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?
Jörn
--
The wise man seeks everything in himself; the ignorant man tries to get
everything from somebody else.
-- unknown
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