Same here Pasquale, the logs on DEBUG log level could be helpful. My guess would be that the respective tasks are overloaded or there is some resource congestion (network, disk, etc).
You should see in the web UI the number of incoming and outgoing events. It would be good to check that the events are similarly sized and can be computed in roughly the same time. Cheers, Till On Mon, Jan 14, 2019 at 4:07 PM Pasquale Vazzana <p.vazz...@mwam.com> wrote: > 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. 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