Re: Linux-VServer example results for sharing vs. separate mappings ...

On Fri, 23 Mar 2007 20:30:00 +0100 Herbert Poetzl <herbert@xxxxxxxxxxxx> wrote:


Hi Eric!
Hi Folks!

here is a real world example result from one of my tests
regarding the benefit of sharing over separate memory

the setup is quite simple, a typical machine used by
providers all over the world, a dual Pentium D 3.2GHz
with 4GB of memory and a single 160GB SATA disk running
a Linux-VServer kernel (2.6.19.7-vs2.2.0-rc18)

the Guest systems used are Mandriva 2007 guests with
syslog, crond, sshd, apache, postfix and postgresql
installed and running (all in all 17 processes per guest)

the disk space used by one guests is roughly 148MB

in addition to that, a normal host system is running
with a few daemons (like sshd, httpd, postfix ...)


the first test setup is starting 200 of those guests
one after the other and measuring the memory usage
before and after the guest did start, as well as
recording the time used to start them ...

this is done right after the machine was rebooted, in
one test with 200 separate guests (i.e. 200 x 148MB)
and in a second run with 200 unified guests (which
means roughly 138MB of shared files)

Please define your terms. What is a "separated guest", what is a "unified
guest" and how do they differ?

If a "separated" guest is something in which separate guests will use
distinct physical pages to cache the contents of /etc/passwd (ie: a separate
filesystem per guest) then I don't think that's interesting information,
frankly.

Because nobody (afaik) is proposing that pagecache be duplicated across
instances in this fashion.

We obviously must share pagecache across instances - if we didn't want to
do that then we could do something completely dumb such as use
xen/kvm/vmware/etc ;)

The issue with pagecache (afaik) is that if we use containers based on
physical pages (an approach which is much preferred by myself) then we can
get in a situation where a pagecache page is physically in container A, is
not actually used by any process in container A, but is being releatedly
referenced by processes which are in other containers and hence unjustly
consumes resources in container A. How significant a problem this is likely
to be I do not know. And there are perhaps things which we can do about it.

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