Commit c2452f32 authored by Nick Piggin's avatar Nick Piggin Committed by Al Viro
Browse files

shrink struct dentry 



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...

Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent e2b689d8

arch/powerpc/oprofile/cell/spu_task_sync.c

+1 −1
Original line number Diff line number Diff line
@@ -297,7 +297,7 @@ static inline unsigned long fast_get_dcookie(struct path *path) 
{

	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;

drivers/oprofile/buffer_sync.c

+1 −1
Original line number Diff line number Diff line
@@ -200,7 +200,7 @@ static inline unsigned long fast_get_dcookie(struct path *path) 
{

	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;

fs/dcache.c

+0 −4
Original line number Diff line number Diff line
@@ -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 * parent, const struct qstr *name) 
	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);

fs/dcookies.c

+19 −9
Original line number Diff line number Diff line
@@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcookie(struct path *path) 
{

	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);
@@ -119,15 +124,15 @@ int get_dcookie(struct path *path, unsigned long *cookie) 
		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;

		}

	}



	*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);

}

include/linux/dcache.h

+14 −7
Original line number Diff line number Diff line
@@ -75,14 +75,22 @@ full_name_hash(const unsigned char *name, unsigned int len) 
	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;

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