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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Brandon Casey <casey@nrlssc.navy.mil>
Cc: Alex Riesen <raa.lkml@gmail.com>,
	Bevan Watkiss <bevan.watkiss@cloakware.com>,
	Git Mailing List <git@vger.kernel.org>
Subject: Re:
Date: Sat, 9 May 2009 09:44:28 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LFD.2.01.0905090926300.3586@localhost.localdomain> (raw)
In-Reply-To: <twBG1KnSrgPNk7NoVey4mgig1BeAk7e1GHOT90PSV9ZGTs-zCWYdtA@cipher.nrlssc.navy.mil>



On Fri, 8 May 2009, Brandon Casey wrote:
> 
> btw, I've since done some more testing on some centos5.3 boxes we have.
> I get similar results (less ancient kernel 2.6.18).

Yes, 2.6.18 is still much too old to matter from a locking standpoint. 

When people initially worried about scalability, the issues were more 
about server side stuff and the cached cases. NFS (as a client) is 
certainly used on the server side too, but it tends to be a somewhat 
secondary worry where only specific parts really matter. So people worked 
a lot more on the core kernel, and on local high-performance filesystem 
scaling.

Only lately have we been pretty aggressive about finally really getting 
rid of the old "single big lock" (BKL) model entirely, or moving outwards 
from the core.

And while we removed the BKL from the normal NFS read/write paths long 
long ago, all the name lookup and directory handling code still had it 
until a year ago.

That, btw, is directly explained by perceived scalability issues: NFS is 
fairly often used as the backing store for a database and scaling thus 
matters there. But databases tend to keep their few big files open and use 
pread/pwrite - so pathname lookup is not nearly as significant for server 
ops as plain read/write.

(Pathname lookup is important for things like web servers etc, but they 
rely heavily on caching for that, and the cached case scales fine).

> I've also scanned through the errata announcements that RedHat has 
> released for their kernel updates.  A few of them involve NFS.  
> Possibly, whatever RedHat modified in the 5.X kernel was also backported 
> to the 4.X kernel.

That is very possibly the case. Expanding the BKL usage in some case could 
easily trigger the lock getting contention - and the way lock contention 
works, once you get a just even a small _hint_ of contention, things often 
fall off a cliff. The contention slows locking down, which in turn causes 
more CPU usage, which in turn causes _more_ contention.

So even a small amount of extra locking - or even just slowing down some 
code that was inside the lock - can have catastrophic behavioural changes 
when the lock is close to being a problem. You do not get a nice gradual 
slowdown at all - you just hit a hard wall.

I guess I should really try to set up some fileserver here at home to 
improve my test coverage. And to do better backups (or the little private 
data I have that I can't just mirror out to the world by turning it into 
an open-source project ;^)

				Linus

  reply	other threads:[~2009-05-09 16:47 UTC|newest]

Thread overview: 83+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-05-07 17:01 (unknown), Bevan Watkiss
2009-05-07 17:13 ` Alex Riesen
2009-05-07 17:26   ` Bevan Watkiss
2009-05-07 18:18     ` Alex Riesen
2009-05-07 18:48       ` Bevan Watkiss
2009-05-07 19:56         ` Björn Steinbrink
2009-05-07 18:56     ` Linus Torvalds
2009-05-07 19:37       ` RE: Bevan Watkiss
2009-05-07 20:07         ` RE: Linus Torvalds
2009-05-07 20:20           ` RE: Linus Torvalds
2009-05-07 20:43             ` Junio C Hamano
2009-05-07 21:33               ` Re: Linus Torvalds
2009-05-07 21:55             ` Linus Torvalds
2009-05-07 22:27               ` RE: david
2009-05-07 22:36                 ` RE: Linus Torvalds
2009-05-07 22:43                   ` RE: david
2009-05-07 23:00                     ` RE: Linus Torvalds
2009-05-07 23:07                       ` RE: david
2009-05-07 23:18                         ` RE: Linus Torvalds
2009-05-07 23:31                           ` RE: david
2009-05-07 23:57                             ` Johan Herland
2009-05-08 16:14                             ` Bevan Watkiss
2009-05-08  8:17               ` Alex Riesen
2009-05-08 14:39                 ` Re: Linus Torvalds
2009-05-08 15:51                   ` Re: Brandon Casey
2009-05-08 16:15                     ` Re: Linus Torvalds
2009-05-08 17:27                       ` Re: Brandon Casey
2009-05-08 17:43                         ` Re: Brandon Casey
2009-05-08 21:49                           ` Re: Linus Torvalds
2009-05-08 23:04                             ` Re: Brandon Casey
2009-05-09 16:44                               ` Linus Torvalds [this message]
2009-05-08 17:44                         ` Re: Linus Torvalds
2009-05-08 16:47               ` 'git checkout' and unlink() calls (was: Re: ) Kjetil Barvik
2009-05-08 17:57                 ` Linus Torvalds
     [not found] <20220301070226.2477769-1-jaydeepjd.8914>
2022-03-06 11:10 ` Jaydeep P Das
2022-03-06 11:22   ` Jaydeep Das
  -- strict thread matches above, loose matches on Subject: below --
2021-08-21 14:40 TECOB270_Ganesh Pawar
2021-08-21 23:52 ` Jeff King
2019-11-15 16:03 Martin Nicolay
2019-11-15 16:29 ` Martin Ågren
2019-11-15 16:37   ` Re: Martin Ågren
2019-08-20 17:23 William Baker
2019-08-20 17:27 ` Yagnatinsky, Mark
2019-03-05 14:57 [GSoC][PATCH v2 3/3] t3600: use helpers to replace test -d/f/e/s <path> Eric Sunshine
2019-03-05 23:38 ` Rohit Ashiwal
2019-01-23 10:50 Christopher Hagler
2019-01-23 14:16 ` Cody Kratzer
2019-01-23 14:25   ` Re: Thomas Braun
2019-01-23 16:00   ` Re: Christopher Hagler
2019-01-23 16:35     ` Randall S. Becker
2019-01-24 17:11   ` Johannes Schindelin
2018-10-08 13:33 Netravnen
2018-10-08 13:34 ` Inderpreet Saini
2018-04-27  0:54 [PATCH v3 2/3] merge: Add merge.renames config setting Ben Peart
2018-04-27 18:19 ` Elijah Newren
2018-04-30 13:11   ` Ben Peart
2018-04-30 16:12     ` Re: Elijah Newren
2018-05-02 14:33       ` Re: Ben Peart
2018-02-27  1:18 Alan Gage
2018-02-27 10:26 ` René Scharfe
2017-11-20 15:10 Viet Nguyen
2017-11-20 20:07 ` Stefan Beller
2017-11-12  2:21 hsed
2017-11-13 18:56 ` Stefan Beller
2017-01-25  0:11 [PATCH 7/7] completion: recognize more long-options Cornelius Weig
2017-01-25  0:21 ` Stefan Beller
2017-01-25  0:43   ` Cornelius Weig
2017-01-25  0:52     ` Re: Stefan Beller
2017-01-25  0:54   ` Re: Linus Torvalds
2017-01-25  1:32     ` Re: Eric Wong
2016-04-11 19:04 (unknown), miwilliams
2016-04-12  4:33 ` Stefan Beller
2015-08-19 19:41 Re: christain147
2015-08-19 11:09 Re: christain147
2015-08-05 12:47 (unknown) Ivan Chernyavsky
2015-08-15  9:19 ` Duy Nguyen
2015-08-17 17:49   ` Re: Junio C Hamano
2015-04-08 20:44 (unknown), Mamta Upadhyay
2015-04-08 21:58 ` Thomas Braun
2015-04-09 11:27   ` Re: Konstantin Khomoutov
     [not found] <CANSxx61FaNp5SBXJ8Y+pWn0eDcunmibKR5g8rttnWGdGwEMHCA@mail.gmail.com>
2015-03-18 20:45 ` Re: Junio C Hamano
2015-03-18 21:06   ` Re: Stefan Beller
2015-03-18 21:17     ` Re: Jeff King
2015-03-18 21:28       ` Re: Jeff King
2015-03-18 21:33         ` Re: Junio C Hamano
2015-03-18 21:45           ` Re: Stefan Beller
2015-03-13  1:34 (unknown) cody.taylor
2015-03-13  2:00 ` Duy Nguyen
2014-09-08 11:36 (unknown), R. Klomp
     [not found] ` <CAOqJoqGSRUw_UT4LhqpYX-WX6AEd2ReAWjgNS76Cra-SMKw3NQ@mail.gmail.com>
2014-09-08 14:36   ` R. Klomp
2014-09-10  0:00     ` Re: David Aguilar
2014-09-15 15:10       ` Re: R. Klomp
2014-02-06 11:54 "Sparse checkout leaves no entry on working directory" all the time on Windows 7 on Git 1.8.5.2.msysgit.0 konstunn
2014-02-06 13:20 ` Johannes Sixt
2014-02-06 19:56   ` Constantine Gorbunov
2012-06-12 21:12 (unknown), rohit sood
2012-06-12 23:51 ` Erik Faye-Lund
2009-11-18  5:03 Re: Anna
2009-05-11 18:57 (unknown) Don Slutz
2009-05-11 20:48 ` Johannes Schindelin
2009-05-12 12:45   ` Re: Don Slutz
2009-03-30  5:03 (unknown), David Aguilar
2009-03-30  7:02 ` Markus Heidelberg
2009-03-30  8:46   ` Re: Junio C Hamano
2007-11-01 20:44 (unknown), Francesco Pretto
2007-11-01 20:48 ` Francesco Pretto
2006-02-02  0:39 [RFC & PATCH] Solaris 8: ENOSYS when mkdir applied to automount., Jason Riedy
2006-02-02  4:18 ` H. Peter Anvin
2005-04-22 22:19 (unknown), atani
2005-04-22 23:16 ` Martin Schlemmer

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