On Thu, 14 September 2006 11:31:23 +0200, Jörn Engel wrote: > > After taking a look at struct dentry, Arnd noted an alignment > problem. The first four fields currently are: > atomic_t d_count; > unsigned int d_flags; /* protected by d_lock */ > spinlock_t d_lock; /* per dentry lock */ > struct inode *d_inode; /* Where the name belongs to - NULL is > * negative */ > On 64bit architectures, the first three take 12 bytes and d_inode is > not naturally aligned, so it can be aligned to byte 16. This grows a > struct dentry from 196 to 200 Bytes (assuming no funky config options > like DEBUG_*, PROFILING or PREEMT && SMP are set). > > One possible solution would be to exchange d_inode with d_mounted, but > I fear that d_inode would move from a hot cacheline to a cold one, > reducing performance. Could there be a good solution or would any > rearrangement here only cause regressions? > > Also, both 196 and 200 bytes are fairly close to 192 bytes, so I could > imagine performance improvements on 64bit machines with 64 Byte > cachelines. Might it make sense to trim DNAME_INLINE_LEN_MIN by 4 or > 8 bytes for such machines? And here is a patch for those who prefer talking code Jörn -- Joern's library part 8: http://citeseer.ist.psu.edu/plank97tutorial.html --- slab/include/linux/dcache.h~dentry_alignment 2006-09-14 10:52:51.000000000 +0200 +++ slab/include/linux/dcache.h 2006-09-14 12:44:56.000000000 +0200 @@ -77,14 +77,17 @@ full_name_hash(const unsigned char *name struct dcookie_struct; +#if BITS_PER_LONG == 64 +#define DNAME_INLINE_LEN_MIN 32 +#else #define DNAME_INLINE_LEN_MIN 36 +#endif struct dentry { atomic_t d_count; unsigned int d_flags; /* protected by d_lock */ spinlock_t d_lock; /* per dentry lock */ - struct inode *d_inode; /* Where the name belongs to - NULL is - * negative */ + int d_mounted; /* * The next three fields are touched by __d_lookup. Place them here * so they all fit in a cache line. @@ -93,6 +96,8 @@ struct dentry { struct dentry *d_parent; /* parent directory */ struct qstr d_name; + struct inode *d_inode; /* Where the name belongs to - NULL is + * negative */ struct list_head d_lru; /* LRU list */ /* * d_child and d_rcu can share memory @@ -110,7 +115,6 @@ struct dentry { #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 */ }; - 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
