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From Jungtaek Lim <kabh...@gmail.com>
Subject Re: Complete latency calculation
Date Wed, 01 Feb 2017 00:48:16 GMT
Thomas,

I ported back STORM-1742 <https://issues.apache.org/jira/browse/STORM-1742>
to 1.0.x branch since I got many questions regarding complete latency from
user@, dev@, and Stack Overflow.

Release vote for Storm 1.0.3 RC1 is starting today (link
<http://mail-archives.apache.org/mod_mbox/storm-dev/201701.mbox/%3C1BD83321-658D-42E7-A88E-7A75D8105240%40gmail.com%3E>),
so you can wait for 1.0.3 or even use 1.0.3 RC1 before releasing officially.
I'd encourage you to help testing 1.0.3 RC1 if you have time.

Andrew,
STORM-1742 <https://issues.apache.org/jira/browse/STORM-1742> changes the
way to calculate complete latency:
- start time: Acker receiving ACK_INIT from Spout
- end time: Acker receiving ACK message which make the tuple tree completed

yes I was also aware of clock difference on nodes so I picked this way
though it doesn't account the latency from Spout to Acker. If you're
interested, please refer the issue and relevant discussion link in
description. Feedbacks are always appreciated!

Thanks,
Jungtaek Lim (HeartSaVioR)

2017년 2월 1일 (수) 오전 8:12, Andrew Xor <andreas.grammenos@gmail.com>님이
작성:

Hello,

 Unfortunately this is a bit more complicated than it might initially seem;
first of all where acks are processed, if at all, depends on the level of
the flow consistency you have implemented for your Spout; which applies to
Trident. In regular Storm, it might be perfectly OK to discard missed
messages, thereby ignoring the "acks" for each of the tuples. In the other
extreme Trident might even stall all of the incoming batches if a tuple
fails to be ack'ed and will have to be replayed, ack'ed and then resume the
computation -- this is done to enforce the in-order exactly once
processing. Additional factor might also impact your latency as network
speed used, packet size and so much more... I think Nick from Upenn did a
similar study and had a more thorough investigation regarding latency
tracking so if he's still subscribed in this list maybe can chip in to give
you more details.

>From my experience the timing is vastly impacted by each node "clock", thus
it cannot really be "trusted" because this can cause big deviations in the
latency measurements -- and to be fair to Storm devs do say its an
approximation!

Hope this helped...

On Tue, Jan 31, 2017 at 3:55 PM, Thomas Cooper (PGR) <
t.cooper@newcastle.ac.uk> wrote:

Hi,


First a bit of background:


I am a PhD student working on modelling the performance of Storm
topologies. I am having reasonable success in modelling the complete
latency, however depending on the load (throughput) I can be up to 50% out.


After exhausting all other sources of possible latency (mostly remote
transfer delays between workers on separate machines) it seems the final
path of ack tuples from the end component to the Acker and through to the
Spout is the last source of unknown latency. Under heavy load and a
relatively low number of spout tasks, each task will be busy calling
next_tuple and acking, so ack complete messages may back up at the spout.
This will artificially extend the complete latency. As the spout does not
report metrics for the delay/processing of acks, I cannot account for this
effect in my models.


I thought I might have to resort to implementing my own spout (a custom
Storm fork is something I would prefer to avoid). However, after seeing
issue 1742 (https://issues.apache.org/jira/browse/STORM-1742) it seems Jungtaek
Lim and the Storm devs have already spotted this problem and implemented a
solution in the master and 1.x branches. Having the Ackers stop the
complete latency clock makes more sense (particularly under heavy load) and
makes the complete latency match more closely that of the sojourn time
(spout to final component) through the whole topology.


However, I was hoping to get these models working with the latest storm
release (1.0.2). It doesn't appear that these changes have been backported
to the 1.0.x branch yet?


My Question (TL;DR):


Where in the 1.0.x codebase does the ack_ack message to the spout tasks get
processed? I know that implementations of ISpout have an ack() method that
gets called. However, in my test topologies when I leave this method
unimplemented the system still reports a complete latency for that spout?
The timestamp in the ack_ack message must be getting processes somewhere,
but I am struggling to identify where.


Any help locating this would be most appreciated.


Regards,


Thomas Cooper
PhD Student
Newcastle University, School of Computer Science
W: http://www.tomcooper.org.uk | A: 4th Floor, The Core, Science Central,
Bath Lane, Newcastle upon Tyne, NE4 5TF

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