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Date: Wed, 13 Mar 2013 13:02:38 -0700 (PDT)
From: Dan Magenheimer <dan.magenheimer@oracle.com>
To: Robert Jennings <rcj@linux.vnet.ibm.com>
Cc: minchan@kernel.org, sjenning@linux.vnet.ibm.com,
        Nitin Gupta <nitingupta910@gmail.com>,
        Konrad Wilk <konrad.wilk@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Bob Liu <lliubbo@gmail.com>,
        Luigi Semenzato <semenzato@google.com>, Mel Gorman <mgorman@suse.de>
Subject: RE: zsmalloc limitations and related topics
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 <20130313151359.GA3130@linux.vnet.ibm.com>
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> From: Robert Jennings [mailto:rcj@linux.vnet.ibm.com]
> Subject: Re: zsmalloc limitations and related topics

Hi Robert --

Thanks for the well-considered reply!
 
> * Dan Magenheimer (dan.magenheimer@oracle.com) wrote:
> > Hi all --
> >
> > I've been doing some experimentation on zsmalloc in preparation
> > for my topic proposed for LSFMM13 and have run across some
> > perplexing limitations.  Those familiar with the intimate details
> > of zsmalloc might be well aware of these limitations, but they
> > aren't documented or immediately obvious, so I thought it would
> > be worthwhile to air them publicly.  I've also included some
> > measurements from the experimentation and some related thoughts.
> >
> > (Some of the terms here are unusual and may be used inconsistently
> > by different developers so a glossary of definitions of the terms
> > used here is appended.)
> >
> > ZSMALLOC LIMITATIONS
> >
> > Zsmalloc is used for two zprojects: zram and the out-of-tree
> > zswap.  Zsmalloc can achieve high density when "full".  But:
> >
> > 1) Zsmalloc has a worst-case density of 0.25 (one zpage per
> >    four pageframes).
> 
> The design of the allocator results in a trade-off between best case
> density and the worst-case which is true for any allocator.  For zsmalloc,
> the best case density with a 4K page size is 32.0, or 177.0 for a 64K page
> size, based on storing a set of zero-filled pages compressed by lzo1x-1.

Right.  Without a "representative workload", we have no idea
whether either my worst-case or your best-case will be relevant.

(As an aside, I'm measuring zsize=28 bytes for a zero page...
Seth has repeatedly said 103 bytes and I think this is
reflected in your computation above.  Maybe it is 103 for your
hardware compression engine?  Else, I'm not sure why our
numbers would be different.)
 
> > 2) When not full and especially when nearly-empty _after_
> >    being full, density may fall below 1.0 as a result of
> >    fragmentation.
> 
> True and there are several ways to address this including
> defragmentation, fewer class sizes in zsmalloc, aging, and/or writeback
> of zpages in sparse zspages to free pageframes during normal writeback.

Yes.  And add pageframe-reclaim to this list of things that
zsmalloc should do but currently cannot do.

> > 3) Zsmalloc has a density of exactly 1.0 for any number of
> >    zpages with zsize >= 0.8.
> 
> For this reason zswap does not cache pages which in this range.
> It is not enforced in the allocator because some users may be forced to
> store these pages; users like zram.

Again, without a "representative" workload, we don't know whether
or not it is important to manage pages with zsize >= 0.8.  You are
simply dismissing it as unnecessary because zsmalloc can't handle
them and because they don't appear at any measurable frequency
in kernbench or SPECjbb.  (Zbud _can_ efficiently handle these larger
pages under many circumstances... but without a "representative" workload,
we don't know whether or not those circumstances will occur.)

> > 4) Zsmalloc contains several compile-time parameters;
> >    the best value of these parameters may be very workload
> >    dependent.
> 
> The parameters fall into two major areas, handle computation and class
> size.  The handle can be abstracted away, eliminating the compile-time
> parameters.  The class-size tunable could be changed to a default value
> with the option for specifying an alternate value from the user during
> pool creation.

Perhaps my point here wasn't clear so let me be more blunt:
There's no way in hell that even a very sophisticated user
will know how to set these values.  I think we need to
ensure either that they are "always right" (which without
a "representative workload"...) or, preferably, have some way
so that they can dynamically adapt at runtime.

> > If density == 1.0, that means we are paying the overhead of
> > compression+decompression for no space advantage.  If
> > density < 1.0, that means using zsmalloc is detrimental,
> > resulting in worse memory pressure than if it were not used.
> >
> > WORKLOAD ANALYSIS
> >
> > These limitations emphasize that the workload used to evaluate
> > zsmalloc is very important.  Benchmarks that measure data
> > throughput or CPU utilization are of questionable value because
> > it is the _content_ of the data that is particularly relevant
> > for compression.  Even more precisely, it is the "entropy"
> > of the data that is relevant, because the amount of
> > compressibility in the data is related to the entropy:
> > I.e. an entirely random pagefull of bits will compress poorly
> > and a highly-regular pagefull of bits will compress well.
> > Since the zprojects manage a large number of zpages, both
> > the mean and distribution of zsize of the workload should
> > be "representative".
> >
> > The workload most widely used to publish results for
> > the various zprojects is a kernel-compile using "make -jN"
> > where N is artificially increased to impose memory pressure.
> > By adding some debug code to zswap, I was able to analyze
> > this workload and found the following:
> >
> > 1) The average page compressed by almost a factor of six
> >    (mean zsize == 694, stddev == 474)
> > 2) Almost eleven percent of the pages were zero pages.  A
> >    zero page compresses to 28 bytes.
> > 3) On average, 77% of the bytes (3156) in the pages-to-be-
> >    compressed contained a byte-value of zero.
> > 4) Despite the above, mean density of zsmalloc was measured at
> >    3.2 zpages/pageframe, presumably losing nearly half of
> >    available space to fragmentation.
> >
> > I have no clue if these measurements are representative
> > of a wide range of workloads over the lifetime of a booted
> > machine, but I am suspicious that they are not.  For example,
> > the lzo1x compression algorithm claims to compress data by
> > about a factor of two.
> 
> I'm suspicious of the "factor of two" claim.  The reference
> (http://www.oberhumer.com/opensource/lzo/lzodoc.php) for this would appear
> to be the results of compressing the Calgary Corpus.  This is fine for
> comparing compression algorithms but I would be hesitant to apply that
> to this problem space.  To illustrate the affect of input set, the newer
> Canterbury Corpus compresses to ~43% of the input size using LZO1X-1.

Yes, agreed, we have no idea if the Corpus is representative of
this problem space... because we have no idea what would
be a "representative workload" for this problem space.

But for how I was using "factor of two", a factor of 100/43=~2.3 is
close enough.  I was only trying to say "factor of two" may be
more "representative" than the "factor of six" in kernbench.

(As an aside, I like the data Nitin collected here:
http://code.google.com/p/compcache/wiki/CompressedLengthDistribution 
as it shows how different workloads can result in dramatically
different zsize distributions.  However, this data includes
all the pages in a running system, including both anonymous
and file pages, and doesn't include mean/stddev.)

> In practice the average for LZO would be workload dependent, as you
> demonstrate with the kernel build.  Swap page entropy for any given
> workload will not necessarily fit the distribution present in the
> Calgary Corpus.  The high density allocation design in zsmalloc allows
> for workloads that can compress to factors greater than 2 to do so.

Exactly.  But at what cost on other workloads?  And how do we evaluate
the cost/benefit of that high density? (... without a "representative
workload" ;-)

> > I would welcome ideas on how to evaluate workloads for
> > "representativeness".  Personally I don't believe we should
> > be making decisions about selecting the "best" algorithms
> > or merging code without an agreement on workloads.
> 
> I'd argue that there is no such thing as a "representative workload".
> Instead, we try different workloads to validate the design and illustrate
> the performance characteristics and impacts.

Sorry for repeatedly hammering my point in the above, but
there have been many design choices driven by what was presumed
to be representative (kernbench and now SPECjbb) workload
that may be entirely wrong for a different workload (as
Seth once pointed out using the text of Moby Dick as a source
data stream).

Further, the value of different designs can't be measured here just
by the workload because the pages chosen to swap may be completely
independent of the intended workload-driver... i.e. if you track
the pid of the pages intended for swap, the pages can be mostly
pages from long-running or periodic system services, not pages
generated by kernbench or SPECjbb.  So it is the workload PLUS the
environment that is being measured and evaluated.  That makes
the problem especially tough.

Just to clarify, I'm not suggesting that there is any single
workload that can be called representative, just that we may
need both a broad set of workloads (not silly benchmarks) AND
some theoretical analysis to drive design decisions.  And, without
this, arguing about whether zsmalloc is better than zbud or not
is silly.  Both zbud and zsmalloc have strengths and weaknesses.

That said, it should also be pointed out that the stream of
pages-to-compress from cleancache ("file pages") may be dramatically 
different than for frontswap ("anonymous pages"), so unless you
and Seth are going to argue upfront that cleancache pages should
NEVER be candidates for compression, the evaluation criteria
to drive design decisions needs to encompass both anonymous
and file pages.  It is currently impossible to evaluate that
with zswap.

> > PAGEFRAME EVACUATION AND RECLAIM
> >
> > I've repeatedly stated the opinion that managing the number of
> > pageframes containing compressed pages will be valuable for
> > managing MM interaction/policy when compression is used in
> > the kernel.  After the experimentation above and some brainstorming,
> > I still do not see an effective method for zsmalloc evacuating and
> > reclaiming pageframes, because both are complicated by high density
> > and page-crossing.  In other words, zsmalloc's strengths may
> > also be its Achilles heels.  For zram, as far as I can see,
> > pageframe evacuation/reclaim is irrelevant except perhaps
> > as part of mass defragmentation.  For zcache and zswap, where
> > writethrough is used, pageframe evacuation/reclaim is very relevant.
> > (Note: The writeback implemented in zswap does _zpage_ evacuation
> > without pageframe reclaim.)
> 
> zswap writeback without guaranteed pageframe reclaim can occur during
> swap activity.  Reclaim, even if it doesn't free a physical page, makes
> room in the page for incoming swap.  With zswap the writeback mechanism
> is driven by swap activity, so a zpage freed through writeback can be
> back-filled by a newly compressed zpage.  Fragmentation is an issue when
> processes exit and block zpages are invalidated and becomes an issue when
> zswap is idle.  Otherwise the holes provide elasticity to accommodate
> incoming pages to zswap.  This is the case for both zswap and zcache.
> 
> At idle we would want defragmentation or aging, either of which has
> the end result of shrinking the cache and returning pages to the
> memory manager.  The former only reduces fragmentation while the
> later has the additional benefit of returning memory for other uses.
> By adding aging, through periodic writeback, zswap becomes a true cache,
> it eliminates long-held allocations, and addresses fragmentation for
> long-held allocations.

We are definitely on different pages here.  You are still trying to
push zswap as a separate subsystem that can independently decide how
to size itself.  I see zcache (and zswap) as a "helper" for the MM
subsystem which allow MM to store more anonymous/pagecache pages in
memory than otherwise possible.  This becomes more obvious when
considering policy for both anonymous AND pagecache pages... and zswap
is not handling both.

> Because the return value of zs_malloc() is not a pointer, but an opaque
> value that only has meaning to zsmalloc, the API zsmalloc already has
> would support the addition of an abstraction layer that would accommodate
> allocation migration necessary for defragmentation.

While what you say is theoretically true and theoretically a very nice
feature to have, the current encoding of a zsmalloc handle does not
appear to be support your argument.  (And, btw, zbud does a very similar
opaque encoding.)

> > CLOSING THOUGHT
> >
> > Since zsmalloc and zbud have different strengths and weaknesses,
> > I wonder if some combination or hybrid might be more optimal?
> > But unless/until we have and can measure a representative workload,
> > only intuition can answer that.

You didn't respond to this, but I am increasingly inclined to
believe that the truth lies here, and the path to success lies
in working together rather than in battling/forking.  From what we jointly
have learned, if we were locked together in the same room and asked
to jointly design a zpage allocator from scratch, I suspect the result
would be quite different from either zsmalloc or zbud.

Thanks,
Dan

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