RE: zsmalloc limitations and related topics

From: Dan Magenheimer
Date: Thu Feb 28 2013 - 17:01:51 EST

> From: Dan Magenheimer
> Subject: zsmalloc limitations and related topics
> :
> 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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