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From Chesnay Schepler <chesnay.schep...@fu-berlin.de>
Subject Re: Python API - Weird Performance Issue
Date Wed, 27 Aug 2014 20:48:46 GMT
Hey Stephan,

I'd like to point out right away that the code related to your questions 
is shared by both programs.

regarding your first point: i have a byte[] into which i serialize the 
data first using a ByteBuffer, and then write that data to a 
MappedByteBuffer.

regarding synchronization: i couldn't find a way to use elaborate things 
like semaphores or similar that work between python and java alike.

the data exchange is currently completely synchronous. java writes a 
record, sets an "isWritten" bit and then repeatedly checks this bit 
whether it is 0. python repeatedly checks this bit whether it is 1. once 
that happens, it reads the record, sets the bit to 0 which tells java 
that it has read the record and can write the next one. this scheme 
works the same way the other way around.

*NOW,* this may seem ... inefficient, to put it slightly. it is (or 
rather should be...) way faster (5x) that what we had so far though 
(asynchronous pipes).
(i also tried different schemes that all had no effect, so i decided to 
stick with the easiest one)

on to your last point: I'm gonna check for that tomorrow.



On 27.8.2014 20:45, Stephan Ewen wrote:
> Hi Chesnay!
>
> That is an interesting problem, though hard to judge with the information
> we have.
>
> Can you elaborate a bit on the following points:
>
>   - When putting the objects from the Java Flink side into the shared
> memory, you need to serialize them. How do you do that? Into a buffer, then
> copy that into the shared memory ByteBuffer? Directly?
>
>   - Shared memory access has to be somehow controlled. The pipes give you
> flow control for free (blocking write calls when the stream consumer is
> busy). What do you do for the shared memory? Usually, one uses semaphores,
> or, in java File(Range)Locks to coordinate access and block until memory
> regions are made available. Can you check if there are some busy waiting
> parts in you code?
>
>   - More general: The code is slower, but does it burn CPU cycles in its
> slowness or is it waiting for locks / monitors / conditions ?
>
> Stephan
>
>
>
> On Wed, Aug 27, 2014 at 8:34 PM, Chesnay Schepler <
> chesnay.schepler@fu-berlin.de> wrote:
>
>> Hello everyone,
>>
>> This will be some kind of brainstorming question.
>>
>> As some of you may know I am currently working on the Python API. The most
>> crucial part here is how the data is exchanged between Java and Python.
>> Up to this point we used pipes for this, but switched recently to memory
>> mapped files in hopes of increasing the (lacking) performance.
>>
>> Early (simplified) prototypes (outside of Flink) showed that this would
>> yield a significant increase. yet when i added the code to flink and ran a
>> job, there was
>> no effect. like at all. two radically different schemes ran in /exactly/
>> the same time.
>>
>> my conclusion was that code already in place (and not part of the
>> prototypes) is responsible for this.
>> so i went ahead and modified the prototypes to use all relevant code from
>> the Python API in order to narrow down the culprit. but this time, the
>> performance increase was there.
>>
>> Now here's the question: How can the /very same code/ perform so much
>> worse when integrated into flink? if the code is not the problem, what
>> could be it?
>>
>> i spent a lot of time looking for that one line of code that cripples the
>> performance, but I'm pretty much out of places to look.
>>
>>


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