Within a given task, a UDF is only instantiated once. For maps and
reduces this should mean one per map or reduce. Since the combiner
can be run multiple times there can be multiple instantiations per
combine. But the warning on number of instantiations is about how
many times the UDF is constructed on the front end (by which I mean
compile time). Pig use to construct the UDF multiple times in the
front end. Now we have it down to one construction on the front end
and one per task in the backend.
Alan.
On Mar 2, 2011, at 9:11 AM, Lai Will wrote:
I understand that the is not inter-task communication at all.
However my question arises within one task. The documentation says
that we should not make any assumptions on how may EvalFunc
instances (of the same class) are instantiated.
Therefore I assume that within the same task, there might be several
instances of my EvalFunc and if every one of them is doing the
parsing of resource files into data structures a lot of memory and
computing power would be wasted.. so it's not about inter-task
communication but about inter-instance communication.
Thank you for your help.
Best,
Will
-----Original Message-----
From: Alan Gates [mailto:[email protected]]
Sent: Wednesday, March 02, 2011 5:17 PM
To: [email protected]
Subject: Re: Shared resources
There is no shared inter-task processing in Hadoop. Each task runs
in a separate JVM and is locked off from all other tasks. This is
partly because you do not know a priori which tasks will run
together in which nodes, and partly for security. Data can be
shared by all tasks on a node via the distributed cache. If all
your work could be done once on the front end and then serialized to
be later read by all tasks you could use this mechanism to share
it. With the code in trunk UDFs can store data to the distributed
cache, though this feature is not in a release yet.
Alan.
On Mar 2, 2011, at 7:54 AM, Lai Will wrote:
So I still get the redundant work whenever the same clusternode/vm
creates multiple instances of my EvalFunc?
And is it usual to have several instance of the EvalFunc on the same
clusternode/vm?
Will
-----Original Message-----
From: Alan Gates [mailto:[email protected]]
Sent: Wednesday, March 02, 2011 4:49 PM
To: [email protected]
Subject: Re: Shared resources
There is no method in the eval func that gets called on the backend
before any exec calls. You can keep a flag that tracks whether you
have done the initialization so that you only do it the first time.
Alan.
On Mar 2, 2011, at 5:29 AM, Lai Will wrote:
Hello,
I wrote a EvalFunc implementation that
1) Parses a SQL Query
2) Scans a folder for resource files and creates an index on
these files
3) According to certain properties of the SQL Query accesses
the corresponding file and creates a Java objects holding relevant
the information of the file (for reuse).
4) Does some computation with the SQL Query and the information
found in the file
5) Outputs a transformed SQL Query
Currently I'm doing local tests without Hadoop and the code works
fine.
The problem I see, is that right now I initialize my parser in the
EvalFunc, so that every time It gets instantiated a new instance of
the parser is generated. Ideally only on instance per machine would
be created.
Even worse right now I create the index and parse the corresponding
resource file once per call exec in EvalFunc and therefore do a lot
of redundant computation.
Just because I don't know where and how to put this shared
computation.
Does anybody have a solution on that?
Best,
Will
