Return-Path: linux-nfs-owner@vger.kernel.org Received: from cantor2.suse.de ([195.135.220.15]:43267 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750896AbaA2HZo (ORCPT ); Wed, 29 Jan 2014 02:25:44 -0500 Date: Wed, 29 Jan 2014 18:25:32 +1100 From: NeilBrown To: Trond Myklebust Cc: "tigran.mkrtchyan@desy.de" , Jim Rees , linux-nfs Subject: Re: readdir vs. getattr Message-ID: <20140129182532.7479eeda@notabene.brown> In-Reply-To: <1365090480.10726.22.camel@leira.trondhjem.org> References: <20130404151507.GA8484@umich.edu> <1365090480.10726.22.camel@leira.trondhjem.org> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/e18p1X4/BDy6BlYjXCbarxT"; protocol="application/pgp-signature" Sender: linux-nfs-owner@vger.kernel.org List-ID: --Sig_/e18p1X4/BDy6BlYjXCbarxT Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable (resent with correct address for Trond) On Thu, 4 Apr 2013 15:48:01 +0000 "Myklebust, Trond" wrote: > On Thu, 2013-04-04 at 17:38 +0200, Tigran Mkrtchyan wrote: > > On Thu, Apr 4, 2013 at 5:15 PM, Jim Rees wrote: > > > Tigran Mkrtchyan wrote: > > > > > > we have a directory with 50K (number of ) files in it. > > > The user does a 'ls' and I can see READDIR4. To > > > get the complete listing a client need to send ~380 requests. > > > Now user does yet another 'ls' in the same directory. > > > The client sends a GETATTR on directorie's FH > > > (actually two of GETATTRS - why?!!) and discovers that a > > > directory didn't change and re-uses existing listing, BUT!!! > > > for each file in the directory it sends a GETATTR to discover > > > is the file's attributes are changed. For 50K files it's a 50K req= uests. > > > > > > So is this a "ls -l"? Because for "ls" it shouldn't stat all the file= s. > >=20 > > I believe it's 'ls -l'. Well, you probably want to say that it's ls > > calling stat on each file. Nevertheless client still should re-use > > cached information. >=20 > What makes you think that it isn't using cached information? I'm > guessing you just need to adjust the values of acregmin and acregmax > upwards. >=20 > That said, we might be able to be a little more intelligent about how we > use the NFS_INO_ADVISE_RDPLUS hint, and have it blow out the readdir > cache when we find ourselves doing lots of lookup revalidates. >=20 Pop. I recently had a customer raise exactly this issue with me, so I've been looking into it. I don't think it can really be fixed by adjusting acregmin/acregmax. Once you have done READDIRPLUS, you have the directory contents in the page-cache and will continue to use those contents until they drop out of t= he cache, or until the directory changes in some way. Meanwhile the stat information from the READDIRPLUS was used to create/upda= te info in the inode table and that will eventually become stale. As soon as = it becomes stale you get a GETATTR storm on the next "ls -l" instead of a few READDIRPLUS calls. By increasing acregmin you can delay that storm, but you can put it off forever. Fixing this is tricky. We really want to know on the first nfs_readdir() c= all whether it will be followed by lookups or not. If it won't, then using the cached data is fine. If it will, then we really want a READDIRPLUS. The only way I can see to address this is for nfs_advise_use_readdirplus (= or code near where that is called) to notice that a readdir is currently active on the same directory and is using cached data, and to re-use that 'open' of the directory to do a readdirplus. This would update the stat info for the current inode and all the other inodes for the directory. This is fairly horrible. The 'struct file' used by the readdir would need = to be stored somewhere so that nfs_lookup_revalidate can use it (if process permissions allow). If multiple processes were doing a readdir at the same time .... I would certainly get confused. However I cannot think of anything else that would even come close to being= a real solution. Any solution that just modified nfs_readdir() could only avoid the GETATTR storm by largely ignoring the cached information and (almost) always calling READDIRPLUS. Does anyone have any other ideas? Or do you think it is worth trying to implement the above "horrible" idea. I did start working on it, but only g= ot far enough to understand the full extend of what is required. Thanks, NeilBrown --Sig_/e18p1X4/BDy6BlYjXCbarxT Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) iQIVAwUBUuis7Dnsnt1WYoG5AQLHJRAAihkhlpgJOQ+mH5vn0JA8BEzFh6eaK7PJ m+n2sh37ypkEGMLVTa36uM0iLRu2kXzlZeOdy7/ayWmj0EEcw/04gXaCwf0BJc+K W71nE2X9vpMh42VbWUGeQJfLJo71ul0I7kFKN5l0nQHGhKfSjhA6Xt9CtZl92kCM nCx5FZvgYW4k/fzMfS0V0PEYcrp8eg5du9FshXabjlR5peW7QVjJ1beuGRaEYkfX WmAy6EGIvtdORFxOjFs8VEA8O7yuwWQSRh0YO4guzetKRDuQIK6RQ0oLIdQb5Hdi eEUrirQRIw3TrbVrEOwvq8ci0Z2OWkoGug1DTou9tnysCi1XKSksAkdDMJbHon/P YbgfAyEQ+xwRhjXaHbgHZlqc6dhYPt1zIl5bqaCz9eBifPK9u1QGI7oA075qTwjA cRnXv2rHHLg/y7YvQAx27Yye3VQFdc4pfsr1Nd4/odFk4miJeVpH/hfmCvCsh7JM Ds/6bRL7eJw7Ki83ZEY27ZhdCi/ZdPfkSNl0kjJdkR3c7z6tEnt9jmYB10MmhjrQ nDOIRGhyRlc5+DHEyrHvkrnQf2zyQJt6A9FWpuL+z9L1vf6q9Nl6umbGj6j3lp9P +xVgCoemwoZm4qcPfg8zmEcI7PYiceFqwftWFkamBu3DCTHSSPU/puvEzGHm6T/r 7TpYjH5dDJc= =UbtI -----END PGP SIGNATURE----- --Sig_/e18p1X4/BDy6BlYjXCbarxT--