[PATCH] speed up writes

Speeds up generic_file_write() by not calling mark_inode_dirty() when
the mtime and ctime didn't change.

There may be concerns over the fact that this restricts mtime and ctime
updates to one-second resolution.  But the interface doesn't support
that anyway - all the filesystem knows is that its dirty_inode()
superop was called.  It doesn't know why.

So filesystems which support high-resolution timestamps already need to
make their own arrangements.  We need an update_mtime i_op to support
those properly.

time to write a one megabyte file one-byte-at-a-time:

Before:
	ext3:		24.8 seconds
	ext2:		 4.9 seconds
	reiserfs:	17.0 seconds
After:
	ext3:		22.5 seconds
	ext2:		4.8  seconds
	reiserfs:	11.6 seconds

Not much improvement because we're also calling expensive
mark_inode_dirty() functions when i_size is expanded.  So compare the
overwrite case:

time dd if=/dev/zero of=foo bs=1 count=1M conv=notrunc

ext3 before:	20.0 seconds
ext3 after:	9.7  seconds

diff --git a/mm/filemap.c b/mm/filemap.c
index 14148545163a0e75f3336b95af442baf3e2e4acd..ed7dd33f86e2be65650dc56ddcf7299f74aef2ed 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2098,6 +2098,7 @@ generic_file_write(struct file *file, const char *buf,
        ssize_t         written;
        int             err;
        unsigned        bytes;
+       time_t          time_now;
 
        if (unlikely((ssize_t) count < 0))
                return -EINVAL;
@@ -2195,9 +2196,12 @@ generic_file_write(struct file *file, const char *buf,
                goto out;
 
        remove_suid(file->f_dentry);
-       inode->i_ctime = CURRENT_TIME;
-       inode->i_mtime = CURRENT_TIME;
-       mark_inode_dirty_sync(inode);
+       time_now = CURRENT_TIME;
+       if (inode->i_ctime != time_now || inode->i_mtime != time_now) {
+               inode->i_ctime = time_now;
+               inode->i_mtime = time_now;
+               mark_inode_dirty_sync(inode);
+       }
 
        if (unlikely(file->f_flags & O_DIRECT)) {
                written = generic_file_direct_IO(WRITE, file,