[ovs-discuss] [ACLv2 18/19] hmap: Add HMAP_FOR_EACH_WITH_HASH_SAFE.

[Ben Pfaff]

Jesse Gross <jesse at nicira.com> writes:

> Add a _SAFE version of the macro to allow callers to delete entries
> as they are iterating over matching entries.

I'm really nervous about this.  It only makes sense to allow
duplicates in an hmap if the maximum number of duplicates for a
given key is bounded by a very small constant and if it is
unusual for there to be duplicates.  For example, if occasionally
there might be exactly 2 entries for a given key but there are
normally only one, then it's OK.  But if there might be dozens or
hundreds of entries with a given key, then that's going to kill
hash performance and it's a much better idea to use a single
hmap_node that keeps a list of the duplicate items, e.g.:

        struct my_node {
            struct hmap_node hmap_node;
            int my_key; /* or whatever */
            struct list list;
        };

Then the performance will be much better.