On 04/06/2018 10:02 AM, Alan Bateman wrote:
On 05/04/2018 22:45, Tony Printezis wrote:
Hi all,

We recently hit another interesting issue with the NIO thread-local
DirectByteBuffer cache. One of our services seems to create and drop
threads at regular intervals. I assume this is due to a thread pool
dynamically resizing itself.

Let's say a thread starts, lives long enough for its Thread object to be
promoted to the old gen (along with its cached direct buffer), then exits.
This will result in its cached direct buffer(s) being unreachable in the
old gen and will only be reclaimed after the next full GC / concurrent GC

Right, if a short lived thread does I/O with a buffer backed by an array in the heap then any direct buffers in its thread local cache won't be freed until there is a GC and reference processing. It's something that I've prototyped a few times and always leaned towards not exposing anything in the API, instead just hooking into the thread exit to clear the buffer cache. One thing to watch out for is that the buffer cache may not exist (as the thread didn't do any I/O with heap buffers) so you'll end up creating (an empty) buffer cache at thread exit. That is benign of course but still unsettling to have additional code executing at this time.


An internal method, let's say ThreadLocal.probe(), that would return thread-local value only if it has already been initialized, would be helpfull. Maybe it could be exposed as new public API too. I remember that I needed it in the past:

    public T probe() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                T result = (T)e.value;
                return result;
        return null;

Regards, Peter

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