I have the same problem, even more impactful. Some subtasks stall forever quite consistently. I am using Flink 1.7.1, but I've tried downgrading to 1.6.3 and it didn't help. The Backend doesn't seem to make any difference, I've tried Memory, FS and RocksDB back ends but nothing changes. I've also tried to change the medium, local spinning disk, SAN or mounted fs but nothing helps. Parallelism is the only thing which mitigates the stalling, when I set 1 everything works but if I increase the number of parallelism then everything degrades, 10 makes it very slow 30 freezes it. It's always one of two subtasks, most of them does the checkpoint in few milliseconds but there is always at least one which stalls for minutes until it times out. The Alignment seems to be a problem. I've been wondering whether some Kafka partitions where empty but there is not much data skew and the keyBy uses the same key strategy as the Kafka partitions, I've tried to use murmur2 for hashing but it didn't help either. The subtask that seems causing problems seems to be a CoProcessFunction. I am going to debug Flink but since I'm relatively new to it, it might take a while so any help will be appreciated.
Pasquale From: Till Rohrmann <trohrm...@apache.org> Sent: 08 January 2019 17:35 To: Bruno Aranda <bara...@apache.org> Cc: user <user@flink.apache.org> Subject: Re: Subtask much slower than the others when creating checkpoints Hi Bruno, there are multiple reasons wh= one of the subtasks can take longer for checkpointing. It looks as if the=e is not much data skew since the state sizes are relatively equal. It als= looks as if the individual tasks all start at the same time with the chec=pointing which indicates that there mustn't be a lot of back-pressure =n the DAG (or all tasks were equally back-pressured). This narrows the pro=lem cause down to the asynchronous write operation. One potential problem =ould be if the external system to which you write your checkpoint data has=some kind of I/O limit/quota. Maybe the sum of write accesses deplete the =aximum quota you have. You could try whether running the job with a lower =arallelism solves the problems. For further debug=ing it could be helpful to get access to the logs of the JobManager and th= TaskManagers on DEBUG log level. It could also be helpful to learn which =tate backend you are using. Cheers, Til= On Tue, Jan 8,=2019 at 12:52 PM Bruno Aranda <mailto:bara...@apache.org> wrote: Hi, We are using Flink =.6.1 at the moment and we have a streaming job configured to create a chec=point every 10 seconds. Looking at the checkpointing times in the UI, we c=n see that one subtask is much slower creating the endpoint, at least in i=s "End to End Duration", and seems caused by a longer "Chec=point Duration (Async)". For instance, in th= attach screenshot, while most of the subtasks take half a second, one (an= it is always one) takes 2 seconds. But we have w=rse problems. We have seen cases where the checkpoint times out for one ta=ks, while most take one second, the outlier takes more than 5 minutes (whi=h is the max time we allow for a checkpoint). This can happen if there is =ack pressure. We only allow one checkpoint at a time as well. Why could one subtask take more time? This jobs read from kafk= partitions and hash by key, and we don't see any major data skew betw=en the partitions. Does one partition do more work? We do have a cluster of 20 machines, in EMR, with TMs that have multiple=slots (in legacy mode). Is this something that co=ld have been fixed in a more recent version? Than=s for any insight! Bruno This e-mail and any attachments are confidential to the addressee(s) and may contain information that is legally privileged and/or confidential. Please refer to http://www.mwam.com/email-disclaimer-uk for important disclosures regarding this email. If we collect and use your personal data we will use it in accordance with our privacy policy, which can be reviewed at https://www.mwam.com/privacy-policy . Marshall Wace LLP is authorised and regulated by the Financial Conduct Authority. Marshall Wace LLP is a limited liability partnership registered in England and Wales with registered number OC302228 and registered office at George House, 131 Sloane Street, London, SW1X 9AT. If you are receiving this e-mail as a client, or an investor in an investment vehicle, managed or advised by Marshall Wace North America L.P., the sender of this e-mail is communicating with you in the sender's capacity as an associated person of and on behalf of Marshall Wace North America L.P., which is registered with the US Securities and Exchange Commission as an investment adviser.
