Re: [Xen-devel] Memory Sharing on HVM guests

[Eric Shelton <eshelton@xxxxxxxxx>]

Thank you for taking the time; it answered a number of lingering questions.

If anyone is successfully using blkback2 with memshr, I would
appreciate hearing about it.

> > 6) Does OS caching need to be disabled, or more likely dialed down to
> > a small maximum, to ensure free memory gets shared and relinquished?
>
> OS guest side caching is irrelevant. OS dom0 caching is disabled in most cases
> when using blkback or blktap. I am not sure what you are talking about here
> though: you are linking caching to freeing memory to sharing. Caching is a 
> good
> thing and that's precisely what tools/memshr, the page cache on the OS guest 
> side,
> aims to share.

Maybe an example will avoid abusing some of those terms:
Given a standard Linux system with swap disabled, a set of processes
will have certain minimum RAM requirement for instructions and data (I
have seen the term "working set" applied) - for example, more than 1GB
of RAM will be needed to run a large full-chip simulation.

Generally there is more than that minimum amount of RAM.  Over time,
except with a small working set of programs and data, all or most the
excess RAM tends to be put to use in the page cache (rather than being
entirely unused), but is readily available to be directly assigned to
processes.  I assume pages in the page cache would contain useful,
non-zero data, such as recent block I/O data.  This appears to exclude
zero-page sharing, and may not be all that successful for same-page
sharing.

In general, I probably would find it more useful if (most of) the
excess pages were available for other domains via sharing.  For
example, maybe it would be better for the excess pages to be part of
the page cache for a storage domain common to the other domains,
allowing it to make more holistic caching decisions and hopefully
already have the more active blocks in its cache - perhaps affording
some TMEM-like benefits to non-TMEM-capable OSes (which is pretty much
anything other than Linux?).

The question was mainly: if I lazily/conservatively overallocate
excess memory to domains, and hope page sharing will automagically
minimize their footprint, will the use and dirtying of excess pages by
the page cache cripple their sharing?  If so, I am curious if would
make sense to cap the page cache, if possible, to say 100MB.  I
suspect total disabling of the page cache is impossible or destroys
performance.

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