Teach cat(1) and cp(1) to use a larger buffer if enough memory is present

in the system. A simple heuristics is used to detect what is "enough"
memory: if number of physmem pages is greater than 32k (equalling 128 MB
on machines with 4 kB pages).

Typical immediate result of these changes is reduction in context switches
and the goal is to increase efficiency by using large buffers:
before: /usr/bin/time -hlp cat file1 > file2
...
       163  voluntary context switches
     11194  involuntary context switches
after: /usr/bin/time -hlp ./cat file1 > file2
...
       417  voluntary context switches
       272  involuntary context switches

Reviewed by:    hackers@ (no objections to earlier version of cat patch)
Approved by:    gnn (mentor)
MFC after:      4 months

1 files changed, 21 insertions, 2 deletions

diff --git a/bin/cat/cat.c b/bin/cat/cat.c
index 9acd68f..d338742 100644
--- a/bin/cat/cat.c
+++ b/bin/cat/cat.c
@@ -77,6 +77,17 @@ static void raw_cat(int);
 static int udom_open(const char *path, int flags);
 #endif
 
+/* Memory strategy threshold, in pages: if physmem is larger then this, use a 
+ * large buffer */
+#define PHYSPAGES_THRESHOLD (32*1024)
+
+/* Maximum buffer size in bytes - do not allow it to grow larger than this */
+#define BUFSIZE_MAX (2*1024*1024)
+
+/* Small (default) buffer size in bytes. It's inefficient for this to be
+ * smaller than MAXPHYS */
+#define BUFSIZE_SMALL (MAXPHYS)
+
 int
 main(int argc, char *argv[])
 {
@@ -247,9 +258,17 @@ raw_cat(int rfd)
 	if (buf == NULL) {
 		if (fstat(wfd, &sbuf))
 			err(1, "%s", filename);
-		bsize = MAX(sbuf.st_blksize, 1024);
+		if (S_ISREG(sbuf.st_mode)) {
+			/* If there's plenty of RAM, use a large copy buffer */
+			if (sysconf(_SC_PHYS_PAGES) > PHYSPAGES_THRESHOLD)
+				bsize = MIN(BUFSIZE_MAX, MAXPHYS*8);
+			else
+				bsize = BUFSIZE_SMALL;
+		} else
+			bsize = MAX(sbuf.st_blksize, 
+					(blksize_t)sysconf(_SC_PAGESIZE));
 		if ((buf = malloc(bsize)) == NULL)
-			err(1, "buffer");
+			err(1, "malloc() failure of IO buffer");
 	}
 	while ((nr = read(rfd, buf, bsize)) > 0)
 		for (off = 0; nr; nr -= nw, off += nw)