Re: write performance difference 3.18.21/4.2.1

[Chao Yu <chao2.yu@samsung.com>]

Hi Marc Jaegeuk,

> -----Original Message-----
> From: Marc Lehmann [mailto:schmorp@schmorp.de]
> Sent: Friday, September 25, 2015 2:51 PM
> To: Jaegeuk Kim
> Cc: linux-f2fs-devel@lists.sourceforge.net
> Subject: Re: [f2fs-dev] write performance difference 3.18.21/4.2.1
> 
> On Thu, Sep 24, 2015 at 11:28:36AM -0700, Jaegeuk Kim <jaegeuk@kernel.org> wrote:
> > One thing that we can try is to run the latest f2fs source in v3.18.
> > This branch supports f2fs for v3.18.
> 
> Ok, please bear with me, the last time I built my own kernel was during
> the 2.4 timeframe, and this is a ubuntu kernel. What I did is this:
> 
>    git clone -b linux-3.18 git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git
>    cd f2fs/fs/f2fs
>    rsync -avPR include/linux/f2fs_fs.h include/trace/events/f2fs.h
> /usr/src/linux-headers-3.18.21-031821/.
>    make -C /lib/modules/3.18.21-031821-generic/build/ M=$PWD modules modules_install
> 
> I then rmmod f2fs/insmod the resulting module, and tried to mount my
> existing f2fs fs for a quick test, but got a null ptr exception on "mount":
> 
> http://ue.tst.eu/e4628dcee97324e580da1bafad938052.txt

This is my fault, sorry about introducing this oops. :(

Please revert the commit 7c5e466755ff ("f2fs: readahead cp payload
pages when mount") since in this commit we try to access invalid
SIT_I(sbi)->sit_base_addr which should be inited later.

Thanks,

> 
> Probably caused me not building a full kernel, but recreating how ubuntu
> build their kernels on a debian system isn't something I look forward to.
> 
> > For example, if I can represent blocks like:
> [number of logs discussion]
> 
> Thanks for this explanation - two logs doesn't look so bad, from a
> locality viewpoint (not a big issue for flash, but a big issue for
> rotational devices - I also realised I can't use dmcache as dmcache, even
> in writethrough mode, writes back all data after an unclean shutdown,
> which would positively kill the disk).
> 
> Since whatever speed difference I saw with two logs wasn't big, you
> completely sold me on 6 logs, or 4 (especially if it seepds up the gc,
> which I haven't much tested yet). Two logs was merely a test anyway (the
> same with no_heap, I don't know what it does, but I thought it is worth
> a try, as metadata + data nearer together is better than having them at
> opposite ends of the log or so).
> 
> --
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>       ---==---(_)__  __ ____  __      Marc Lehmann
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> 
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