initial commit

3 files changed, 148 insertions, 0 deletions

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..1998cb7
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,6 @@
+fmlock: fmlock.c
+
+clean:
+	-rm fmlock
+
+.PHONY: clean
diff --git a/README b/README
new file mode 100644
index 0000000..684a286
--- /dev/null
+++ b/README
@@ -0,0 +1,12 @@
+== fmlock -- map a file into memory and lock it to RAM ==
+
+fmlock is a small tool intended to load a file into the
+block I/O cache and lock it there. This is done by mmap-ing
+the whole file and calling mlock on the address space
+occupied by the file. Then the program puts itself to sleep
+until a signal arrives. The purpose of this is that (large)
+files can be preloaded into RAM, specifically the block I/O
+cache for other processes to read from. This is for example
+required to measure the RAM bandwidth and CPU limited
+throughput of data processing code, working on real data
+without getting effected by I/O bottlenecks.
diff --git a/fmlock.c b/fmlock.c
new file mode 100644
index 0000000..4ea68d0
--- /dev/null
+++ b/fmlock.c
@@ -0,0 +1,130 @@
+#include <sys/types.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stddef.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <string.h>
+
+int heat_the_cache(int fd)
+{
+	char buf[1024];
+	int rv;
+	do {
+	retry_read:
+		rv = read(fd, buf, sizeof(buf));
+		if( -1 == rv
+		 && EINTR == errno ) {
+			goto retry_read;
+		}
+	} while( 0 < rv );
+
+	return rv != 0;
+}
+
+int main(int argc, char *argv[])
+{
+	int i, fd_null, n_fileslocked;
+	fd_set phony_fdset;
+	char *buf;
+
+	if( 2 > argc ) {
+		fprintf(stderr,
+			"Usage:\n%s [filenames]\n",
+			argv[0] );
+		return 1;
+	}
+
+	n_fileslocked = 0;
+	for(i = 1; i < argc; ++i) {
+		char const * const filename = argv[i];
+		int fd;
+		struct stat st;
+		void *ptr;
+
+	retry_open:
+		fd = open(filename, O_RDONLY);
+		if( -1 == fd ) {
+			if( EINTR == errno ) {
+				goto retry_open;
+			} else
+			{
+				fprintf(stderr,
+					"error open('%s'): %s\n",
+					filename,
+					strerror(errno) );
+				continue;
+			}
+		}
+
+		if( -1 == fstat(fd, &st) ) {
+			fprintf(stderr,
+				"error fstat(fd['%s']): %s\n",
+				filename,
+				strerror(errno) );
+			goto finish_file;
+		}
+
+		ptr = mmap(
+			NULL,
+			st.st_size,
+			PROT_READ,
+			MAP_SHARED | MAP_LOCKED,
+			fd,
+			0 );
+		if( MAP_FAILED == ptr ) {
+			fprintf(stderr,
+				"error mmap(fd['%s'], 0..%lld): %s\n",
+				filename,
+				(long long)st.st_size,
+				strerror(errno) );
+			goto finish_file;
+		}
+
+		if( -1 == mlock(ptr, st.st_size) ) {
+			fprintf(stderr,
+				"error mlock(ptr[fd['%s']]=%p): %s\n",
+				filename,
+				ptr,
+				strerror(errno) );
+			goto finish_file;
+		}
+		++n_fileslocked;
+
+		heat_the_cache(fd);
+
+	finish_file:
+		close(fd);
+	}
+
+	if( !n_fileslocked ) {
+		return 1;
+	}
+
+	/* At this point the program shall sleep until a terminating
+	 * signal arrives. To do so a nice side effect of the definition
+	 * of /dev/null behavior is used: read on a /dev/null fd always
+	 * return 0, which correspond to EOF which is a "no content
+	 * available for read (yet)" situation for which select waits.
+	 * So by selecting for a read a fd on /dev/null we can put the
+	 * process to sleep. */
+	fd_null = open("/dev/null", O_RDONLY);
+	if( -1 == fd_null ) {
+		fprintf(stderr,
+			"error open('/dev/null'): %s\n",
+			strerror(errno) );
+		return 1;
+	}
+	FD_ZERO(&phony_fdset);
+	FD_SET(fd_null, &phony_fdset);
+	if( -1 == select(fd_null, &phony_fdset, NULL, NULL, NULL) ) {
+		fprintf(stderr,
+			"error select(...): %s\n",
+			strerror(errno) );
+		return 1;
+	}
+
+	return 0;
+}