merlimat opened a new pull request, #3837:
URL: https://github.com/apache/bookkeeper/pull/3837
### Motivation
Note: this is stacked on top of #3830 & #3835
This change improves the way the AddRequests responses are send to client.
The current flow is:
* The journal-force-thread issues the fsync on the journal file
* We iterate over all the entries that were just synced and for each of
them:
1. Trigger channel.writeAndFlus()
2. This will jump on the connection IO thread (Netty will use a
`write()` to `eventfd` to post the task and wake the epoll)
3. Write the object in the connection and trigger the serialization
logic
4. Grab a `ByteBuf` from the pool and write ~20 bytes with the response
5. Write and flush the buffer on the channel
6. With the flush consolidator we try to group multiple buffer into a
single `writev()` syscall, though each call will have a long list of buffer,
making the memcpy inefficient.
7. Release all the buffers and return them to the pool
All these steps are quite expensive when the bookie is receiving a lot of
small requests.
This PR changes the flow into:
1. journal fsync
2. go through each request and prepare the response into a per-connection
`ByteBuf` which is not written on the channel as of yet
3. after preparing all the responses, we flush them at once: Trigger an
event on all the connections that will write the accumulated buffers.
The advantages are:
1. 1 ByteBuf allocated per connection instead of 1 per request
1. Less allocations and stress of buffer pool
2. More efficient socket write() operations
3. 1 task per connection posted on the Netty IO threads, instead of 1 per
request.
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