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Re: [DISCUSS] Improve broadcast serialization


Hi 

> 1. I want to define a new AbstractRecordWriter as base class which defines some abstract methods and utility codes. The current RecordWriter used for other partitioner and new BroadcastRecordWriter used only for BroadcastPartitioner will both extend AbstractRecordWriter. The fields in BroadcastPartitioner are extactly as you showed below, but for current RecordWriter it also only needs one RecordSerializer if we make the RecordSerializer has no internal state.

Lets first discuss what we would like to have/implement on higher level and later focus on implementation details. Regarding making RecordSerializer stateless, there were some discussions about it previously and it was on our TODO list but I don’t remember what was holding us back. Maybe Nico will remember?

> 
> 2. You pointed the key problem that how to handle `randomEmit` in BroadcastRecordWriter, and I think this process may resue the `emit` logic in current RecordWriter. Then the `emit` and `broadcastEmit` logics in BroadcastRecordWriter will serialize data only once and copy to BufferBuilder only once. So this improvement is deterministic for BroadcastPartitioner.
> 

What logic to reuse do you have in mind? 
 
> 
> 4. As for 'broadcastEmit` improvement in RecordWriter for non-broadcast partitioner, we can also do as you suggested in option [2], but it has to finish/flush the previous BufferBuilder generated by common `emit` operation. So it may bring bad impacts on buffer utility which was improved well in event-driven flush feature. So I am not sure whether it is worth doing `broadcastEmit` improvement in RecordWriter.
> 

The whole point of my proposal [c] was to avoid the need to flush. Code would need a little bit more refactoring but it should look something like this:

void broadcastEmit(record):
	serializedRecord = serializer.serialize(record)
	for bufferBuilder in bufferBuilders:
		bufferBuilder.append(serializedRecord)
		// if we overfilled bufferBuilder, finish it, request new one and continue writing

void emit(record, channel)
	serializedRecord = serializer.serialize(record)
	bufferBuilders[channel].append(serializedRecord)
	// if we overfilled bufferBuilder, finish it, request new one and continue writing

I do not see here a need for additional flushes and it should be strict improvement over current code base.

> 
> I already realized the demo covering above 1,2,5 before. I can create jiras after we reach a final agreement, then maybe you can help review PR if have time. :)
> 

Sure :)

Piotrek
> 
> Best,
> 
> Zhijiang
> ------------------------------------------------------------------
> 发件人:Piotr Nowojski <piotr@xxxxxxxxxxxxxxxxx>
> 发送时间:2018年7月18日(星期三) 16:37
> 收件人:dev <dev@xxxxxxxxxxxxxxxx>; Zhijiang(wangzhijiang999) <wangzhijiang999@xxxxxxxxxx>
> 主 题:Re: [DISCUSS] Improve broadcast serialization
> 
> Hi,
> 
> Couple of more thoughts
> 
> a) I’m not sure if you would have to modify current RecordWriter at all. You could extract interface from current RecordWriter and just provide two implementations: current one and BroadcastRecordWriter. I’m not sure, but it doesn’t seem like they would duplicate/share lots of code. BroadcastRecordWriter would have fields:
> 
> private final RecordSerializer<T> serializers;
> 
> private final Optional<BufferBuilder> bufferBuilder;
> 
> Compared to RecordWriter’s arrays.
> 
> b) One thing that I noticed now are latency markers and randomEmit method. It prevents us from implementing option [1]. BroadcastRecordWriter would have to flush all channels on randomEmit (as I proposed in option [2]).
> 
> c) Another option to optimise broadcast writes (or for that matter all multi channel writes), would be to serialise record only once to SpanningRecordSerializer#serializationBuffer, but copy it multiple times to separate BufferBuilders. That would save us much more then half of the overhead (serialisation is more costly compared to data copying), while we would avoid problems with uneven state of channels. There would be no problems with mixed broadcast/non broadcast writes, this option could support both of them at the same time - in other words, it would be as generic as the current one.
> 
> d) Regarding StreamRecordWriter, other option is, that it could be refactored to a class implementing extracted RecordWriter interface and being a proxy/wrapper around another RecordWriter instance:
> 
> Class StreamRecordWriter implements RecordWriter {
>   private final RecordWrtier recordWriter; //either broadcast or non broadcast 
>   public void foo() {
>     recordWriter.foo();
>   }
> }
> 
> To be honest I’m not sure at the moment which one would be better [2] or [c]. In ideal world, we might want to replace current RecordWriter with [c] and after that (if that’s not enough) to implement [2] on top of [c]. 
> 
> Piotrek
> 
> > On 18 Jul 2018, at 05:36, Zhijiang(wangzhijiang999) <wangzhijiang999@xxxxxxxxxx.INVALID> wrote:
> > 
> > Hi Piotr,
> > 
> > Thanks for your replies and professional suggestions!
> > 
> > My initial thought is just as you said in first suggestion. The current RecordWriter will emit StreamRecord to some subpartition via ChannelSelector or broadcast events/watermark to all subpartitions directly.
> > If the ChannelSelector implementation is BroadcastPartitioner, then we can create a specialized BroadcastRecordWriter to handle the 'emit', 'broadcastEmit', 'broadcastEvent', etc.
> > To make it seems not tricky, I want to abstract the RecordWriter as a plugin, then implement a BroadcastRecordWriter and NonBroadcastRecordWriter separately to extend abstract RecordWriter. That means we divide the RecordWriter by ChannelSelector, and also we may remove current StreamRecordWriter to uniform the RecordWriter criteria in both stream and batch mode.
> > 
> > Considering specific implementations, I think one RecordSerializer can work for both BroadcastRecordWriter and NonBroadcastRecordWriter, but the precondition is making the RecordSerializer has no internal state, so we have to remove the BufferBuilder variable from SpanningRecordSerializer and pass it via addRecord/continueWritingWithNextBufferBuilder
> > methods from RecordWriter. BroadcastRecordWriter only needs maintain one BufferBuilder for all subpartitions, and NonBroadcastRecordWriter may need maintain one BufferBuilder per subpartition.
> > 
> > Another issue is whether this improvement is suitable for broadcastEmit(watermark) in NonBroadcastRecordWriter as you said in suggestion 2,3. I wonder it may decrease the buffer utilization if switch between broadcast and non-broadcast modes, even it may seem more tricky in implementation. I am still thinking of it.
> > 
> > Maybe we can implement the improvement for BroadcastPartitioner in first step and make sure one RecordSerializer for all subpartitions. That can reduce the memory overhead in RecordSerializer and the time cost in broadcast serialization scenarios.
> > 
> > Best,
> > 
> > Zhijiang
> > 
> > 
> > ------------------------------------------------------------------
> > 发件人:Piotr Nowojski <piotr@xxxxxxxxxxxxxxxxx>
> > 发送时间:2018年7月17日(星期二) 23:31
> > 收件人:dev <dev@xxxxxxxxxxxxxxxx>; Zhijiang(wangzhijiang999) <wangzhijiang999@xxxxxxxxxx>
> > 主 题:Re: [DISCUSS] Improve broadcast serialization
> > 
> > Hi
> > 
> > Generally speaking this would be a nice optimisation, however it might be tricky to implement. The thing to keep in mind is that currently interface allow to interleave broadcasting and normal sending, because of that at any given time some serialisers can have more data then others. For example when we have two output channels and we are looping following writes:
> > 
> > Write sth to 1. Channel
> > Broadcast to all channels
> > Write sth to 1. Channel
> > Broadcast to all channels
> > Write sth to 1. Channel
> > Broadcast to all channels
> > (…)
> > 
> > Thus buffers of different channels can fill out with different rates.
> > 
> >> In theory every record can be serialized only once and referenced for all the subpartitions in broadcast mode.
> > 
> > The problem here is that after records serialising, the only unit that can be referenced afterwards is “Buffer”. So that would leave us now with couple of options:
> > 
> > 1. Create a specialised BroadcastRecordWriter that supports ONLY broadcasting, guaranteeing that all channels always receive the same data. Here you could serialise records only once, to one BufferBuilder that could be shared and referenced by multiple BufferConsumers from different channels. Any non broadcast write would have to fail.
> > 
> > 2. Similar as above, but specialised in MOSTLY broadcasting. Operate as in 1. for broadcasts, but for any non broadcast write: finish current broadcasting BufferBuilder, flush all data on all channels, serialise single record to single channel using newly create BufferBuilder and also immediately finish/flush it, so that any subsequent broadcasts will work again as in 1.:
> > 
> > 3. Similar as 2, but lazily switch between broadcasting and non-broadcasting modes. It would have two modes of operating that could be switched back and forth: the same as currently implemented for non-broadcasted and optimised broadcast mode
> > 
> > Broadcast to all channels
> > Broadcast to all channels
> > Broadcast to all channels
> > Broadcast to all channels
> > Write sth to X Channel // this flushes all channels and clears/finishes previous BufferBuilder 
> > Write sth to Y Channel
> > Write sth to Y Channel
> > Write sth to Y Channel
> > Write sth to X Channel 
> > Broadcast to all channels // this flushes all channels and clears/finishes previous BufferBuilders, 
> > Broadcast to all channels
> > Broadcast to all channels
> > (…)
> > 
> > However both in 2. and 3. there would be very big penalty for mixing broadcast with normal writes.  
> > 
> > Piotrek
> > 
> >> On 13 Jul 2018, at 09:44, Zhijiang(wangzhijiang999) <wangzhijiang999@xxxxxxxxxx.INVALID> wrote:
> >> 
> >> Hi all,
> >> 
> >> In current implementation, the RecordSerializer is created separately for each subpartition in RecordWriter, that means the number of serializers equals to the number of subpartitions.
> >> For broadcast partitioner, every record will be serialized many times in all the subpartitions, and this may bring bad performance to some extent.
> >> In theory every record can be serialized only once and referenced for all the subpartitions in broadcast mode.
> >> 
> >> To do so, I propose the following changes:
> >> 1. Create and maintain only one serializer in RecordWriter, and it will serialize the record for all the subpartitions. It makes sense for any partitioners, and the memory overhead can be also decreased, because every serializer will maintain some separate byte buffers internally.
> >> 2. Maybe we can abstract the RecordWriter as a base class used for other partitioner mode and implement a BroadcastRecordWriter for BroadcastPartitioner. And this new implementation will add buffer references based on the number of subpartitions before adding into subpartition queue.
> >> 3. Maybe we can remove StreamRecordWriter by migrating flusher from it to RecordWriter, then the uniform RecordWriter can be used for both stream and batch. The above BroadcastRecordWriter can aslo uniform for both stream and batch.
> >> 
> >> I am not sure whether this improvement is proposed before and what do you think of it?
> >> If necessary I can create JIRAs to contirbute it, and may need one commiter cooperate with me.
> >> 
> >> Best,
> >> 
> >> Zhijiang
> > 
>