nio package and asynchronous io

Mon Feb 4 14:36:00 GMT 2002

Mark Wielaard wrote:
 >>>>do you have any plan on adding nio support and asynchronous io to the
 >>>>classpath project?
 >>>>if yes what is your agenda on this subject.
 >>>>
 >>>I have done a little bit of work for GCJ with this.
 >>>
 >
 >I have read the online 1.4 specs for java.nio and thought about how to
 >implement it but I have done no actual work. We will get it in time, but
 >nobody is actively working on it at the moment.
I've started to implement some of the Buffer and mapped I/O stuff in my
GCJ tree too.
Its relatively simple to implement for CNI, however I think we'll need
to look at adding some special support for buffers in GCJ in order to
get really good performance from zero-copy I/O. Thats because the idea
is to write things like:
while (buf.hasRemaining())
 ... = buf.get();
in order to avoid copying data into an array during I/O, but unless they
get inlined the call overhead is going to kill any performance gain
obtained from not copying. Inlining the hasRemaining() and all its
friends is easy because they are final, but the buf.get() is a bit
harder because its a virtual method. However if we taught GCJ that the
various methods in nio usually/always return a GCJMappedByteBuffer or
whatever, it could inline the get() too and the whole things becomes no
less efficient than a regular bounds-checked array acccess.
Its just a shame they didn't include Buffer methods like:
byte[] get(int length); // get bytes from the buffer as a shared byte
array.
I guess allowing that sort of thing would raise aliasing issues for the
JITs, and you couldn't do it for a read-only buffer without changing the
semantics of arrays.
regards
Bryce.
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