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xen-devel
[Xen-devel] RE: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") f
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To: |
Rik van Riel <riel@xxxxxxxxxx> |
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Subject: |
[Xen-devel] RE: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") for Linux |
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From: |
Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> |
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Date: |
Tue, 7 Jul 2009 12:53:06 -0700 (PDT) |
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Cc: |
npiggin@xxxxxxx, akpm@xxxxxxxx, jeremy@xxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, tmem-devel@xxxxxxxxxxxxxx, kurt.hackel@xxxxxxxxxx, Russell <rusty@xxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, dave.mccracken@xxxxxxxxxx, linux-mm@xxxxxxxxx, Rusty, sunil.mushran@xxxxxxxxxx, Avi Kivity <avi@xxxxxxxxxx>, Schwidefsky <schwidefsky@xxxxxxxxxx>, Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>, Marcelo Tosatti <mtosatti@xxxxxxxxxx>, alan@xxxxxxxxxxxxxxxxxxx, chris.mason@xxxxxxxxxx |
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Tue, 07 Jul 2009 12:54:25 -0700 |
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> From: Rik van Riel [mailto:riel@xxxxxxxxxx]
> Dan Magenheimer wrote:
> > "Preswap" IS persistent, but for various reasons may not always be
> > available for use, again due to factors that may not be
> visible to the
> > kernel (but, briefly, if the kernel is being "good" and has
> shared its
> > resources nicely, then it will be able to use preswap, else
> it will not).
> > Once a page is put, a get on the page will always succeed.
>
> What happens when all of the free memory on a system
> has been consumed by preswap by a few guests?
> Will the system be unable to start another guest,
The default policy (and only policy implemented as of now) is
that no guest is allowed to use more than max_mem for the
sum of directly-addressable memory (e.g. RAM) and persistent
tmem (e.g. preswap). So if a guest is using its default
memory==max_mem and is doing no ballooning, nothing can
be put in preswap by that guest.
> or is there some way to free the preswap memory?
Yes and no. There is no way externally to free preswap
memory, but an in-guest userland root service can write to sysfs
to affect preswap size. This essentially does a partial
swapoff on preswap if there is sufficient (directly addressable)
guest RAM available. (I have this prototyped as part of
the xenballoond self-ballooning service in xen-unstable.)
Dan
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