Hi Vignesh,
In addition to what Ron has said, there are a number of options to consider,
depending on the nature of your calculations:
Given that your main focus seems to be latency:
* As Ron has said, Flink manages parallelism in a coarse grained way that
is optimized for spending as little time as possible in synchronization, and
takes away the need to manually synchronize
* If you spawn your own threads (it’s possible) you need to manage
synchronization yourself and this can add considerable to latency and create
back-pressure
* You would combine two implementations of parallelism and probably end
up in threads stealing CPU resources from each other
* When planning for horizontal scalability in Flink, plan CPU resources so
they can manage the workload
Fanout and collection pattern in general is a good idea, given that in order to
allow for horizontal scaling you need to have at least one network shuffle
anyway
* Make sure you use the best serializer possible for your data.
* Out of the box, Pojo serializer is hard to top, a hand coded serializer
might help (keep this for later in you dev process)
* If you can arrange you problem so that operators can be chained into a
single chain you can avoid serialization within the chain
* In Flink, if you union() or connect() multiple streams, chaining is
interrupted and this adds considerably to latency
There is a neat trick to combine the parallelism-per-key-group and the
parallelism-per-algorithm into a single implementation and end up with single
chains with little de-/serialization except for the fanout
* One of my students has devised this scheme in his masters thesis (see [1]
chapter 4.4.1 pp. 69)
* With his implementation we reduces back-pressure and latency
significantly for some orders of magnitude
I hope this helps, feels free to discuss details 😊
Thias
[1] Master Thesis, Dominik Bünzli, University of Zurich, 2021:
https://www.merlin.uzh.ch/contributionDocument/download/14168
From: liu ron <ron9....@gmail.com>
Sent: Dienstag, 15. August 2023 03:54
To: Vignesh Kumar Kathiresan <vkath...@yahooinc.com>
Cc: user@flink.apache.org
Subject: Re: Recommendations on using multithreading in flink map functions in
java
Hi, Vignesh
Flink is a distributed parallel computing framework, each MapFunction is
actually a separate thread. If you want more threads to process the data, you
can increase the parallelism of the MapFunction without having to use multiple
threads in a single MapFunction, which in itself violates the original design
intent of Flink.
Best,
Ron
Vignesh Kumar Kathiresan via user
<user@flink.apache.org<mailto:user@flink.apache.org>> 于2023年8月15日周二 03:59写道：
Hello All,
Problem statement
For a given element, I have to perform multiple(lets say N) operations on it.
All the N operations are independent of each other. And for achieving lowest
latency, I want to do them concurrently. I want to understand what's the best
way to perform it in flink?.
I understand flink achieves huge parallelism across elements. But is it
anti-pattern to do parallel processing in a map func at single element level? I
do not see anything on the internet for using multithreading inside a map
function.
I can always fan out with multiple copies of the same element and send them to
different operators. But it incurs at the least a serialize/deserialize cost
and may also incur network shuffle. Trying to see if a multithreaded approach
is better.
Thanks,
Vignesh
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