Hi,
Comments?
Thanks,
Nick
--
struct dentry is one of the most critical structures in the kernel. So it's
sad to see it going neglected.
With CONFIG_PROFILING turned on (which is probably the common case at least
for distros and kernel developers), sizeof(struct dcache) == 208 here
(64-bit). This gives 19 objects per slab.
I packed d_mounted into a hole, and took another 4 bytes off the inline
name length to take the padding out from the end of the structure. This
shinks it to 200 bytes. I could have gone the other way and increased the
length to 40, but I'm aiming for a magic number, read on...
I then got rid of the d_cookie pointer. This shrinks it to 192 bytes. Rant:
why was this ever a good idea? The cookie system should increase its hash
size or use a tree or something if lookups are a problem. Also the "fast
dcookie lookups" in oprofile should be moved into the dcookie code -- how
can oprofile possibly care about the dcookie_mutex? It gets dropped after
get_dcookie() returns so it can't be providing any sort of protection.
At 192 bytes, 21 objects fit into a 4K page, saving about 3MB on my system
with ~140 000 entries allocated. 192 is also a multiple of 64, so we get
nice cacheline alignment on 64 and 32 byte line systems -- any given dentry
will now require 3 cachelines to touch all fields wheras previously it
would require 4.
I know the inline name size was chosen quite carefully, however with the
reduction in cacheline footprint, it should actually be just about as fast
to do a name lookup for a 36 character name as it was before the patch (and
faster for other sizes). The memory footprint savings for names which are
<= 32 or > 36 bytes long should more than make up for the memory cost for
33-36 byte names.
Performance is a feature...
---
arch/powerpc/oprofile/cell/spu_task_sync.c | 2 +-
drivers/oprofile/buffer_sync.c | 2 +-
fs/dcache.c | 4 ----
fs/dcookies.c | 28 +++++++++++++++++++---------
include/linux/dcache.h | 21 ++++++++++++++-------
5 files changed, 35 insertions(+), 22 deletions(-)
Index: linux-2.6/include/linux/dcache.h
===================================================================
--- linux-2.6.orig/include/linux/dcache.h
+++ linux-2.6/include/linux/dcache.h
@@ -75,14 +75,22 @@ full_name_hash(const unsigned char *name
return end_name_hash(hash);
}
-struct dcookie_struct;
-
-#define DNAME_INLINE_LEN_MIN 36
+/*
+ * Try to keep struct dentry aligned on 64 byte cachelines (this will
+ * give reasonable cacheline footprint with larger lines without the
+ * large memory footprint increase).
+ */
+#ifdef CONFIG_64BIT
+#define DNAME_INLINE_LEN_MIN 32 /* 192 bytes */
+#else
+#define DNAME_INLINE_LEN_MIN 40 /* 128 bytes */
+#endif
struct dentry {
atomic_t d_count;
unsigned int d_flags; /* protected by d_lock */
spinlock_t d_lock; /* per dentry lock */
+ int d_mounted;
struct inode *d_inode; /* Where the name belongs to - NULL is
* negative */
/*
@@ -107,10 +115,7 @@ struct dentry {
struct dentry_operations *d_op;
struct super_block *d_sb; /* The root of the dentry tree */
void *d_fsdata; /* fs-specific data */
-#ifdef CONFIG_PROFILING
- struct dcookie_struct *d_cookie; /* cookie, if any */
-#endif
- int d_mounted;
+
unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
};
@@ -177,6 +182,8 @@ d_iput: no no no yes
#define DCACHE_INOTIFY_PARENT_WATCHED 0x0020 /* Parent inode is watched */
+#define DCACHE_COOKIE 0x0040 /* For use by dcookie subsystem */
+
extern spinlock_t dcache_lock;
extern seqlock_t rename_lock;
Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c
+++ linux-2.6/fs/dcache.c
@@ -34,7 +34,6 @@
#include <linux/bootmem.h>
#include "internal.h"
-
int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
@@ -948,9 +947,6 @@ struct dentry *d_alloc(struct dentry * p
dentry->d_op = NULL;
dentry->d_fsdata = NULL;
dentry->d_mounted = 0;
-#ifdef CONFIG_PROFILING
- dentry->d_cookie = NULL;
-#endif
INIT_HLIST_NODE(&dentry->d_hash);
INIT_LIST_HEAD(&dentry->d_lru);
INIT_LIST_HEAD(&dentry->d_subdirs);
Index: linux-2.6/fs/dcookies.c
===================================================================
--- linux-2.6.orig/fs/dcookies.c
+++ linux-2.6/fs/dcookies.c
@@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcoo
{
struct dcookie_struct *dcs = kmem_cache_alloc(dcookie_cache,
GFP_KERNEL);
+ struct dentry *d;
if (!dcs)
return NULL;
- path->dentry->d_cookie = dcs;
+ d = path->dentry;
+ spin_lock(&d->d_lock);
+ d->d_flags |= DCACHE_COOKIE;
+ spin_unlock(&d->d_lock);
+
dcs->path = *path;
path_get(path);
hash_dcookie(dcs);
@@ -119,14 +124,14 @@ int get_dcookie(struct path *path, unsig
goto out;
}
- dcs = path->dentry->d_cookie;
-
- if (!dcs)
+ if (path->dentry->d_flags & DCACHE_COOKIE) {
+ dcs = find_dcookie((unsigned long)path->dentry);
+ } else {
dcs = alloc_dcookie(path);
-
- if (!dcs) {
- err = -ENOMEM;
- goto out;
+ if (!dcs) {
+ err = -ENOMEM;
+ goto out;
+ }
}
*cookie = dcookie_value(dcs);
@@ -251,7 +256,12 @@ out_kmem:
static void free_dcookie(struct dcookie_struct * dcs)
{
- dcs->path.dentry->d_cookie = NULL;
+ struct dentry *d = dcs->path.dentry;
+
+ spin_lock(&d->d_lock);
+ d->d_flags &= ~DCACHE_COOKIE;
+ spin_unlock(&d->d_lock);
+
path_put(&dcs->path);
kmem_cache_free(dcookie_cache, dcs);
}
Index: linux-2.6/drivers/oprofile/buffer_sync.c
===================================================================
--- linux-2.6.orig/drivers/oprofile/buffer_sync.c
+++ linux-2.6/drivers/oprofile/buffer_sync.c
@@ -200,7 +200,7 @@ static inline unsigned long fast_get_dco
{
unsigned long cookie;
- if (path->dentry->d_cookie)
+ if (path->dentry->d_flags & DCACHE_COOKIE)
return (unsigned long)path->dentry;
get_dcookie(path, &cookie);
return cookie;
Index: linux-2.6/arch/powerpc/oprofile/cell/spu_task_sync.c
===================================================================
--- linux-2.6.orig/arch/powerpc/oprofile/cell/spu_task_sync.c
+++ linux-2.6/arch/powerpc/oprofile/cell/spu_task_sync.c
@@ -297,7 +297,7 @@ static inline unsigned long fast_get_dco
{
unsigned long cookie;
- if (path->dentry->d_cookie)
+ if (path->dentry->d_flags & DCACHE_COOKIE)
return (unsigned long)path->dentry;
get_dcookie(path, &cookie);
return cookie;
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
