From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:46872 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751795AbdIVTFT (ORCPT ); Fri, 22 Sep 2017 15:05:19 -0400 Date: Fri, 22 Sep 2017 21:05:28 +0200 From: Greg KH To: Eduardo Valentin Cc: Willy Tarreau , stable@vger.kernel.org, shli@fb.com, neilb@suse.com, colyli@suse.de Subject: Re: [stable v4.9.y] Backports to fix fstrim time / CPU load on raid0 Message-ID: <20170922190528.GA17075@kroah.com> References: <20170922171606.GA22274@u40b0340c692b58f6553c.ant.amazon.com> <20170922173145.GB6480@kroah.com> <20170922174048.GB11575@u40b0340c692b58f6553c.ant.amazon.com> <20170922181122.GA13515@1wt.eu> <20170922183859.GC11575@u40b0340c692b58f6553c.ant.amazon.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170922183859.GC11575@u40b0340c692b58f6553c.ant.amazon.com> Sender: stable-owner@vger.kernel.org List-ID: On Fri, Sep 22, 2017 at 11:38:59AM -0700, Eduardo Valentin wrote: > Hello Willy, > > On Fri, Sep 22, 2017 at 08:11:22PM +0200, Willy Tarreau wrote: > > On Fri, Sep 22, 2017 at 10:40:48AM -0700, Eduardo Valentin wrote: > > > On Fri, Sep 22, 2017 at 07:31:45PM +0200, Greg KH wrote: > > > > On Fri, Sep 22, 2017 at 10:16:06AM -0700, Eduardo Valentin wrote: > > > > > Hello GregKH, > > > > > > > > > > I have been seeing several reports of performance issue with raid0 while performing fstrim on v4.9.y. > > > > > Currently, if one performs: > > > > > > > > > > # fio --name fio_test_file --direct=1 --rw=randwrite --bs=4k --size=5G --numjobs=8 --group_reporting --directory=/mount/raid0 > > > > > # rm -rf /media/nvme-raid0 > > > > > # time fstrim -vvv -a > > > > > real 3m41.102s > > > > > user 0m0.000s > > > > > sys 0m4.964s > > > > > > > > Also, is this a regression from older kernels? > > > > > > I personally did not try older than 4.9 kernels. but looking at git history, > > > and the commit message of the fix, looks like this is long known issue that > > > got fixed on v4.12. > > > > Or it may simply be something that couldn't be achieved without significantly > > improving the underlying infrastructure using the patches you've spotted (and > > possibly a few others that you didn't notice but are required for stability > > or to avoid breaking other subsystems). > > > > While I use 4.9 on many of my machines, I'd rather favor maximal stability > > over an optimization for some operations that don't appear *that* often. > > That said if the relevant maintainers consider it safe to backport, I'll > > certainly welcome some performance improvements on my machines, but that's > > not what I'm primarily looking for in stable kernels. > > > > And if we start to backport performance improvements into LTS kernels, > > what will encourage users to upgrade to the next LTS ? > > Yeah, I pondered for several days before opening up this backport > request exactly because of your concerns above. > > But what made me to decide to share the backport for the community > consideration is the way I see this. To me, this is *not only* a > matter of performance boost, but also a real fix. Mainly because the > systems with v4.9.y become unresponsive while performing fstrim > because of kernel CPU consumption with the fstrim implementation in > raid0. Meaning, depending on how this is deployed, how you use your > fs, and how you deploy your raid, this can be a real problem on > production systems. > > So, yeah, I hope you consider this not only from a performance > perspective, but a fix for a real issue. It might be a "fix", but given that this isn't a regression, the reason for backporting this is pretty low. If you want faster I/O, upgrade to a newer kernel, sounds like a good reason to me (not to mention more features and more bugfixes as well!) :) So I would need some big explainations by the developers involved here as to why I should take it (which is what has caused me to take such things in the past for other subsystems...) thanks, greg k-h