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[Thread Pool](https://www.elastic.co/guide/en/elasticsearch/reference/6.3/modules-threadpool.html?nsukey=W48pJFXgvLgtyfM%2FXI%2FQ8BhcPYE0WjNFGwKD%2Bv0cjUkocQhh1FCPQwklwVBOec541GcUkX899f%2FQv5ikjA8ckqdcrlVHpjATTKbJarNA1Gr%2BltpTiDJe%2FMuqrZGa0CAkbLfIGrYqMyc0s%2BlkxFr%2BzEo2hmWsgMaPZvK9K6f9Xugz0k0v1Wb2nfeex8ZQn0Y9SFEHwRaYV5uzw8rnH9I%2BGg%3D%3D)

From this document, we can find that the bulk thread pool setting is deleted.

write
For single-document index/delete/update and bulk requests. Thread pool type is fixed with a size of # of available processors, queue_size of 200. The maximum size for this pool is 1 + # of available processors.

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