Throw exception when start broker or historical using default config.

[denglingang]

Throw exception when start broker or historical using default config, exception is as follows,
1) Not enough direct memory. Please adjust -XX:MaxDirectMemorySize, druid.processing.buffer.sizeBytes, druid.processing.numThreads, or druid.processing.numMergeBuffers: maxDirectMemory[4,294,967,296], memoryNeeded[5,368,709,120] = druid.processing.buffer.sizeBytes[536,870,912] * (druid.processing.numMergeBuffers[2] + druid.processing.numThreads[7] + 1)
I think druid should be friendlier to first-used user.

[leventov]

With what Xmx do you start?

[denglingang]

Thanks for your review, -Xmx8g by default.

[denglingang]

@leventov Pls review again, thx.

[leventov]

👎 I don't like the idea of reducing default buffer sizing that much by default, 99% of users won't bother changing defaults, may have awful performance because of buffer sizing, and blame Druid slow.

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