RE: [Xen-devel] [PATCH] balloon: selfballooning and post memory info via xenbus

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Awfully quiet out there... comments please?

Here's a second version.  This one adds hysteresis, both
"up" and "down".

This smooths changes in balloon size to better accomodate
spiky or sine-wave-ish memory loads.

It can also be used as a rudimentary QoS device to modify
the "eagerness" of some domains in attempting to increase
or decrease memory.

By setting downhysteresis (when
selfballooning) to a value of n between 2 and 99,
memory will be reduced by (current - selftarget)/n
every second whenever current > selftarget.  By
setting uphysteresis (when selfballooning) to a value
of n between 2 and 99, memory will be increased by
(current - selftarget)/n every second whenever
current < selftarget.  Both default off.  It is
expected that downhysterisis will often be a largish
value and uphysteresis will often be a smallish value,
so as to respond rapidly to increases in memory usage
and more gradually to decreases in memory usage, which
is as desired.

Dan

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Dan
> Magenheimer
> Sent: Monday, May 05, 2008 4:45 PM
> To: Xen-Devel (E-mail)
> Subject: [Xen-devel] [PATCH] balloon: selfballooning and post memory
> info via xenbus
> 
> 
> OK, here's the promised patch.  The overall objective of the
> patch is to enable limited memory load-balancing capabilities
> as a step toward allowing limited memory overcommit.  With
> this and some other minor hackery, I was able to run as
> many as 15 lightly loaded 512MB domains on a 2GB system
> (yes, veerrrryyy slooowwwlly).
> 
> Review/comments appreciated.
> 
> With this patch, balloon.c communicates (limited) useful
> memory usage information via xenbus.  It also implements
> "selfballooning" which applies the memory information
> locally to immediately adjust the balloon, giving up memory
> when it is not needed and asking for it back when it is needed,
> implementing a first-come-first-served system-wide ballooning
> "policy".  When a domain asks for memory but none is available,
> it must use its own configured swap disk, resulting in
> (potentially severe) performance degradation.  Naturally,
> it is not recommended to turn on selfballooning in a domain
> that has no swap disk or if performance is more important
> than increasing the number of VMs runnable on a physical machine.
> 
> A key assumption is that the Linux variable vm_committed_space
> is a reasonable first approximation of memory needed by a domain.
> This approximation will probably improve over time, but is
> a good start for now.  The variable is bound on the lower end
> by the recently submitted minimum_target() algorithm patch;
> thus O-O-M conditions should not occur.
> 
> The code is a bit complicated in a couple of places because of
> race conditions involving xenstored startup relative to
> turning on selfballooning locally.  Because the key variable
> (vm_committed_space) is not exported by Linux, I implemented
> a horrible hack which still allows the code to work in a
> module, however I fully expect that this part of the patch
> will not be accepted (which will limit the functionality to
> pvm domains only... probably OK for now).
> 
> Existing balloon functionality which is unchanged:
> - Set target for VM from domain0
> - Set target inside VM by writing to /proc/xen/balloon
> Existing balloon info on xenbus which is unchanged:
> - /local/domain/X/memory/target
> To turn on selfballooning:
> - Inside a VM: "echo 1 > /proc/xen/balloon"
> - From domain0: "xenstore-write /local/domain/X/memory/selfballoon 1"
> To turn off selfballooning:
> - Inside a VM: "echo 0 > /proc/xen/balloon"
> - From domain0: "xenstore-write /local/domain/X/memory/selfballoon 0"
> New balloon info now on xenbus:
> - /local/domain/X/memory/selfballoon [0 or 1]
> - /local/domain/X/memory/actual [kB] *
> - /local/domain/X/memory/minimum [kB] *
> - /local/domain/X/memory/selftarget [kB] * (only valid if
>   selfballoon==1)
>  * writeable only by balloon driver in X when either
>    selfballooning is first enabled, or target is changed
>    by domain0
> 
> Thanks,
> Dan
> 
> ===================================
> Thanks... for the memory
> I really could use more / My throughput's on the floor
> The balloon is flat / My swap disk's fat / I've OOM's in store
> Overcommitted so much
> (with apologies to the late great Bob Hope)
>

Attachment: balloon2.patch
Description: Binary data

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