Extreme console latency during disk IO (8.0-RC1, previous releases also affected according to others)

Robert N. M. Watson rwatson at freebsd.org
Tue Oct 13 13:42:35 UTC 2009

On 13 Oct 2009, at 14:33, Ivan Voras wrote:

>> If (1) is highly variable during I/O, it's almost certainly a  
>> property of
>> the VM technology you're using, and there's nought to be done about  
>> it in
>> the guest OS.
> Here's an example of a ping session with 0.1s resolution during a few
> seconds-stall in ssh:
> 64 bytes from icmp_seq=1576 ttl=64 time=0.383 ms
> 64 bytes from icmp_seq=1577 ttl=64 time=0.405 ms
> 64 bytes from icmp_seq=1578 ttl=64 time=0.360 ms
> 64 bytes from icmp_seq=2304 ttl=64 time=4.194 ms
> 64 bytes from icmp_seq=2305 ttl=64 time=0.454 ms
> 64 bytes from icmp_seq=2306 ttl=64 time=0.376 ms
> note huge packet loss. It looks like it's VM fault or something like  
> it.

It sounds like the VM is failing to execute the guest during certain  
types of I/O. A bit of scheduler tracing in the host OS probably  
wouldn't go amiss to confirm that the VM really is suspending the  
guest at about the same time ICMP latency goes up. However, given the  
above I think I you can reasonable assume that the 4ms jump you're  
seeing there is due to global host OS/VM scheduling, and not FreeBSD  


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