Re: more dput lock contentions in 2.6.38-rc?
From: Nick Piggin
Date: Mon Jan 24 2011 - 20:05:06 EST
On Tue, Jan 25, 2011 at 11:35 AM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> we are testing dbench benchmark and see big drop of 2.6.38-rc compared
> to 2.6.37 in several machines with 2 sockets or 4 sockets. We have 12
> disks mount to /mnt/stp/dbenchdata/sd*/ and dbench runs against data of
> the disks. According to perf, we saw more lock contentions:
> In 2.6.37: 13.00% dbench [kernel.kallsyms] [k] _raw_spin_lock
> In 2.6.38-rc: 69.45% dbench [kernel.kallsyms] [k]_raw_spin_lock
> - 69.45% dbench [kernel.kallsyms] [k] _raw_spin_lock
> - _raw_spin_lock
> - 48.41% dput
> - 61.17% path_put
> - 60.47% do_path_lookup
> + 53.18% user_path_at
> + 42.13% do_filp_open
> + 4.69% user_path_parent
What filesystems are mounted on the path?
> - 35.56% d_path
This guy is from glibc's statvfs call that dbench uses. It
parses /proc/mounts for mount flags which is racy (and
not a good idea to do with any frequency).
A patch went into the kernel that allows glibc to get the
flags directly. Not sure about glibc status, I imagine it
will get there in another decade or two... Can you try
commenting it out of dbench source code?
> + 2.17% do_filp_open
> + 1.72% mounts_release
> + 38.69% link_path_walk
> + 30.21% path_get
> + 19.08% nameidata_drop_rcu
> + 0.83% __d_lookup
> it appears there are heavy lock contention when dput release '/', 'mnt',
> 'stp', 'dbenchdata' and 'proc' when dbench is running.
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