sun.nio.ch.Util: Don't cache an unlimited amount of memory

[Alan Bateman]

On 27/12/2015 20:35, Evan Jones wrote:
> Summary: nio Util caches an unlimited amount of memory for temporary 
> direct ByteBuffers, which effectively is a native memory leak. 
> Applications that do large I/Os can inadvertently waste gigabytes of 
> native memory or run out of memory. I suggest it should only cache a 
> "small" amount of memory per-thread (e.g. 1 MB), and maybe have a flag 
> to allow changing the limit for applications where this causes a 
> performance regression.
>
> 1. Would JDK committers support this change?
> 2. If so, any suggestions for the default and/or how to override it 
> with a flag?
>
> Tony Printezis (CCed here) added a flag to Twitter's internal JVM/JDK 
> to limit the size of this cache, which we could probably use that as a 
> starting point for a patch.
Limiting the size of the buffer cache might help in some scenarios, it 
just means a bit more complexity and yet another tuning option.

Do you do scatter/gather I/O? The current implementation will cache up 
to IOV_MAX buffers per thread but if you aren't doing scatter/gather I/O 
then caching a maximum of one buffer per thread should reduce the memory 
usage. It wouldn't be hard to modify the BufferCache implementation to 
track the number of per-thread buffers in use so that this count is the 
maximum cached rather than IOV_MAX. I'm curious if you've looked into 
doing anything along those lines.

-Alan