mina sshd Nio2Acceptor seems like memory peak

I am using sshd 1.1.0 as an sftp server running on a Red Hat os. I found this program often produces hprof file. I use MAT to analysis this file and find this kind of prompt:
"org.apache.sshd.common.io.nio2.Nio2Acceptor" instance "0x761551a68" occupies 3,577,793,864 (98.18%) byte. Its main accumulation of memory is "java.util.concurrent.ConcurrentHashMap$Segment[]" instance "0x7608721c8" loaded by "bootstrap class loader" .
Futher I found Nio2Accepter's sessions is so huge, seen in the attachment1. I doubt the method "sessionClosed" in Nio2Service is not executed when a client disconnect abnormally. So I want to ask your help to postion this problem. Thank you.