linux-kernel - Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU symlinks

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Date:	Sat, 16 May 2015 09:39:13 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Neil Brown <neilb@...e.de>
Subject: Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU
 symlinks

On Fri, May 15, 2015 at 03:15:48PM -0600, Andreas Dilger wrote:
> On May 14, 2015, at 5:23 AM, Dave Chinner <david@...morbit.com> wrote:
> > 
> > On Wed, May 13, 2015 at 08:52:59PM -0700, Linus Torvalds wrote:
> >> On Wed, May 13, 2015 at 8:30 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >>> 
> >>> Maybe...  I'd like to see the profiles, TBH - especially getxattr() and
> >>> access() frequency on various loads.  Sure, make(1) and cc(1) really care
> >>> about stat() very much, but I wouldn't be surprised if something like
> >>> httpd or samba would be hitting getxattr() a lot...
> >> 
> >> So I haven't seen samba profiles in ages, but iirc we have more
> >> serious problems than trying to speed up basic filename lookup.
> >> 
> >> At least long long ago, inode semaphore contention was a big deal,
> >> largely due to readdir().
> > 
> > It still is - it's the prime reason people still need to create
> > hashed directory structures so that they can get concurrency in
> > directory operations.  IMO, concurrency in directory operations is a
> > more important problem to solve than worrying about readdir speed;
> > in large filesystems readdir and lookup are IO bound operations and
> > so everything serialises on the IO as it's done with the i_mutex
> > held....
> 
> We've had a patch[*] to add ext4 parallel directory operations in Lustre for
> a few years, that adds separate locks for each internal tree and leaf block
> instead of using i_mutex, so it scales as the size of the directory grows.
> This definitely improved many-threaded directory create/lookup/unlink
> performance (rename still uses a single lock).

Yup, we can do the same to XFS to implement concurrent modifications.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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