RE: frontswap/zcache: xvmalloc discussion

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> > One neat feature of frontswap (and the underlying Transcendent
> > Memory definition) is that ANY PUT may be rejected**.  So zcache
> > could keep track of the distribution of "zsize" and if the number
> > of pages with zsize>PAGE_SIZE/2 greatly exceeds the number of pages
> > with "complementary zsize", the frontswap code in zcache can reject
> > the larger pages until balance/sanity is restored.
> >
> > Might that help?
> 
> We could do that, but I imagine that would let a lot of pages through
> on most workloads.  Ideally, I'd like to find a solution that would
> capture and (efficiently) store pages that compressed to up to 80% of
> their original size.

After thinking about this a bit, I have to disagree.  For workloads
where the vast majority of pages have zsize>PAGE_SIZE/2, this would
let a lot of pages through.  So if you are correct that LZO
is poor at compression and a large majority of pages are in
this category, some page-crossing scheme is necessary.  However,
that isn't what I've seen... the zsize of many swap pages is
quite small.

So before commencing on a major compression rewrite, it might
be a good idea to measure distribution of zsize for swap pages
on a large variety of workloads.  This could probably be done
by adding a code snippet in the swap path of a normal (non-zcache)
kernel.  And if the distribution is bad, replacing LZO with a
higher-compression-but-slower algorithm might be the best answer,
since zcache is replacing VERY slow swap-device reads/writes with
reasonably fast compression/decompression.  I certainly think
that an algorithm approaching an average 50% compression ratio
should be the goal.

Dan

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