[Xen-users] heavy write crashing, was: process limit

[Tim Freeman <tfreeman@xxxxxxxxxxx>]

On Fri, 06 Jan 2006 15:51:18 -0600
Charles Duffy <cduffy@xxxxxxxxxxx> wrote:

> Anand wrote:
> > Yes it makes sense, however i was wondering if something like this is
> > possible or not. Lets say a heavy disk io process goes wild and keeps
> > on writing to disk and span multiple processes, the dom0 can come to a
> > grinding halt. (for that matter there is no way to do disk io
> > scheduling like cpu scheduling :( ) 
> 
> IIRC, disk I/O scheduling is a TODO for Xen and should be supported in
> the future.
> 
> That said, have you actually seen this case (where the Dom0 comes to a
> complete halt, I/O blocked on account of disk usage by the DomUs)?

You can experience some nasty hangs with Xen 3.0.0 when running certain stress
invocations in domU, particularly with the --hdd option: "spawn N workers
spinning on write()/unlink()" (default is 1GB write()). 

This is just some casual experience:

In a loopback mounted domU, I ran "stress -v --hdd 1" and it will almost
immediately kill the machine. 

I increased dom0's RAM to try and help dom0's kernel with the writing and after
giving dom0 a whole gigabyte, it lasted longer but still locks up.

I gave dom0 much more CPU (almost half), kept its high RAM allocation, and it
did a lot better in this situation, lasted a lot longer, but eventually the
system became unresponsive.  Here, console toggling from the keyboard of the
real machine still worked up until the time my patience ran out... 

Tim 


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