Re: [PATCH 0/3] Allow ZRAM to use any zpool-compatible backend
From: Minchan Kim
Date: Tue Oct 15 2019 - 16:00:51 EST
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  and is notable for the heated discussions it has caused.
> > >
> > > The patchset in  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 . 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.
> > >
> > >  https://lkml.org/lkml/2015/9/14/356
> > >  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
> 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
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
> > 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
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.