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Message-ID: <20211118112315.GD13047@quack2.suse.cz>
Date: Thu, 18 Nov 2021 12:23:15 +0100
From: Jan Kara <jack@...e.cz>
To: Chengguang Xu <cgxu519@...ernel.net>
Cc: Miklos Szeredi <miklos@...redi.hu>, Jan Kara <jack@...e.cz>,
Amir Goldstein <amir73il@...il.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
overlayfs <linux-unionfs@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
operation
On Thu 18-11-21 14:32:36, Chengguang Xu wrote:
>
> ---- 在 星期三, 2021-11-17 14:11:29 Chengguang Xu <cgxu519@...ernel.net> 撰写 ----
> > ---- 在 星期二, 2021-11-16 20:35:55 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
> > > On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@...ernel.net> wrote:
> > > >
> > > > ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
> > > > > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@...ernel.net> wrote:
> > > > > > > However that wasn't what I was asking about. AFAICS ->write_inode()
> > > > > > > won't start write back for dirty pages. Maybe I'm missing something,
> > > > > > > but there it looks as if nothing will actually trigger writeback for
> > > > > > > dirty pages in upper inode.
> > > > > > >
> > > > > >
> > > > > > Actually, page writeback on upper inode will be triggered by overlayfs ->writepages and
> > > > > > overlayfs' ->writepages will be called by vfs writeback function (i.e writeback_sb_inodes).
> > > > >
> > > > > Right.
> > > > >
> > > > > But wouldn't it be simpler to do this from ->write_inode()?
> > > > >
> > > > > I.e. call write_inode_now() as suggested by Jan.
> > > > >
> > > > > Also could just call mark_inode_dirty() on the overlay inode
> > > > > regardless of the dirty flags on the upper inode since it shouldn't
> > > > > matter and results in simpler logic.
> > > > >
> > > >
> > > > Hi Miklos，
> > > >
> > > > Sorry for delayed response for this, I've been busy with another project.
> > > >
> > > > I agree with your suggesion above and further more how about just mark overlay inode dirty
> > > > when it has upper inode? This approach will make marking dirtiness simple enough.
> > >
> > > Are you suggesting that all non-lower overlay inodes should always be dirty?
> > >
> > > The logic would be simple, no doubt, but there's the cost to walking
> > > those overlay inodes which don't have a dirty upper inode, right?
> >
> > That's true.
> >
> > > Can you quantify this cost with a benchmark? Can be totally synthetic,
> > > e.g. lookup a million upper files without modifying them, then call
> > > syncfs.
> > >
> >
> > No problem, I'll do some tests for the performance.
> >
>
> Hi Miklos,
>
> I did some rough tests and the results like below. In practice, I don't
> think that 1.3s extra time of syncfs will cause significant problem.
> What do you think?
Well, burning 1.3s worth of CPU time for doing nothing seems like quite a
bit to me. I understand this is with 1000000 inodes but although that is
quite a few it is not unheard of. If there would be several containers
calling sync_fs(2) on the machine they could easily hog the machine... That
is why I was originally against keeping overlay inodes always dirty and
wanted their dirtiness to at least roughly track the real need to do
writeback.
Honza
> Test bed: kvm vm
> 2.50GHz cpu 32core
> 64GB mem
> vm kernel 5.15.0-rc1+ (with ovl syncfs patch V6)
>
> one millon files spread to 2 level of dir hierarchy.
> test step:
> 1) create testfiles in ovl upper dir
> 2) mount overlayfs
> 3) excute ls -lR to lookup all file in overlay merge dir
> 4) excute slabtop to make sure overlay inode number
> 5) call syncfs to the file in merge dir
>
> Tested five times and the reusults are in 1.310s ~ 1.326s
>
> root@...144-4-centos test]# time ./syncfs ovl-merge/create-file.sh
> syncfs success
>
> real 0m1.310s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-merge/create-file.sh
> syncfs success
>
> real 0m1.326s
> user 0m0.001s
> sys 0m0.000s
> [root@...144-4-centos test]# time ./syncfs ovl-merge/create-file.sh
> syncfs success
>
> real 0m1.321s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-merge/create-file.sh
> syncfs success
>
> real 0m1.316s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-merge/create-file.sh
> syncfs success
>
> real 0m1.314s
> user 0m0.001s
> sys 0m0.001s
>
>
> Directly run syncfs to the file in ovl-upper dir.
> Tested five times and the reusults are in 0.001s ~ 0.003s
>
> [root@...144-4-centos test]# time ./syncfs a
> syncfs success
>
> real 0m0.002s
> user 0m0.001s
> sys 0m0.000s
> [root@...144-4-centos test]# time ./syncfs ovl-upper/create-file.sh
> syncfs success
>
> real 0m0.003s
> user 0m0.001s
> sys 0m0.000s
> [root@...144-4-centos test]# time ./syncfs ovl-upper/create-file.sh
> syncfs success
>
> real 0m0.001s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-upper/create-file.sh
> syncfs success
>
> real 0m0.001s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-upper/create-file.sh
> syncfs success
>
> real 0m0.001s
> user 0m0.000s
> sys 0m0.001s
> [root@...144-4-centos test]# time ./syncfs ovl-upper/create-file.sh
> syncfs success
>
> real 0m0.001s
> user 0m0.000s
> sys 0m0.001
>
>
>
>
>
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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