Re: [PATCH 0/3] Allow ZRAM to use any zpool-compatible backend

[Minchan Kim]

On Tue, Oct 15, 2019 at 09:39:35AM +0200, Vitaly Wool wrote:
> Hi Minchan,
> 
> On Mon, Oct 14, 2019 at 6:41 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >
> > On Thu, Oct 10, 2019 at 11:04:14PM +0300, Vitaly Wool wrote:
> > > The coming patchset is a new take on the old issue: ZRAM can currently be used only with zsmalloc even though this may not be the optimal combination for some configurations. The previous (unsuccessful) attempt dates back to 2015 [1] and is notable for the heated discussions it has caused.
> > >
> > > The patchset in [1] had basically the only goal of enabling ZRAM/zbud combo which had a very narrow use case. Things have changed substantially since then, and now, with z3fold used widely as a zswap backend, I, as the z3fold maintainer, am getting requests to re-interate on making it possible to use ZRAM with any zpool-compatible backend, first of all z3fold.
> > >
> > > The preliminary results for this work have been delivered at Linux Plumbers this year [2]. The talk at LPC, though having attracted limited interest, ended in a consensus to continue the work and pursue the goal of decoupling ZRAM from zsmalloc.
> > >
> > > The current patchset has been stress tested on arm64 and x86_64 devices, including the Dell laptop I'm writing this message on now, not to mention several QEmu confugirations.
> > >
> > > [1] https://lkml.org/lkml/2015/9/14/356
> > > [2] https://linuxplumbersconf.org/event/4/contributions/551/
> >
> > Please describe what's the usecase in real world, what's the benefit zsmalloc
> > cannot fulfill by desgin and how it's significant.
> 
> I'm not entirely sure how to interpret the phrase "the benefit
> zsmalloc cannot fulfill by design" but let me explain.
> First, there are multi multi core systems where z3fold can provide
> better throughput.

Please include number in the description with workload.

> Then, there are low end systems with hardware
> compression/decompression support which don't need zsmalloc
> sophistication and would rather use zbud with ZRAM because the
> compression ratio is relatively low.

I couldn't imagine how it's bad with zsmalloc. Could you be more
specific?

> Finally, there are MMU-less systems targeting IOT and still running
> Linux and having a compressed RAM disk is something that would help
> these systems operate in a better way (for the benefit of the overall
> Linux ecosystem, if you care about that, of course; well, some people
> do).

Could you write down what's the problem to use zsmalloc for MMU-less
system? Maybe, it would be important point rather other performance
argument since other functions's overheads in the callpath are already
rather big.

> 
> > I really don't want to make fragmentaion of allocator so we should really see
> > how zsmalloc cannot achieve things if you are claiming.
> 
> I have to say that this point is completely bogus. We do not create
> fragmentation by using a better defined and standardized API. In fact,
> we aim to increase the number of use cases and test coverage for ZRAM.
> With that said, I have hard time seeing how zsmalloc can operate on a
> MMU-less system.
> 
> > Please tell us how to test it so that we could investigate what's the root
> > cause.
> 
> I gather you haven't read neither the LPC documents nor my
> conversation with Sergey re: these changes, because if you did you
> wouldn't have had the type of questions you're asking. Please also see
> above.

Please include your claims in the description rather than attaching
file. That's the usualy way how we work because it could make easier to
discuss by inline.

> 
> I feel a bit awkward explaining basic things to you but there may not
> be other "root cause" than applicability issue. zsmalloc is a great
> allocator but it's not universal and has its limitations. The
> (potential) scope for ZRAM is wider than zsmalloc can provide. We are
> *helping* _you_ to extend this scope "in real world" (c) and you come
> up with bogus objections. Why?

Please add more detail to convince so we need to think over why zsmalloc
cannot be improved for the usecase.