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SubjectRE: [RFC] transcendent memory for Linux


> From: Linus Walleij [mailto:linus.ml.walleij@gmail.com]
> Sent: Saturday, June 27, 2009 7:19 AM
> Subject: Re: [RFC] transcendent memory for Linux
>
> > We call this latter class "transcendent memory" and it
> > provides an interesting opportunity to more efficiently
> > utilize RAM in a virtualized environment.  However this
> > "memory but not really memory" may also have applications
> > in NON-virtualized environments, such as hotplug-memory
> > deletion, SSDs, and page cache compression.  Others have
> > suggested ideas such as allowing use of highmem memory
> > without a highmem kernel, or use of spare video memory.
>
> Here is what I consider may be a use case from the embedded
> world: we have to save power as much as possible, so we need
> to shut off entire banks of memory.
>
> Currently people do things like put memory into self-refresh
> and then sleep, but for long lapses of time you would
> want to compress memory towards lower addresses and
> turn as many banks as possible off.
>
> So we have something like 4x16MB banks of RAM = 64MB RAM,
> and the most necessary stuff easily fits in one of them.
> If we can shut down 3x16MB we save 3 x power supply of the
> RAMs.
>
> However in embedded we don't have any swap, so we'd need
> some call that would attempt to remove a memory by paging
> out code and data that has been demand-paged in
> from the FS but no dirty pages, these should instead be
> moved down to memory which will be retained, and the
> call should fail if we didn't succeed to migrate all
> dirty pages.
>
> Would this be possible with transcendent memory?

Yes, I think this would work nicely as a use case for tmem.

As Avi points out, you could do this with memory defragmentation,
but if you know in advance that you will be frequently
powering on and off a bank of RAM, you could put only
ephemeral memory into it (enforced by a kernel policy and
the tmem API), then defragmentation (and compression towards
lower addresses) would not be necessary, and you could power
off a bank with no loss of data.

One issue though: I would guess that copying pages of memory
could be very slow in an inexpensive embedded processor.

Dan
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