I’ve been trying to track down a NIO memory leak that occurs in a Netty
application I am porting from Netty 3 to Netty 4. This leak does not occur
in the Netty 3 version of the application.
For now, I’m using only unpooled heap buffers in Netty 4, but NIO buffers
do come into play for socket communication.
I’ve captured a few heap dumps from affected instances, and in each it
appears that the leaked DirectByteBuf java objects are rooted in an
io.netty.util.Recycler.
These buffers remain indefinitely: I can disable the application to drain
traffic and force GCs, but the # of NIO buffers and NIO allocated space
stays flat.
The issue is likely related to slow readers. However, the leak persists
long after all channels have been closed.
I implemented a writability listener and the leak does appear to go away if
I stop writing to a channel after it goes unwritable. This is good, but I’m
still worried that this just makes the problem less likely since it’s still
possible to write/flush and have pending data: writability just limits how
much data will be buffered.
Digging into ChannelOutBoundBuffer I see the following stanza in close:
// Release all unflushed messages.
try {
Entry e = unflushedEntry;
while (e != null) {
// Just decrease; do not trigger any events via
decrementPendingOutboundBytes()
int size = e.pendingSize;
TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, -size);
if (!e.cancelled) {
ReferenceCountUtil.safeRelease(e.msg);
safeFail(e.promise, cause);
}
e = e.recycleAndGetNext();
}
} finally {
inFail = false;
}
clearNioBuffers();
This seems a bit curious to me: why are flushed buffers not released here?
Since the leak seems to be rooted in the Recycler, this could be the
culprit…What do you think?
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