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<div class="title">C++ Performance Notes </div>  </div>
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<div class="textblock"><h2>Streaming write buffering</h2>
<p>Generally, each write operation (Write(), WritesDone()) implies a syscall. gRPC will try to batch together separate write operations from different threads, but currently cannot automatically infer batching in a single stream.</p>
<p>If message k+1 in a stream does not rely on responses from message k, it's possible to enable write batching by passing a WriteOptions argument to Write with the buffer_hint set:</p>
<div class="fragment"><div class="line">stream_writer-&gt;Write(message, WriteOptions().set_buffer_hint());</div></div><!-- fragment --><p>The write will be buffered until one of the following is true:</p><ul>
<li>the per-stream buffer is filled (controllable with the channel argument GRPC_ARG_HTTP2_WRITE_BUFFER_SIZE) - this prevents infinite buffering leading to OOM</li>
<li>a subsequent Write without buffer_hint set is posted</li>
<li>the call is finished for writing (WritesDone() called on the client, or Finish() called on an async server stream, or the service handler returns for a sync server stream)</li>
</ul>
<h2>Completion Queues and Threading in the Async API</h2>
<p>Right now, the best performance trade-off is having numcpu's threads and one completion queue per thread. </p>
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