Answers inline. — Hitesh
On Oct 29, 2014, at 2:33 AM, Fabio <anyte...@gmail.com> wrote: > Thanks Bikas for your answer and suggestion, actually my work deals more with > high level modeling/behavior/performance of Tez, but there is another guy who > is goign to handle Tez sources, I will suggest him to contribute. > I've just found many commented configuration parameters in > org.apache.tez.dag.api.TezConfiguration that I didn't even know, they will > help. > > Right now I have another question that came to my mind while modeling Tez: > Situation: I have a DAG with 2 tasks waiting to run, the cluster is quite > overloaded. The Tez AM will ask for 2 containers at the Resource Manager and > wait for them. At some point a single container becomes available and a task > can run and finish, so Tez (I guess) will exploit that same container for > reuse, but what about the other request sent to the RM? Is it somehow > actively voided by Tez or at some point it will just get another container > that wont be used (and possibly discarded afterward)? I don't even know if > YARN have such a feature for removing a previously submitted request to the > RM. > [Hitesh] Tez will always ask the RM for as many containers as the tasks it needs to run. In cases when a task is scheduled to run on an existing available container, it will do so based on certain conditions such as checking if the data needed by the task is available on the same node and/or rack as that of the existing container. In terms of the RM request management, the protocol between the RM and an ApplicationMaster is more or less an update protocol ( and not an incremental one ). Based on your example, Tez would first ask the RM for 2 containers. Once it gets one, it will keep on telling the RM that it now needs one. If the previously assigned container is also used for the 2nd task, it will update the ask to the RM to 0 containers. There is obviously a minor race condition where the RM may have already allocated the container before Tez is able to tell it that it does not need the additional container. In such cases, Tez will get an additional allocation which it does not need but release it in due time ( the YARN protocol supports releasing containers without using them ). > I would keep this thread for future generic questions about Tez behavior if > it's ok. > > Thanks so far :) > > Fabio > > On 10/27/2014 05:48 PM, Bikas Saha wrote: >> Also, any contributions to the project via your thesis work would be >> welcome. Please do first open a jira and provide a design overview before >> submitting code. >> >> From: Bikas Saha [mailto:bi...@hortonworks.com] >> Sent: Monday, October 27, 2014 9:47 AM >> To: user@tez.apache.org >> Subject: RE: Questions about Tez under the hood >> >> Answers inline. >> >> From: Fabio C. [mailto:anyte...@gmail.com] >> Sent: Monday, October 27, 2014 7:08 AM >> To: user@tez.apache.org >> Subject: Questions about Tez under the hood >> >> Hi guys, I'm currently working at my master degree thesis on Tez, and I am >> trying to understand how Tez works under the hood. I have some questions, I >> hope someone can help with this: >> >> 1) How does Tez handle containers for reuse? Are they kept for some seconds >> (how long?) in a sort of buffer waiting for tasks which will need them? Or a >> container is sent back to the RM if no task is immediately ready to take it? >> >> [Bikas] Yes they wait around for a buffer period of time. Idle containers >> are released back the RM randomly between a mix and a max release time until >> a minimum held container threshold is met. So the behavior can be customized >> using the min/max timeouts and the min held threshold. >> >> 2) Let's say I have a DAG with two branches proceeding in parallel before >> joining in a root node (such as the example on the tez home >> pagehttp://tez.apache.org/images/PigHiveQueryOnTez.png ). In this case, we >> will have both branches running at the same time. At some point we may have >> the first branch that is almost complete, while the second is still at an >> early stage. In this case, does Tez knows that "soon or later" the two >> branches will merge, thus there will be a common consumer waiting for the >> slower branch to complete? Actually the real question is: does Tez >> prioritize the scheduling/resource allocation of tasks belonging to slower >> branches? If yes, what kind of policy is adopted? Is it configurable? >> >> [Bikas] Currently the priority of a vertex is the distance from the source >> of the DAG. So vertices can run in parallel. On the roadmap are items like >> critical path scheduling where the vertex that is holding up the job the >> most or that’s going to unblock the most amount of downstream work to be >> given higher priority. >> >> 3) tez.am.shuffle-vertex-manager.min-src-fraction: if I have a dag made of >> two producer vertexes, each one running 10 tasks, and below them a consumer >> vertex, let's say running 5 tasks, so if this property is set to 0.2, does >> it mean that before running any consumer task we need 2 producer tasks to >> complete for each of the producer vertexes? Or are they considered as a >> whole and we need just 4 tasks completed (even just from one vertex)? >> >> [Bikas] It currently looks at the fraction of the whole (both combined) but >> we are going to change it to look at the fraction per source vertex. Again, >> this is just a hint. (With auto-parallelism on) the vertex also looks at >> whether enough data has been produced before triggering the tasks because >> the real intention is to have enough data available for the reduce to pull >> so that it can overlap the pull with the completion of the map tasks. >> >> 4) As far as I understand, a single Tez Application Master can handle >> multiple DAGs at the same time, but only if the user-application has been >> coded to do so (for example, if I run two wordcount with the same user, it >> simply creates two different Tez App Master). Is this correct? >> >> [Bikas] If the TezClient is started in session mode then it re-uses the App >> Master for multiple DAGs. The code is the same in session and non-session >> mode. The behavior can be changed via configuration (or hard coded in the >> code if desired). So you can use both modes with the same code. To be clear, >> the AppMaster does not run dags concurrently. It runs one DAG at a time. >> >> Thanks in advance >> >> Fabio >> >> CONFIDENTIALITY NOTICE >> NOTICE: This message is intended for the use of the individual or entity to >> which it is addressed and may contain information that is confidential, >> privileged and exempt from disclosure under applicable law. If the reader of >> this message is not the intended recipient, you are hereby notified that any >> printing, copying, dissemination, distribution, disclosure or forwarding of >> this communication is strictly prohibited. If you have received this >> communication in error, please contact the sender immediately and delete it >> from your system. Thank You. > >
