Messages in this thread Patch in this message | | | From | Eric Dumazet <> | Subject | [PATCH] struct vfsmount : keep mnt_count & mnt_expiry_mark away from mnt_flags | Date | Fri, 15 Dec 2006 18:51:58 +0100 |
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I noticed cache misses in touch_atime() that can be avoided if we keep mnt_count & mnt_expiry_mark in a different cache line than mnt_flags (mostly read)
mnt_count & mnt_expiry_mark are modified each time a file is opened/closed in a file system.
touch_atime() is called each time a file is read, and generally needs to read mnt_flags.
Other fields of struct vfsmount are mostly read so I chose to move mnt_count & mnt_expiry_mark at the end of struct vfsmount. And adding a comment so that nobody tries to re-arrange fields to fill the holes :)
On 64bits platforms, the new offsetof(mnt_count) is 0xC0 On 32bits platforms, it is 0x60, so I didnot add a ____cacheline_aligned_in_smp because it would have a too big impact on the size of this object (in particular if CONFIG_X86_L1_CACHE_SHIFT=7)
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
--- linux-2.6.20-rc1-mm1/include/linux/mount.h 2006-12-14 02:14:23.000000000 +0100 +++ linux-2.6.20-rc1-mm1-ed/include/linux/mount.h 2006-12-15 19:30:54.000000000 +0100 @@ -43,9 +43,8 @@ struct vfsmount { struct super_block *mnt_sb; /* pointer to superblock */ struct list_head mnt_mounts; /* list of children, anchored here */ struct list_head mnt_child; /* and going through their mnt_child */ - atomic_t mnt_count; int mnt_flags; - int mnt_expiry_mark; /* true if marked for expiry */ + /* 4 bytes hole on 64bits arches */ char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */ struct list_head mnt_list; struct list_head mnt_expire; /* link in fs-specific expiry list */ @@ -54,6 +53,13 @@ struct vfsmount { struct list_head mnt_slave; /* slave list entry */ struct vfsmount *mnt_master; /* slave is on master->mnt_slave_list */ struct mnt_namespace *mnt_ns; /* containing namespace */ + /* + * We put mnt_count & mnt_expiry_mark at the end of struct vfsmount + * to let these frequently modified fields in a separate cache line + * (so that reads of mnt_flags wont ping-pong on SMP machines) + */ + atomic_t mnt_count; + int mnt_expiry_mark; /* true if marked for expiry */ int mnt_pinned; }; | |