSTACKSTR(p);
 	lastnonifs = lastnonifsws = -1;
+	fdctx_init(STDIN_FILENO, &fdctx);
 	for (;;) {
-		nread = read(STDIN_FILENO, &c, 1);
+		nread = fdgetc(&fdctx, &c);
 		if (nread == -1) {
 			if (errno == EINTR) {
 				sig = pendingsig;
@@ -260,6 +331,7 @@ readcmd(int argc __unused, char **argv __unused)
 		STARTSTACKSTR(p);
 		lastnonifs = lastnonifsws = -1;
 	}
+	fdctx_destroy(&fdctx);
 	STACKSTRNUL(p);
 
 	/*