RE: zsmalloc limitations and related topics

[Dan Magenheimer]

> From: Dan Magenheimer
> Subject: zsmalloc limitations and related topics
> 
> WORKLOAD ANALYSIS
>   :
> 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 realized that with a small hack in zswap, I could simulate the
effect on zsmalloc of a workload with very different zsize
distribution, one with a much higher mean, by simply doubling
(and tripling) the zsize passed to zs_malloc.  The results:

Unchanged: mean=694 stddev=474 -> mean density = 3.2
Doubled:   mean=1340 stddev=842 -> mean density = 1.9
Tripled:   mean=1636 stddev=1031 -> mean density = 1.6

Note that even tripled, the mean of the simulated
distribution is still much lower than PAGE_SIZE/2,
which is roughly the published expected compression for
lzo1x.  So one would still expect a mean density greater
than two but, apparently, one-third of available space is
lost to fragmentation.

Without a "representative" workload, I still have no clue
as to whether this simulated distribution is relevant,
but it is interesting to note that, for a workload with
lower mean compressibility, zsmalloc's reputation as
"high density" may be undeserved.

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