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Add an optimization tips document. (#1024)


NIO doesn't offer any tips of on how to write performant code. After
discussing with @weissi we decided it would be useful to gather and
document some useful tips and tricks.


Started an optimization tips document.


Easier for NIO users to write performant code.
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glbrntt authored and Lukasa committed Jun 5, 2019
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Swift NIO Optimization Tips

The following document is a list of tips and advice for writing high performance
code using Swift NIO.

General Optimization Tips

Although not specific to NIO, the [Swift GitHub repository][swift-repo] has a
list of tips and tricks for [writing high performance Swift code][swift-optimization-tips]
and is a great place to start.

Wrapping types in `NIOAny`

Anything passing through a NIO pipeline that is not special-cased by `NIOAny`
(i.e. is not one of `ByteBuffer`, `FileRegion`, `IOData` or
`AddressedEnvelope<ByteBuffer>`) needs to have careful attention paid to its
size. If the type isn't one of the aforementioned special cases then it must be
no greater than 24 bytes in size to avoid a heap-allocation each time it is
added to a `NIOAny`. The size of a type can be checked using

If the type being wrapped is a value type then it can be narrowed by placing it
in a copy-on-write box. Alternatively, if the type is an `enum` with associated
data, then it is possible for the associated data to be boxed by the compiler by
making it an `indirect case`.


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