Re: [RFC] mm: add support for zsmalloc and zcache

[Mel Gorman]

On Mon, Sep 24, 2012 at 01:36:48PM -0700, Dan Magenheimer wrote:
> > From: Mel Gorman [mailto:mgorman@xxxxxxx]
> > Subject: Re: [RFC] mm: add support for zsmalloc and zcache
> > 
> > On Sat, Sep 22, 2012 at 02:18:44PM -0700, Dan Magenheimer wrote:
> > > > From: Mel Gorman [mailto:mgorman@xxxxxxx]
> > > > Subject: Re: [RFC] mm: add support for zsmalloc and zcache
> > > >
> > > > On Fri, Sep 21, 2012 at 01:35:15PM -0700, Dan Magenheimer wrote:
> > > > > > From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> > > > > > Subject: Re: [RFC] mm: add support for zsmalloc and zcache
> > > > > The two proposals:
> > > > > A) Recreate all the work done for zcache2 as a proper sequence of
> > > > >    independent patches and apply them to zcache1. (Seth/Konrad)
> > > > > B) Add zsmalloc back in to zcache2 as an alternative allocator
> > > > >    for frontswap pages. (Dan)
> > > >
> > > > Throwing it out there but ....
> > > >
> > > > C) Merge both, but freeze zcache1 except for critical fixes. Only allow
> > > >    future work on zcache2. Document limitations of zcache1 and
> > > >    workarounds until zcache2 is fully production ready.
> > >
> > What would the impact be if zcache2 and zcache1 were mutually exclusive
> > in Kconfig and the naming was as follows?
> > 
> > CONFIG_ZCACHE_DEPRECATED	(zcache1)
> > CONFIG_ZCACHE			(zcache2)
> > 
> > That would make it absolutely clear to distributions which one they should
> > be enabling and also make it clear that all future development happen
> > on zcache2.
> > 
> > I know it looks insane to promote something that is instantly deprecated
> > but none of the other alternatives seem to be gaining traction either.
> > This would at least allow the people who are currently heavily behind
> > zcache1 to continue supporting it and applying critical fixes until they
> > move to zcache2.
> 
> Just wondering... how, in your opinion, is this different from
> leaving zcache1 (or even both) in staging? 

Because leaving it in staging implies it is not supported. What I'm
suggesting is that zcache1 be promoted but marked deprecated. Seth and the
embedded people that use it should continue to support it as it currently
stands and fix any critical bugs that are reported but avoid writing new
features for it. The limitations of it should be documented.

> "Tainting" occurs
> either way, it's just a matter of whether or not there is a message
> logged by the kernel that it is officially tainted, right?
> 

Using a deprecated interface does not necessarily taint the kernel.

> However, it _is_ another attempt at compromise and, if this
> is the only solution that allows the debate to end, and it
> is agreed on by whatever maintainer is committed to pull
> both (be it you, or Andrew, or Konrad, or Linux), I would
> agree to your "C-prime" proposal.
>  

And bear in mind that I do not any sort of say in what happens
ultimately. I'm just suggesting alternatives here that may potentially
keep everyone happy (or at least stop it going in circles).

> > > I use the terms "zcache1" and "zcache2" only to clarify which
> > > codebase, not because they are dramatically different. I estimate
> > > that 85%-90% of the code in zcache1 and zcache2 is identical, not
> > > counting the allocator or comments/whitespace/janitorial!
> > 
> > If 85-90% of the code is identicial then they really should be sharing
> > the code rather than making copies. That will result in some monolithic
> > patches but it's unavoidable. I expect it would end up looking like
> > 
> > Patch 1		promote zcache1
> > Patch 2		promote zcache2
> > Patch 3		move shared code for zcache1,zcache2 to common files
> > 
> > If the shared code is really shared and not copied it may reduce some of
> > the friction between the camps.
> 
> This part I would object to... at least I would object to signing
> up to do Patch 3 myself.  Seems like a lot of busywork if zcache1
> is truly deprecated.
> 

It'd help the path to truly deprecating it.

1. Fixes in common code only have to be applied once. This avoids a
   situation where zcache1 gets a fix and zcache2 misses it and vice-versa.
   In a related note it makes it a bit more obvious is a new feature is
   attempted to be merged to zcache1

2. It forces the zcache2 and zcache1 people to keep more or less in sync
   with each other and limit API breakage between components.

3. It makes it absolutely clear what the differences between zcache1 and
   zcache2 are at any given time.

My expectation is that the zcache1-specific components would shrink over
time with zcache2 taking over responsibility. Ideally the end result
would be that zcache1 is just an alias for the zcache2 code.

I recognise that this is a lot of busy work and time-consuming but it's
at least *a* path that allows zcache1 to migrate to zcache2. Of course
if the zcache1 people do not support the idea in principal then it goes
back to square one.

> > zcache1 does appear to have a few snarls that would make me wary of having
> > to support it. I don't know if zcache2 suffers the same problems or not
> > as I have not read it.
> > 
> > Unfortunately, I'm not going to get the chance to review [zcache2] in the
> > short-term. However, if zcache1 and zcache2 shared code in common files
> > it would at least reduce the amount of new code I have to read :)
> 
> Understood, which re-emphasizes my point about how the presence
> of both reduces the (to date, very limited) MM developer time available
> for either.
> 

While that may be true, it's not looking like that one side will accept the
complete deletion of zcache1 on day 1. On the flip-side, they have a point
that zcache1 has been tested by more people even if there are some serious
limitations in the code.

> > > Seth (and IBM) seems to have a bee in his bonnet that the existing
> > > zcache1 code _must_ be promoted _soon_ with as little change as possible.
> > > Other than the fact that he didn't like my patching approach [1],
> > > the only technical objection Seth has raised to zcache2 is that he
> > > thinks zsmalloc is the best choice of allocator [2] for his limited
> > > benchmarking [3].
> > 
> > FWIW, I would fear that kernbench is not that interesting a benchmark for
> > something like zcache. From an MM perspective, I would be wary that the
> > data compresses too well and fits too neatly in the different buckets and
> > make zsmalloc appear to behave much better than it would for a more general
> > workload.  Of greater concern is that the allocations for zcache would be
> > too short lived to measure if external fragmentation was a real problem
> > or not. This is pure guesswork as I didn't read zsmalloc but this is the
> > sort of problem I'd be looking out for if I did review it. In practice,
> > I would probably prefer to depend on zbud because it avoids the external
> > fragmentation problem even if it wasted memory but that's just me being
> > cautious.
> 
> Your well-honed intuition is IMHO exactly right.
> 
> But my compromise proposal would allow the allocator decision to be delayed
> until a broader set of workloads are brought to bear.
> 

If the API to the underlying allocator is fixed it should be at least
possible to load either. It does not feel like an issue that should
completely hold up everything.

It may be the case that on day 1 that zcache2 cannot use zsmalloc but then
I'd expect that at least the zsmalloc allocator would be the first block
of code shared by both zcache1 and zcache2.

> > > I've offered to put zsmalloc back in to zcache2 as an optional
> > > (even default) allocator, but that doesn't seem to be good enough
> > > for Seth.  Any other technical objections to zcache2, or explanation
> > > for his urgent desire to promote zcache1, Seth (and IBM) is keeping
> > > close to his vest, which I find to be a bit disingenuous.
> > 
> > I can only guess what the reasons might be for this and none of the
> > guesses will help resolve this problem.
> 
> Me too.  Given the amount of time already spent on this discussion
> (and your time reviewing, IMHO, old code), I sure hope the reasons
> are compelling.
> 
> It's awfully hard to determine a compromise when one side
> refuses to budge for unspecified reasons.   And the difference
> between deprecated and in-staging seems minor enough that
> it's hard to believe your modified proposal will make that
> side happy... but we are both shooting in the dark.
> 

This is why I think the compromise is going to be promoting both,
marking deprecated and then share as much code as possible. Without the
sharing the split may remain permanent and just cause more problems in
the future.

> > > So, I'd like to challenge Seth with a simple question:
> > >
> > > If zcache2 offers zsmalloc as an alternative (even default) allocator,
> > > what remaining _technical_ objections do you (Seth) have to merging
> > > zcache2 _instead_ of zcache1?
> > >
> > > If Mel agrees that your objections are worth the costs of bifurcating
> > > zcache and will still endorse merging both into core mm, I agree to move
> > > forward with Mel's alternative (C) (and will then repost
> > > https://lkml.org/lkml/2012/7/31/573).
> > 
> > If you go with C), please also add another patch on top *if possible*
> > that actually shares any common code between zcache1 and zcache2.
> 
> Let's hear Seth's technical objections first, and discuss post-merge
> followon steps later?
> 

Sure, but bear in mind I do not have the final say in this, I'm just making
suggestions on how this logjam could potentially be cleared.

-- 
Mel Gorman
SUSE Labs
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