Hi community,
Recently we have noticed a strange behavior for Flink jobs on Kubernetes
per-job mode: when the parallelism increases, the time it takes for the
TaskManagers to register with *JobManager *becomes abnormally long (for a
task with parallelism of 50, it could take 60 ~ 120 seconds or even longer
for the registration attempt), and usually more than 10 attempts are needed
to finish this registration.
Because of this, we could not submit a job requiring more than 20 slots
with the default configuration, as the TaskManager would say:
> Registration at JobManager
> (akka.tcp://flink@myjob-201076.default:6123/user/rpc/jobmanager_2)
> attempt 9 timed out after 25600 ms
Free slot with allocation id 60d5277e138a94fb73fc6691557001e0 because: The
> slot 60d5277e138a94fb73fc6691557001e0 has timed out.
Free slot TaskSlot(index:0, state:ALLOCATED, resource profile:
> ResourceProfile{cpuCores=1.0000000000000000, taskHeapMemory=1.425gb
> (1530082070 bytes), taskOffHeapMemory=0 bytes, managedMemory=1.340gb
> (1438814063 bytes), networkMemory=343.040mb (359703515 bytes)},
> allocationId: 60d5277e138a94fb73fc6691557001e0, jobId:
> 493cd86e389ccc8f2887e1222903b5ce).
> java.lang.Exception: The slot 60d5277e138a94fb73fc6691557001e0 has timed
> out.
In order to cope with this issue, we have to change the below configuration
parameters:
>
> # Prevent "Could not allocate the required slot within slot request
> timeout. Please make sure that the cluster has enough resources. Stopping
> the JobMaster for job"
> slot.request.timeout: 500000
# Increase max timeout in a single attempt
> cluster.registration.max-timeout: 300000
> # Prevent "free slot (TaskSlot)"
> akka.ask.timeout: 10 min
> # Prevent "Heartbeat of TaskManager timed out."
> heartbeat.timeout: 500000
However, we acknowledge that this is only a temporary dirty fix, which is
not what we want. It could be seen that during TaskManager registration to
JobManager, lots of warning messages come out in logs:
No hostname could be resolved for the IP address 9.166.0.118, using IP
> address as host name. Local input split assignment (such as for HDFS files)
> may be impacted.
Initially we thought this was probably the cause (reverse lookup of DNS
might take up a long time), however we later found that the reverse lookup
only took less than 1ms, so maybe not because of this.
Also, we have checked the GC log of both TaskManagers and JobManager, and
they seem to be perfectly normal, without any signs of pauses. And the
heartbeats are processed as normal according to the logs.
Moreover, TaskManagers register quickly with ResourceManager, but then
extra slow with TaskManager, so this is not because of a slow network
connection.
Here we wonder what could be the cause for the slow registration between
JobManager and TaskManager(s)? No other warning or error messages in the
log (DEBUG level) other than the "No hostname could be resolved" messages,
which is quite weird.
Thanks for the reading, and hope to get some insights into this issues : )
Sincerely,
Weike
