Re: Questions about Tez

[Hitesh Shah]

When comparing just a simple MR job to a Tez dag with 2 vertices, the perf 
improvements are limited (as the plan is pretty much the same and data is 
transferred via a shuffle edge):
   - container re-use
   - pipelined sorter vs the MR sorter ( your mileage may vary here depending 
on the kind of work load )
   - auto-reduce parallelism
   - dynamic splits grouping for the map vertex ( assuming splits are 
calculated in the client )

For the second question, the dag plan/structure and the processor are both 
user-code and therefore which output to write to is driven by user/business 
logic. If you write a tee processor, it could write to all outputs if needed. A 
processor which re-partitions data to different downstream vertices could be 
coded to write diff data to each output if needed. The MapProcessor and 
ReduceProcessor assume MR semantics which means that they always assume one 
input and one output (likewise for the WordCount example). 

thanks
— Hitesh

> On Aug 12, 2016, at 9:54 AM, Madhusudan Ramanna  wrote:
> 
> Hello all,
> 
> I've just started looking at Tez. I've setup Tez locally and have run sample 
> MapReduce job with Tez as a replacement for Yarn MRV2. I plan to use Tez 
> independently (No hive or pig).  I have the following questions
> 
> 1.  Are there performance gains to use Tez for Mapreduce jobs apart from 
> container reuse ? Want to make sure I'm not missing anything
> 
> 2.  More technically, from what should be the behavior of a processor while 
> generating outputs.  Should it write to all KeyValueWriters returned from 
> getOutputs() in the processor.   The WordCount example retrieves an output by 
> name and writes to it.  
> 
> thanks,
> Madhu

Questions about Tez

[Madhusudan Ramanna]

Hello all,
I've just started looking at Tez. I've setup Tez locally and have run sample 
MapReduce job with Tez as a replacement for Yarn MRV2. I plan to use Tez 
independently (No hive or pig).  I have the following questions
1.  Are there performance gains to use Tez for Mapreduce jobs apart from 
container reuse ? Want to make sure I'm not missing anything
2.  More technically, from what should be the behavior of a processor while 
generating outputs.  Should it write to all KeyValueWriters returned from 
getOutputs() in the processor.   The WordCount example retrieves an output by 
name and writes to it.  
thanks,Madhu

Re: Questions about Tez under the hood

[Fabio C.]

Got it. Thanks for the support.

Fabio

On Fri, Jan 30, 2015 at 9:20 PM, Bikas Saha bi...@hortonworks.com wrote:

  There are many nuances like container affinity etc. but broadly this is
 what happens.



 The scheduler loops through all free containers. For each container, at
 node/rack/any level of locality, it picks the current top priority of tasks
 that needs to be assigned. For that priority it asks YARN AMRMClient to
 return pending requests at given locality. These pending requests are
 returned in the order they were received. Thus tasks of same priority from
 different vertices would be received intermingled in the order in which
 they arrived.



 Bikas



 *From:* Fabio C. [mailto:anyte...@gmail.com]
 *Sent:* Friday, January 30, 2015 5:52 AM
 *To:* user@tez.apache.org
 *Subject:* Re: Questions about Tez under the hood



 Thanks a lot.
 Now I was trying to figure out what happens if there are different tasks
 from two different vertexes at the same priority awaiting for resources. If
 a suitable container becomes available, who is going to get it? I'm having
 a hard time finding the code about this. I was thinking that maybe we serve
 at first the first submitted vertex and among its tasks we follow the task
 id order, but it's just a guess.



 On Wed, Jan 28, 2015 at 10:50 PM, Hitesh Shah hit...@apache.org wrote:

 Answers inline.

 — Hitesh

 On Jan 28, 2015, at 3:15 AM, Fabio anyte...@gmail.com wrote:

  Hi everyone,
  I take back this mail since I have a few more questions about Tez. I am
 digging into the internal scheduling policy and I'm trying to fully
 understand how containers are assigned once Tez receive them from the RM.
  I am mainly referring to
 org.apache.tez.dag.app.rm.YarnTaskSchedulerService (I am currently on
 0.5.0, I hope there have been no change) and I am not considering locality
 (let's say I just have one node in the cluster).
 
  Could someone please confirm this?
  - When a response comes from the RM with container allocations for the
 application, those containers are added to the list of delayed containers
 (together with old containers already available for reuse), and their
 scheduling time is set to be 1 ms after the last scheduling time seen so
 far. They are assigned right away only if container reuse is disabled.

 Yes. With container reuse disabled, there should be a 1:1 mapping for a
 pending task to an allocated container. With re-use enabled, existing
 containers are used first to reduce launch cost overheads.

  - The tez scheduler will keep on trying to assign a container at its
 nextScheduleTime.

 Yes - as well as whenever a new pending task comes in, it will try and
 assign a container to it.

  - If we've just got any container from the RM, then the tez scheduler
 will try to assign all delayed containers (old ones first and new ones
 last, since they are ordered according to their next scheduling time)

 Yes - probably something which needs to be fixed. This was being done
 earlier before when a new container would be allocated first instead of a
 re-used container. This may no longer be needed as the scheduling loop
 would get trigged on the next schedule time elapse. This probably
 unintentionally acts as a trigger to run a new matching loop.

  Just out of curiosity, why not ordering the delayed containers according
 to their expiry time?

 The general flow is that on each loop, a container is tried to be assigned
 to a potential matching task. On each loop, its locality constraints are
 relaxed more or more to allow for more matches ( local only on round 1,
 rack or local match on round 2, …). By the time, the container hits its
 final loop, it will match against any pending task. Sorting by next
 schedule time means the thread wakes up when it is time to run the next
 loop for a given container.  Sorting by expiry would imply scanning the
 whole list to find all containers whose schedule time has elapsed.


 
  Thanks
 
  Fabio
 
  On 10/29/2014 04:48 PM, Hitesh Shah wrote:
  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

RE: Questions about Tez under the hood

[Bikas Saha]

There are many nuances like container affinity etc. but broadly this is what 
happens.

The scheduler loops through all free containers. For each container, at 
node/rack/any level of locality, it picks the current top priority of tasks 
that needs to be assigned. For that priority it asks YARN AMRMClient to return 
pending requests at given locality. These pending requests are returned in the 
order they were received. Thus tasks of same priority from different vertices 
would be received intermingled in the order in which they arrived.

Bikas

From: Fabio C. [mailto:anyte...@gmail.com]
Sent: Friday, January 30, 2015 5:52 AM
To: user@tez.apache.org
Subject: Re: Questions about Tez under the hood

Thanks a lot.
Now I was trying to figure out what happens if there are different tasks from 
two different vertexes at the same priority awaiting for resources. If a 
suitable container becomes available, who is going to get it? I'm having a hard 
time finding the code about this. I was thinking that maybe we serve at first 
the first submitted vertex and among its tasks we follow the task id order, but 
it's just a guess.

On Wed, Jan 28, 2015 at 10:50 PM, Hitesh Shah 
hit...@apache.orgmailto:hit...@apache.org wrote:
Answers inline.

— Hitesh

On Jan 28, 2015, at 3:15 AM, Fabio 
anyte...@gmail.commailto:anyte...@gmail.com wrote:

 Hi everyone,
 I take back this mail since I have a few more questions about Tez. I am 
 digging into the internal scheduling policy and I'm trying to fully 
 understand how containers are assigned once Tez receive them from the RM.
 I am mainly referring to org.apache.tez.dag.app.rm.YarnTaskSchedulerService 
 (I am currently on 0.5.0, I hope there have been no change) and I am not 
 considering locality (let's say I just have one node in the cluster).

 Could someone please confirm this?
 - When a response comes from the RM with container allocations for the 
 application, those containers are added to the list of delayed containers 
 (together with old containers already available for reuse), and their 
 scheduling time is set to be 1 ms after the last scheduling time seen so far. 
 They are assigned right away only if container reuse is disabled.

Yes. With container reuse disabled, there should be a 1:1 mapping for a pending 
task to an allocated container. With re-use enabled, existing containers are 
used first to reduce launch cost overheads.

 - The tez scheduler will keep on trying to assign a container at its 
 nextScheduleTime.

Yes - as well as whenever a new pending task comes in, it will try and assign a 
container to it.

 - If we've just got any container from the RM, then the tez scheduler will 
 try to assign all delayed containers (old ones first and new ones last, since 
 they are ordered according to their next scheduling time)

Yes - probably something which needs to be fixed. This was being done earlier 
before when a new container would be allocated first instead of a re-used 
container. This may no longer be needed as the scheduling loop would get 
trigged on the next schedule time elapse. This probably unintentionally acts as 
a trigger to run a new matching loop.

 Just out of curiosity, why not ordering the delayed containers according to 
 their expiry time?

The general flow is that on each loop, a container is tried to be assigned to a 
potential matching task. On each loop, its locality constraints are relaxed 
more or more to allow for more matches ( local only on round 1, rack or local 
match on round 2, …). By the time, the container hits its final loop, it will 
match against any pending task. Sorting by next schedule time means the thread 
wakes up when it is time to run the next loop for a given container.  Sorting 
by expiry would imply scanning the whole list to find all containers whose 
schedule time has elapsed.


 Thanks

 Fabio

 On 10/29/2014 04:48 PM, Hitesh Shah wrote:
 Answers inline.

 — Hitesh

 On Oct 29, 2014, at 2:33 AM, Fabio 
 anyte...@gmail.commailto: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

Re: Questions about Tez under the hood

[Fabio C.]

Thanks a lot.
Now I was trying to figure out what happens if there are different tasks
from two different vertexes at the same priority awaiting for resources. If
a suitable container becomes available, who is going to get it? I'm having
a hard time finding the code about this. I was thinking that maybe we serve
at first the first submitted vertex and among its tasks we follow the task
id order, but it's just a guess.

On Wed, Jan 28, 2015 at 10:50 PM, Hitesh Shah hit...@apache.org wrote:

 Answers inline.

 — Hitesh

 On Jan 28, 2015, at 3:15 AM, Fabio anyte...@gmail.com wrote:

  Hi everyone,
  I take back this mail since I have a few more questions about Tez. I am
 digging into the internal scheduling policy and I'm trying to fully
 understand how containers are assigned once Tez receive them from the RM.
  I am mainly referring to
 org.apache.tez.dag.app.rm.YarnTaskSchedulerService (I am currently on
 0.5.0, I hope there have been no change) and I am not considering locality
 (let's say I just have one node in the cluster).
 
  Could someone please confirm this?
  - When a response comes from the RM with container allocations for the
 application, those containers are added to the list of delayed containers
 (together with old containers already available for reuse), and their
 scheduling time is set to be 1 ms after the last scheduling time seen so
 far. They are assigned right away only if container reuse is disabled.

 Yes. With container reuse disabled, there should be a 1:1 mapping for a
 pending task to an allocated container. With re-use enabled, existing
 containers are used first to reduce launch cost overheads.

  - The tez scheduler will keep on trying to assign a container at its
 nextScheduleTime.

 Yes - as well as whenever a new pending task comes in, it will try and
 assign a container to it.

  - If we've just got any container from the RM, then the tez scheduler
 will try to assign all delayed containers (old ones first and new ones
 last, since they are ordered according to their next scheduling time)

 Yes - probably something which needs to be fixed. This was being done
 earlier before when a new container would be allocated first instead of a
 re-used container. This may no longer be needed as the scheduling loop
 would get trigged on the next schedule time elapse. This probably
 unintentionally acts as a trigger to run a new matching loop.

  Just out of curiosity, why not ordering the delayed containers according
 to their expiry time?

 The general flow is that on each loop, a container is tried to be assigned
 to a potential matching task. On each loop, its locality constraints are
 relaxed more or more to allow for more matches ( local only on round 1,
 rack or local match on round 2, …). By the time, the container hits its
 final loop, it will match against any pending task. Sorting by next
 schedule time means the thread wakes up when it is time to run the next
 loop for a given container.  Sorting by expiry would imply scanning the
 whole list to find all containers whose schedule time has elapsed.

 
  Thanks
 
  Fabio
 
  On 10/29/2014 04:48 PM, Hitesh Shah wrote:
  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

Re: Questions about Tez under the hood

[Hitesh Shah]

Answers inline.

— Hitesh

On Jan 28, 2015, at 3:15 AM, Fabio anyte...@gmail.com wrote:

 Hi everyone,
 I take back this mail since I have a few more questions about Tez. I am 
 digging into the internal scheduling policy and I'm trying to fully 
 understand how containers are assigned once Tez receive them from the RM.
 I am mainly referring to org.apache.tez.dag.app.rm.YarnTaskSchedulerService 
 (I am currently on 0.5.0, I hope there have been no change) and I am not 
 considering locality (let's say I just have one node in the cluster).
 
 Could someone please confirm this?
 - When a response comes from the RM with container allocations for the 
 application, those containers are added to the list of delayed containers 
 (together with old containers already available for reuse), and their 
 scheduling time is set to be 1 ms after the last scheduling time seen so far. 
 They are assigned right away only if container reuse is disabled.

Yes. With container reuse disabled, there should be a 1:1 mapping for a pending 
task to an allocated container. With re-use enabled, existing containers are 
used first to reduce launch cost overheads.

 - The tez scheduler will keep on trying to assign a container at its 
 nextScheduleTime.

Yes - as well as whenever a new pending task comes in, it will try and assign a 
container to it. 

 - If we've just got any container from the RM, then the tez scheduler will 
 try to assign all delayed containers (old ones first and new ones last, since 
 they are ordered according to their next scheduling time)

Yes - probably something which needs to be fixed. This was being done earlier 
before when a new container would be allocated first instead of a re-used 
container. This may no longer be needed as the scheduling loop would get 
trigged on the next schedule time elapse. This probably unintentionally acts as 
a trigger to run a new matching loop. 

 Just out of curiosity, why not ordering the delayed containers according to 
 their expiry time?

The general flow is that on each loop, a container is tried to be assigned to a 
potential matching task. On each loop, its locality constraints are relaxed 
more or more to allow for more matches ( local only on round 1, rack or local 
match on round 2, …). By the time, the container hits its final loop, it will 
match against any pending task. Sorting by next schedule time means the thread 
wakes up when it is time to run the next loop for a given container.  Sorting 
by expiry would imply scanning the whole list to find all containers whose 
schedule time has elapsed. 

 
 Thanks
 
 Fabio
 
 On 10/29/2014 04:48 PM, Hitesh Shah wrote:
 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

Re: Questions about Tez under the hood

[Fabio]

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.


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 
mailto:bi...@hortonworks.com]

*Sent:* Monday, October 27, 2014 9:47 AM
*To:* user@tez.apache.org mailto:user@tez.apache.org
*Subject:* RE: Questions about Tez under the hood

Answers inline.

*From:*Fabio C. [mailto:anyte...@gmail.com mailto:anyte...@gmail.com]
*Sent:* Monday, October 27, 2014 7:08 AM
*To:* user@tez.apache.org mailto: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 
page http://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

Re: Questions about Tez under the hood

[Hitesh Shah]

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

RE: Questions about Tez under the hood

[Bikas Saha]

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 page
http://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

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