Sockets, pipes and anonymous fds have interesting properties. Like other files, they use a dentry and an inode. But dentries for these kind of files are not hashed into dcache, since there is no way someone can lookup such a file in the vfs tree. (/proc/{pid}/fd/{number} uses a different mechanism) Still, allocating and freeing such dentries are expensive processes, because we currently take dcache_lock inside d_alloc(), d_instantiate(), and dput(). This lock is very contended on SMP machines. This patch defines a new DCACHE_SINGLE flag, to mark a dentry as a single one (for sockets, pipes, anonymous fd), and a new d_alloc_single(const struct qstr *name, struct inode *inode) method, called by the three subsystems. Internally, dput() can take a fast path to dput_single() for SINGLE dentries. No more atomic_dec_and_lock() for such dentries. Differences betwen an SINGLE dentry and a normal one are : 1) SINGLE dentry has the DCACHE_SINGLE flag 2) SINGLE dentry's parent is itself (DCACHE_DISCONNECTED) This to avoid taking a reference on sb 'root' dentry, shared by too many dentries. 3) They are not hashed into global hash table (DCACHE_UNHASHED) 4) Their d_alias list is empty (socket8 bench result : from 25s to 19.9s) Signed-off-by: Eric Dumazet --- fs/anon_inodes.c | 16 ------------ fs/dcache.c | 51 +++++++++++++++++++++++++++++++++++++++ fs/pipe.c | 23 +---------------- include/linux/dcache.h | 9 ++++++ net/socket.c | 24 +----------------- 5 files changed, 65 insertions(+), 58 deletions(-)