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

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Date:	Sat, 16 May 2015 09:30:22 +1000
From:	NeilBrown <neilb@...e.de>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	Dave Chinner <david@...morbit.com>,
	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>
Subject: Re: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU
 symlinks

On Fri, 15 May 2015 15:15:48 -0600 Andreas Dilger <adilger@...ger.ca> 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).

.. and I've been wondering what to do about i_mutex and NFS.  I've had
customer reports of slowness in creating files that seems to be due to
i_mutex on the directory being held over the whole 'create' RPC, so only one
of those can be in flight at the one time.
"make  -j" on a large source directory can easily want to create lots of
"*.o" files at "the same time".

And NFS doesn't need i_mutex at all because the server will provide the
needed guarantees.

NeilBrown


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