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[Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") f

Dan Magenheimer wrote:
Tmem [PATCH 0/4] (Take 2): Transcendent memory
Transcendent memory - Take 2
Changes since take 1:
1) Patches can be applied serially; function names in diff (Rik van Riel)
2) Descriptions and diffstats for individual patches (Rik van Riel)
3) Restructure of tmem_ops to be more Linux-like (Jeremy Fitzhardinge)
4) Drop shared pools until security implications are understood (Pavel
   Machek and Jeremy Fitzhardinge)
5) Documentation/transcendent-memory.txt added including API description
   (see also below for API description).

Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>

Normal memory is directly addressable by the kernel, of a known
normally-fixed size, synchronously accessible, and persistent (though
not across a reboot).

What if there was a class of memory that is of unknown and dynamically
variable size, is addressable only indirectly by the kernel, can be
configured either as persistent or as "ephemeral" (meaning it will be
around for awhile, but might disappear without warning), and is still
fast enough to be synchronously accessible?

I have trouble mapping this to a VMM capable of overcommit without just coming back to CMM2.

In CMM2 parlance, ephemeral tmem pools is just normal kernel memory marked in the volatile state, no?

It seems to me that an architecture built around hinting would be more robust than having to use separate memory pools for this type of memory (especially since you are requiring a copy to/from the pool).

For instance, you can mark data DMA'd from disk (perhaps by read-ahead) as volatile without ever bringing it into the CPU cache. With tmem, if you wanted to use a tmem pool for all of the page cache, you'd likely suffer significant overhead due to copying.


Anthony Liguori

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