[Linux-cluster] GFS cluster / DLM locking - Mostly idle but high load

[Gordan Bobic]

Hi,

I have a cluster (3 nodes at the moment, may grow up to 16) for handling a 
lot of small files (Maildir). When I test the system by sending around 3-5 
messages/second I see the load on the cluster nodes go up to about 20-30, 
even though the CPUs on the cluster are about 90% idle at all times.

I am guessing that this is due to the clustered machines waiting for DLM 
locks to be established, which causes a lot of processes to be fighting to 
run, but since they don't get to run very soon, they back up and cause the 
load averages to go up.

Assuming the DLM runs over the interface specified by IP and MAC in 
cluster.conf, it is running over gigabit ethernet.

Are there any configuration changes or tuning parameters I can apply to 
DLM to alleviate this condition? The machine I'm running the test from 
(the one sending messages) is about 1/4 of the spec of each of the cluster 
nodes, and it's running a load average of about 0.4. It seems crazy that a 
single low-spec node should be able to completely overwhelm a cluster 12x 
it's spec several times over.

Thanks.

Gordan