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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