Ingo Molnar a écrit :
* Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
On Thu, Nov 27, 2008 at 12:32:59AM +0100, Eric Dumazet wrote:
This function arms a flag (MNT_SPECIAL) on the vfs, to avoid
refcounting on permanent system vfs.
Use this function for sockets, pipes, anonymous fds.
IMO that's pushing it past the point of usefulness; unless you can show
that this really gives considerable win on pipes et.al. *AND* that it
doesn't hurt other loads...
The numbers look pretty convincing:
(socket8 bench result : from 2.94s to 2.23s)
And i wouldnt expect it to hurt real-filesystem workloads.
Here's the contemporary trace of a typical ext3- sys_open():
0) | sys_open() {
0) | do_sys_open() {
0) | getname() {
0) 0.367 us | kmem_cache_alloc();
0) | strncpy_from_user(); {
0) | _cond_resched() {
0) | need_resched() {
0) 0.363 us | constant_test_bit();
0) 1. 47 us | }
0) 1.815 us | }
0) 2.587 us | }
0) 4. 22 us | }
0) | alloc_fd() {
0) 0.480 us | _spin_lock();
0) 0.487 us | expand_files();
0) 2.356 us | }
0) | do_filp_open() {
0) | path_lookup_open() {
0) | get_empty_filp() {
0) 0.439 us | kmem_cache_alloc();
0) | security_file_alloc() {
0) 0.316 us | cap_file_alloc_security();
0) 1. 87 us | }
0) 3.189 us | }
0) | do_path_lookup() {
0) 0.366 us | _read_lock();
0) | path_walk() {
0) | __link_path_walk() {
0) | inode_permission() {
0) | ext3_permission() {
0) 0.441 us | generic_permission();
0) 1.247 us | }
0) | security_inode_permission() {
0) 0.411 us | cap_inode_permission();
0) 1.186 us | }
0) 3.555 us | }
0) | do_lookup() {
0) | __d_lookup() {
0) 0.486 us | _spin_lock();
0) 1.369 us | }
0) 0.442 us | __follow_mount();
0) 3. 14 us | }
0) | path_to_nameidata() {
0) 0.476 us | dput();
0) 1.235 us | }
0) | inode_permission() {
0) | ext3_permission() {
0) | generic_permission() {
0) | in_group_p() {
0) 0.410 us | groups_search();
0) 1.172 us | }
0) 1.994 us | }
0) 2.789 us | }
0) | security_inode_permission() {
0) 0.454 us | cap_inode_permission();
0) 1.238 us | }
0) 5.262 us | }
0) | do_lookup() {
0) | __d_lookup() {
0) 0.480 us | _spin_lock();
0) 1.621 us | }
0) 0.456 us | __follow_mount();
0) 3.215 us | }
0) | path_to_nameidata() {
0) 0.420 us | dput();
0) 1.193 us | }
0) + 23.551 us | }
0) | path_put() {
0) 0.420 us | dput();
0) | mntput() {
0) 0.359 us | mntput_no_expire();
0) 1. 50 us | }
0) 2.544 us | }
0) + 27.253 us | }
0) + 28.850 us | }
0) + 33.217 us | }
0) | may_open() {
0) | inode_permission() {
0) | ext3_permission() {
0) 0.480 us | generic_permission();
0) 1.229 us | }
0) | security_inode_permission() {
0) 0.405 us | cap_inode_permission();
0) 1.196 us | }
0) 3.589 us | }
0) 4.600 us | }
0) | nameidata_to_filp() {
0) | __dentry_open() {
0) | file_move() {
0) 0.470 us | _spin_lock();
0) 1.243 us | }
0) | security_dentry_open() {
0) 0.344 us | cap_dentry_open();
0) 1.139 us | }
0) 0.412 us | generic_file_open();
0) 0.561 us | file_ra_state_init();
0) 5.714 us | }
0) 6.483 us | }
0) + 46.494 us | }
0) 0.453 us | inotify_dentry_parent_queue_event();
0) 0.403 us | inotify_inode_queue_event();
0) | fd_install() {
0) 0.440 us | _spin_lock();
0) 1.247 us | }
0) | putname() {
0) | kmem_cache_free() {
0) | virt_to_head_page() {
0) 0.369 us | constant_test_bit();
0) 1. 23 us | }
0) 1.738 us | }
0) 2.422 us | }
0) + 60.560 us | }
0) + 61.368 us | }
and here's a sys_close():
0) | sys_close() {
0) 0.540 us | _spin_lock();
0) | filp_close() {
0) 0.437 us | dnotify_flush();
0) 0.401 us | locks_remove_posix();
0) 0.349 us | fput();
0) 2.679 us | }
0) 4.452 us | }
i'd be surprised to see a flag to show up in that codepath. Eric, does
your testing confirm that?
On a socket/pipe, definitly no, because inode->i_sb->s_flags is not contended.
But on a shared inode, it might hurt :
offsetof(struct inode, i_count)=0x24
offsetof(struct inode, i_lock)=0x70
offsetof(struct inode, i_sb)=0x9c
offsetof(struct inode, i_writecount)=0x144
So i_sb sits in a probably contended cache line
I wonder why i_writecount sits so far from i_count, that doesnt make sense.
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