Re: [uml-devel] [PATCH] I/O-Request Token Bucket Limiter

["Christopher S. Aker" <caker@theshore.net>]

> hello christoph,
> could you please explain (in a simple way), what your patch can be used for?
> from my point of understanding, we now can limit the maximum i/o bandwidth a
single uml can use?
> is that right?
> i have difficulties in understanding the meaning of token/bucket/filter.

It allows you to place a limit on the amount of disk I/O that makes it to the host
from each UML.  It is useful in the case where a mis-configured UML is thrashing
its swapfile, and in turn consuming a lot of I/O on the host and starving the
other UMLs of precious disk time.

The useful thing about the TBF is that it allows an initial burstable,
unrestricted rate until the token bucket is empty.  The variable token_max (the
size of the bucket) becomes how much "burstable" I/O to provide to the UML before
it starts limiting the rate.  Each I/O request takes one token from io_tokens, but
every second token_refill is added to io_tokens (to a maximum of token_max).

A way to see this is in action is to apply the patch, run the UML, and then on the
host:

[root@host ~]# watch "uml_mconsole /path/to/mconsole/socket io_status 2>/dev/null
"

And then:

[user@uml ~] dd if=/dev/zero of=/bigfile bs=1M count=200
(or some other I/O generating activity)

and watch the mconsole io_status output...

-Chris



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