Re: [PATCH v7 0/5] zram/zsmalloc promotion

[Minchan Kim <minchan@kernel.org>]

On Wed, Aug 21, 2013 at 06:26:45PM +0800, Bob Liu wrote:
> On 08/21/2013 05:24 PM, Bob Liu wrote:
> > Hi Minchan,
> > 
> > On 08/21/2013 02:16 PM, Minchan Kim wrote:
> >> It's 7th trial of zram/zsmalloc promotion.
> >> I rewrote cover-letter totally based on previous discussion.
> >>
> >> The main reason to prevent zram promotion was no review of
> >> zsmalloc part while Jens, block maintainer, already acked
> >> zram part.
> >>
> >> At that time, zsmalloc was used for zram, zcache and zswap so
> >> everybody wanted to make it general and at last, Mel reviewed it
> >> when zswap was submitted to merge mainline a few month ago.
> >> Most of review was related to zswap writeback mechanism which
> >> can pageout compressed page in memory into real swap storage
> >> in runtime and the conclusion was that zsmalloc isn't good for
> >> zswap writeback so zswap borrowed zbud allocator from zcache to
> >> replace zsmalloc. The zbud is bad for memory compression ratio(2)
> >> but it's very predictable behavior because we can expect a zpage
> >> includes just two pages as maximum. Other reviews were not major. 
> >> http://lkml.indiana.edu/hypermail/linux/kernel/1304.1/04334.html
> >>
> >> Zcache doesn't use zsmalloc either so zsmalloc's user is only
> >> zram now so this patchset moves it into zsmalloc directory.
> >> Recently, Bob tried to move zsmalloc under mm directory to unify
> >> zram and zswap with adding pseudo block device in zswap(It's
> >> very weired to me) but he was simple ignoring zram's block device
> >> (a.k.a zram-blk) feature and considered only swap usecase of zram,
> >> in turn, it lose zram's good concept.
> >>
> > 
> > Yes, I didn't notice the feature that zram can be used as a normal block
> > device.
> > 
> > 
> >> Mel raised an another issue in v6, "maintainance headache".
> >> He claimed zswap and zram has a similar goal that is to compresss
> >> swap pages so if we promote zram, maintainance headache happens
> >> sometime by diverging implementaion between zswap and zram
> >> so that he want to unify zram and zswap. For it, he want zswap
> >> to implement pseudo block device like Bob did to emulate zram so
> >> zswap can have an advantage of writeback as well as zram's benefit.
> > 
> > If consider zram as a swap device only, I still think it's better to add
> > a pseudo block device to zswap and just disable the writeback of zswap.
> > 
> > But I have no idea of zram's block device feature.
> > 
> 
> BTW: I think the original/main purpose that zram was introduced is for
> swapping. Is there any real users using zram as a normal block device

I don't know but when I read http://lwn.net/Articles/334649/, it aimed
for compressing page caches as well as swap pages but it made widespread
hooks in core (I guess that's why zcache had a birth later by Nitin and Dan)
so reviewers guided him to support anon pages only to merge it.
And at that time, it was a specific virtual block device for only supporting
swap. AFAIRC, akpm suggested to make it general block device so other party
can have a benefit.

You can type "zram tmp" in google and will find many article related
to use zram as tmp and I have been received some questions/reports
from anonymous guys by private mail. And Jorome, Redhat guy, has
contributed that part like partial I/O.

> instead of swap?
> For normal usage, maybe we can extend ramdisk with compression feature.

Maybe, but I don't see any advantage. The ramdisk is really simple
and there is no part to share zram code. Morever, zram have a potential
to extend other features like asynchronous, defragmentation, and
multiple compressor. I don't want to make simple ramdisk bloating and
complicated so every distro could enable it.

Another thing is device name in linux is rather straightforward to
understand like "dd if=/dev/zero of=/dev/null" but if we support
compression feature of ramdisk, "dd if=/dev/ram0 of=/dev/ram1"?
Which is compression ramdisk? How could normal user can identify it?

I think there is no benefit.

> 
> -- 
> Regards,
> -Bob
> 
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-- 
Kind regards,
Minchan Kim

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