Reduce size of buffer used to assemble batches

[merlimat]

Motivation

The current implementation is using 128KB buffers to accumulate batches before sending. (128Kb is the max batch size, so the intention here is to avoid reallocations on that buffer as it grows).

The problem starts with (batching + “many” partitions + slow responses from broker, due to network limit or reloads). At that point, the batches are still being prepared “on-time”, eg: every 1ms. With many partitions, the per-partition rate is comparatively low, so we end cutting a batch per each message and queuing them for sending. But each message now is taking 128Kb direct memory, instead of 10K or 1K..

Modifications

Adjust the initial buffer size to the max batch size previously sent out by this particular producer.

[rdhabalia]

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