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Message-ID: <49306EA8.1050801@cosmosbay.com>
Date: Fri, 28 Nov 2008 23:20:24 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: Ingo Molnar <mingo@...e.hu>
CC: Al Viro <viro@...IV.linux.org.uk>,
David Miller <davem@...emloft.net>,
"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
kernel-testers@...r.kernel.org, Mike Galbraith <efault@....de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Linux Netdev List <netdev@...r.kernel.org>,
Christoph Lameter <cl@...ux-foundation.org>,
Christoph Hellwig <hch@...radead.org>, rth@...ddle.net,
ink@...assic.park.msu.ru
Subject: Re: [PATCH 6/6] fs: Introduce kern_mount_special() to mount special
vfs
Ingo Molnar a écrit :
> * Al Viro <viro@...IV.linux.org.uk> 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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