RFR: JDK-8202788: Explicitly reclaim cached thread-local direct buffers at thread exit

[Martin Buchholz]

I'm ignoring the direct buffers problem (sorry Tony) and wearing my
ReentrantReadWriteLock hat.  I still like the idea of adding
ThreadLocal.compute(Function remappingFunction) and perhaps other such
map-inspired methods. RRWL wouldn't benefit much, since it already tries to
minimize use of ThreadLocal, but it would at least allow get() and remove()
to be coalesced on read-unlock.

RRWL never changes from one non-null value to another, it only switches
between absent and present.  I'd like to avoid the two lookups due to get
and remove.  Here's a prototype that does not yet help RRWL, but helps
callers switching between non-null values, and could probably be extended
via surgery to ThreadLocalMap:

    public T compute(
            java.util.function.Function<? super T,? extends T>
remappingFunction) {
        final Thread currentThread = Thread.currentThread();
        final ThreadLocalMap map = getMap(currentThread);
        final ThreadLocalMap.Entry e = (map == null)
            ? null
            : map.getEntry(this);
        final T oldValue = (e == null) ? null : (T)e.value;
        final T newValue = remappingFunction.apply(oldValue);
        if (newValue == null) {
            if (oldValue != null) {
                map.remove(this);
            }
        } else if (e != null) {
            e.value = newValue;
        } else if (map != null) {
            map.set(this, newValue);
        } else {
            createMap(currentThread, newValue);
        }
        return newValue;
    }



On Sun, Jun 17, 2018 at 2:20 PM, Peter Levart <peter.levart at gmail.com>
wrote:

> Update: the discussion on concurrent-interest about possible future
> addition of public ThreadLocal.getIfPresent() or ThreadLocal.isPresent()
> has died out, but it nevertheless reached a point where
> ThreadLocal.isPresent() was found the least problematic. So I would like to
> get further with this proposal using the last webrev.04 version of the
> patch that uses ThreadLocal.isPresent() internally - still package-private
> at the moment.
>
> Here's the webrev (unchanged from the day it was published):
>
> http://cr.openjdk.java.net/~plevart/jdk-dev/DBBCache_Cleanup/webrev.04/
>
> Would this version be OK?
>
> Regards, Peter
>
>
>
> On 06/06/18 20:55, Peter Levart wrote:
>
>