On 03/23/2016 09:40 PM, Kim Barrett wrote:
On Mar 23, 2016, at 3:33 PM, Peter Levart <peter.lev...@gmail.com> wrote:
Hi Kim,
On 03/23/2016 07:55 PM, Kim Barrett wrote:
On Mar 23, 2016, at 10:02 AM, Peter Levart <peter.lev...@gmail.com>
wrote:
...so I checked what it would be needed if there was such
getPendingReferences() native method. It turns out that a single native method
would not be enough to support the precise direct ByteBuffer allocation. Here's
a refactored webrev that introduces a getPendingReferences() method which could
be turned into a native equivalent one day. There is another native method
needed - int awaitEnqueuePhaseStart():
http://cr.openjdk.java.net/~plevart/jdk9-dev/removeInternalCleaner/webrev.09.part2/
I don't think the Reference.awaitEnqueuePhaseStart thing is needed.
Rather, I think the Direct-X-Buffer allocation should conspire with
the the Direct-X-Buffer cleanups directly to manage that sort of
thing, and not add anything to Reference and the reference processing
thread. E.g. the phase and signal/wait are purely part of
Direct-X-Buffer. (I also think something like that could/should have
been done instead of providing Direct-X-Buffer with access to
Reference.tryHandlePending, but that's likely water under the bridge
now.)
Something very roughly like this:
allocating thread, after allocation failed
bool waitForCleanups() {
int epoch = DXB.getCleanupCounter();
long start = startTime();
long timeout = calcTimeout(start)
synchronized (DXB.getCleanupMonitor()) {
while (epoch == DBX.getCleanupCounter()) {
wait(timeout);
timeout = calcTimeout(start);
if (timeout <= 0) break;
}
return epoch != DBX.getCleanupCounter();
}
}
cleanup function, after freeing memory
synchronized (DBX.getCleanupMonitor()) {
DBX.incCleanupCounter();
DBX.getCleanupMonitor().notify_all();
}
Actually, epoch should probably have been obtained *before* the failed
allocation attempt, and should be an argument to waitForCleanups.
That's all quite sketchy, but I need to do other things today.
Peter, care to try filling this in?
There's no need to maintain a special cleanup counter as java.nio.Bits already maintains the amount
of currently allocated direct memory (in bytes). What your suggestion leads to is similar to one of
previous versions of java.nio.Bits which waited for some 'timeout' time after invoking System.gc()
and then re-tried reservation, failing if it didn't succeed. The problem with such
"asynchronous" approach is that there's no right value of 'timeout' for all situations.
If you wait for to short time, you might get OOME although there are plenty unreachable but still
uncleaned direct buffers. If you wait for to long, your throughput will suffer. There has to be
some "feedback" from reference processing to know when there's still beneficial to wait
and when there's no point in waiting any more.
Regards, Peter
I don't think there's any throughput penalty for a long timeout. The
proper response to waitForCleanups returning false (assuming the epoch
was obtained early and passed as an argument) is OOME. I really doubt
the latency for reporting OOME is of critical importance.
That is, the caller looks something like (not even pretending to write
Java)
alloc = tryAllocatation(allocSize)
if alloc != NULL
return alloc
endif
// Maybe add a retry+wait with a short timeout here,
// to allow existing cleanups to run before requesting
// another gc. Not clear that's really worthwhile, as
// it only comes up when we get here just after a gc
// and the resulting cleanups are not yet all processed.
System.gc()
while true
epoch = getEpoch()
alloc = tryAllocation(allocSize)
if alloc != NULL
return alloc
elif !waitForCleanup(epoch)
throw OOME // No cleanup progress for a while
endif
end
Right, this is easier to understand. I already figured out what you
wanted to say the 1st time. I'll try to prepare a prototype along this
idea tomorrow.
Regards, Peter
