add synchronized to protect the map, to fix the bug:

java.util.ConcurrentModificationException
         at java.util.IdentityHashMap.remove(IdentityHashMap.java:729)
         at com.ycache.danga.MemCached.SockIOPool.clearHostFromPool(Unknown Source)
         at com.ycache.danga.MemCached.SockIOPool.createSocket(Unknown Source)
         at com.ycache.danga.MemCached.SockIOPool.getConnection(Unknown Source)
         at com.ycache.danga.MemCached.SockIOPool.getSock(Unknown Source)
         at com.ycache.danga.MemCached.MemCachedClient.get(Unknown Source)
         at com.ycache.danga.MemCached.MemCachedClient.get(Unknown Source)
         at com.yihaodian.common.ycache.memcache.impl.BaseMemcacheProxy.get(Unknown Source)