Re: [9fans] Kernel panic when allocating a huge memory

[erik quanstrom <quanstro@quanstro.net>]

On Sat Nov  3 14:49:39 EDT 2012, charles.forsyth@gmail.com wrote:

> local paging algorithms can avoid thrashing: "the process pages
> against itself".  global paging algorithms typically do not
> (invariably do not, in my experience, but most people use essentially
> the same one, so there might be some that worked).
>
> Wilkes has a nice discussion of paging algorithms as an application of
> control theory in "The Dynamics of Paging".
> http://comjnl.oxfordjournals.org/content/16/1/4.short
>
> "It is notorious that the use of apparently innocuous scheduling and
> paging algorithms can give rise to the type of unstable behaviour
> known as thrashing."

good point.

however, if running 10 copies of the same os install, a common occurance,
and the hypervisor is consolidating identical pages (usually using cas), the
pages an os is likely to free are likely to be duplicated.  unless they all page
out that page, no  memory is saved unless the hypervisor swaps it out.

vmware makes the same point www.vmware.com/files/pdf/mem_mgmt_perf_vsphere5.pdf
p. 13.  but they also make the case that hat the balloon technique can
outpreform host swapping, p. 20, fig. 12.

(vdi (virtual desktop) would be interesting to graph.  couldn't find that.)

p. 27 best practices, bullet 3 basically says, make sure you have have enough
memory because paging sucks.  :-)

- erik

p.s. sharepoint takes 5 vms to run?  really guys?