[ovs-discuss] Questions about Replacement Policy in emc_insert()

cynthiazhang cynthia_zhang123 at 163.com
Tue Jul 4 03:24:41 UTC 2017


Hi:
When I read the code of emc_cache in dips-netdev.c, I could not understand the replacement policy below in emc_insert() as follows():
      /* Replacement policy: put the flow in an empty (not alive) entry, or
         * in the first entry where it can be */
        if (!to_be_replaced
            || (emc_entry_alive(to_be_replaced)
                && !emc_entry_alive(current_entry))
            || current_entry->key.hash < to_be_replaced->key.hash) {
            to_be_replaced = current_entry;
        }
1)The EMC_FOR_EACH_POS_WITH_HASH makes us have two locations to put the flow in. If the first entry is dead and the function emc_cache_slow_sweep() has not clear this entry, and the second entry is also in the same condition. So does this mean that we should compare the hash value of each entry?If so ,why we choose the smaller hash value to put the new flow in? How to understand the "first place" in the Replacement Policy?
2)So the second question is when we should compare the value of the two entries? And why we do this?
3)Besides, why we define EM_FLOW_HASH_SEGS as 2? 


Thanks,
Cynthia.
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