Was there a reason why this was not introduced in the first place?
Tony
—————
Tony Printezis | @TonyPrintezis | tprinte...@twitter.com
On April 6, 2018 at 8:49:17 AM, Peter Levart (peter.lev...@gmail.com) wrote:
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
>> cycle.
>>
> 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.
>
> -Alan
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) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return null;
}
Regards, Peter
