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SubjectRe: how to turn off, or to clear read cache?
Date
Sergey Spiridonov <spiridonov@gamic.com> writes:

> I need to make some performance tests. I need to switch off or to
> clear read cache, so that consequent reading of the same file will
> take the same amount of time.
>
> Is there an easy way to do it, without rebuilding the kernel?

I never found one, so I made this little patch. It adds a mount
option to the ext3 filesystem that makes it drop the cache when a file
is closed. Diff is against 2.4.21. It's just a quick hack that
accomplishes what I needed. I'm sure there's a better, more generic,
way.

Index: include/linux/ext3_fs.h
===================================================================
RCS file: /home/cvs/linux-2.4/include/linux/ext3_fs.h,v
retrieving revision 1.8
diff -u -r1.8 ext3_fs.h
--- include/linux/ext3_fs.h 1 Apr 2003 21:09:23 -0000 1.8
+++ include/linux/ext3_fs.h 17 Jun 2003 09:10:56 -0000
@@ -339,6 +339,7 @@
#define EXT3_MOUNT_WRITEBACK_DATA 0x0C00 /* No data ordering */
#define EXT3_MOUNT_UPDATE_JOURNAL 0x1000 /* Update the journal format */
#define EXT3_MOUNT_NO_UID32 0x2000 /* Disable 32-bit UIDs */
+#define EXT3_MOUNT_NOCACHE 0x4000 /* Free cached blocks on close */

/* Compatibility, for having both ext2_fs.h and ext3_fs.h included at once */
#ifndef _LINUX_EXT2_FS_H
Index: fs/ext3/file.c
===================================================================
RCS file: /home/cvs/linux-2.4/fs/ext3/file.c,v
retrieving revision 1.4
diff -u -r1.4 file.c
--- fs/ext3/file.c 28 Aug 2002 21:11:16 -0000 1.4
+++ fs/ext3/file.c 17 Jun 2003 09:10:57 -0000
@@ -35,6 +35,10 @@
{
if (filp->f_mode & FMODE_WRITE)
ext3_discard_prealloc (inode);
+ if (inode->i_sb->u.ext3_sb.s_mount_opt & EXT3_MOUNT_NOCACHE){
+ write_inode_now(inode, 1);
+ invalidate_inode_pages(inode);
+ }
return 0;
}

Index: fs/ext3/super.c
===================================================================
RCS file: /home/cvs/linux-2.4/fs/ext3/super.c,v
retrieving revision 1.10
diff -u -r1.10 super.c
--- fs/ext3/super.c 28 Apr 2003 21:14:51 -0000 1.10
+++ fs/ext3/super.c 17 Jun 2003 09:10:59 -0000
@@ -654,6 +654,8 @@
if (want_numeric(value, "commit", &v))
return 0;
sbi->s_commit_interval = (HZ * v);
+ } else if (!strcmp (this_char, "nocache")){
+ set_opt (*mount_options, NOCACHE);
} else {
printk (KERN_ERR
"EXT3-fs: Unrecognized mount option %s\n",

--
Måns Rullgård
mru@users.sf.net

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