Merge branch 'n-times-fadvise'

README

@@ -34,6 +34,8 @@ For testing purposes, I included two small tools:
 * `cachedel` calls `posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED)` on
   the file argument. Thus, if the file is not accessed by any other
   application, the pages will be eradicated from the fs cache.
+  Specifying -n <number> will repeat the syscall the given number of
+  times which can be useful in some circumstances (see below).
 * `cachestats` has three modes: In quiet mode (`-q`), the exit status
   is 0 (success) if the file is fully cached. In normal mode,
   the number of cached vs. not-cached pages is printed. In verbose
@@ -94,9 +96,17 @@ There are timing issues to consider, as well. If you consider `nocache
 cat <file>`, in most (all?) cases the cache will not be restored. For
 discussion and possible solutions see <http://lwn.net/Articles/480930/>.
 My experience showed that in many cases you could "fix" this by doing
-the `posix_fadvise` call *twice*. If you want this behaviour, compile
-the library using `make CFLAGS=-DDOUBLEFADVISE`.
+the `posix_fadvise` call *twice*. For both tools `nocache` and
+`cachedel` you can specify the number using `-n`, like so:
 
+    $ nocache -n 2 cat ~/file.mp3
+
+This actually only sets the environment variable `NOCACHE_NR_FADVISE`
+to the specified value, and the shared library reads out this value.
+If test number 3 in `t/basic.t` fails, then try increasing this number
+until it works, e.g.:
+
+    $ env NOCACHE_NR_FADVISE=2 make test
 
 Acknowledgements
 ----------------

cachedel.c

@@ -4,6 +4,7 @@
 #include <error.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 int exiterr(const char *s)
 {
@@ -13,32 +14,40 @@ int exiterr(const char *s)
 
 int main(int argc, char *argv[])
 {
+    int i, n = 1;
     int fd;
+    char *fn;
     struct stat st;
 
-    if(argc != 2) {
-        fprintf(stderr, "usage: %s <file> "
-            "-- call fadvise(DONTNEED) on file", argv[0]);
+    if(argc == 4 && !strcmp("-n", argv[1])) {
+            n = atoi(argv[2]);
+            fn = argv[3];
+    } else if(argc != 2) {
+        fprintf(stderr, "usage: %s [-n <n>] <file> "
+            "-- call fadvise(DONTNEED) <n> times on file", argv[0]);
         exit(1);
+    } else {
+        fn = argv[1];
     }
 
-    fd = open(argv[1], O_RDONLY);
+    fd = open(fn, O_RDONLY);
     if(fd == -1)
         exiterr("open");
 
     if(fstat(fd, &st) == -1)
         exiterr("fstat");
     if(!S_ISREG(st.st_mode)) {
-        fprintf(stderr, "%s: S_ISREG: not a regular file", argv[1]);
+        fprintf(stderr, "%s: S_ISREG: not a regular file", fn);
         return EXIT_FAILURE;
     }
     if(st.st_size == 0) {
-        fprintf(stderr, "%s: file size is 0!\n", argv[1]);
+        fprintf(stderr, "%s: file size is 0!\n", fn);
         return EXIT_FAILURE;
     }
 
-    if(posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED) == -1)
-        exiterr("posix_fadvise");
+    for(i = 0; i < n; i++)
+        if(posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED) == -1)
+            exiterr("posix_fadvise");
 
     return EXIT_SUCCESS;
 }

fcntl_helpers.c

@@ -5,12 +5,12 @@
 /* Since open() and close() are re-defined in nocache.c, it's not
  * possible to include <fcntl.h> there. So we do it here. */
 
-int fadv_dontneed(int fd, off_t offset, off_t len)
+int fadv_dontneed(int fd, off_t offset, off_t len, int n)
 {
-#ifdef DOUBLEFADVISE
-        posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
-#endif
-        return posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
+        int i, ret;
+        for(i = 0, ret = 0; i < n && ret == 0; i++)
+            ret = posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
+        return ret;
 }
 
 int fadv_noreuse(int fd, off_t offset, off_t len)

fcntl_helpers.h

 #ifndef _FCNTL_HELPERS_H
 #define _FCNTL_HELPERS_H
-extern int fadv_dontneed(int fd, off_t offset, off_t len);
+extern int fadv_dontneed(int fd, off_t offset, off_t len, int n);
 extern int fadv_noreuse(int fd, off_t offset, off_t len);
 extern int valid_fd(int fd);
 extern void sync_if_writable(int fd);

man/cachedel.1

@@ -2,6 +2,10 @@
 .SH NAME
 cachedel \- drop pagecache for a file
 .SH SYNOPSIS
-cachedel \fBfile\fR
+cachedel [-n <n>] \fBfile\fR
+.SH OPTIONS
+.TP
+\fB\-n <n>\fR "Repeat system call"
+Will call posix_fadvise(POSIX_FADV_DONTNEED) \fB<n>\fR in a row.
 .SH DESCRIPTION
-Call fadvise(DONTNEED) on file
+Call posix_fadvise(POSIX_FADV_DONTNEED) on file.

man/nocache.1

@@ -2,7 +2,13 @@
 .SH NAME
 nocache \- don't use Linux page cache on given command
 .SH SYNOPSIS
-nocache \fBcommand\fR [argument...]
+nocache [-n <n>] \fBcommand\fR [argument...]
+.SH OPTIONS
+.TP
+\fB\-n <n>\fR "Set number of fadvise calls"
+Execute the `posix_fadvise` system call \fB<n>\fR times in a row.
+Depending on your machine, this might give better results (use it if in
+your tests `nocache` fails to eradicate pages from cache properly).
 .SH DESCRIPTION
 The `nocache` tool tries to minimize the effect an application has on
 the Linux file system cache. This is done by intercepting the `open`

nocache

@@ -8,4 +8,9 @@ else
     export LD_PRELOAD="$libnocache"
 fi
 
+if [ "$1" = "-n" ]; then
+    export NOCACHE_NR_FADVISE="$2"
+    shift 2
+fi
+
 exec "$@"

nocache.c

@@ -63,9 +63,13 @@ static struct fadv_info *fds;
 static size_t PAGESIZE;
 static pthread_mutex_t lock; /* protects access to fds[] */
 
+static char *env_nr_fadvise = "NOCACHE_NR_FADVISE";
+static int nr_fadvise;
+
 static void init(void)
 {
     int i;
+    char *s;
     char *error;
     struct rlimit rlim;
     
@@ -93,7 +97,10 @@ static void init(void)
         exit(EXIT_FAILURE);
     }
 
-
+    if((s = getenv(env_nr_fadvise)) != NULL)
+        nr_fadvise = atoi(s);
+    if(nr_fadvise <= 0)
+        nr_fadvise = 1;
 
     PAGESIZE = getpagesize();
     for(i = 0; i < max_fds; i++)
@@ -308,14 +315,14 @@ static void free_unclaimed_pages(int fd)
     while(j < fds[i].nr_pages) {
         if(fds[i].info[j] & 1) {
             if(start < j)
-                fadv_dontneed(fd, start*PAGESIZE, (j - start) * PAGESIZE);
+                fadv_dontneed(fd, start*PAGESIZE, (j - start) * PAGESIZE, nr_fadvise);
             start = j + 1;
         }
         j++;
     }
 
     /* forget written contents that go beyond previous file size */
-    fadv_dontneed(fd, start < j ? start*PAGESIZE : fds[i].size, 0);
+    fadv_dontneed(fd, start < j ? start*PAGESIZE : fds[i].size, 0, nr_fadvise);
 
     free(fds[i].info);
     fds[i].fd = -1;