1. 08 Dec, 2018 1 commit
    • David Ahern's avatar
      neighbor: Improve garbage collection · 58956317
      David Ahern authored
      The existing garbage collection algorithm has a number of problems:
      
      1. The gc algorithm will not evict PERMANENT entries as those entries
         are managed by userspace, yet the existing algorithm walks the entire
         hash table which means it always considers PERMANENT entries when
         looking for entries to evict. In some use cases (e.g., EVPN) there
         can be tens of thousands of PERMANENT entries leading to wasted
         CPU cycles when gc kicks in. As an example, with 32k permanent
         entries, neigh_alloc has been observed taking more than 4 msec per
         invocation.
      
      2. Currently, when the number of neighbor entries hits gc_thresh2 and
         the last flush for the table was more than 5 seconds ago gc kicks in
         walks the entire hash table evicting *all* entries not in PERMANENT
         or REACHABLE state and not marked as externally learned. There is no
         discriminator on when the neigh entry was created or if it just moved
         from REACHABLE to another NUD_VALID state (e.g., NUD_STALE).
      
         It is possible for entries to be created or for established neighbor
         entries to be moved to STALE (e.g., an external node sends an ARP
         request) right before the 5 second window lapses:
      
              -----|---------x|----------|-----
                  t-5         t         t+5
      
         If that happens those entries are evicted during gc causing unnecessary
         thrashing on neighbor entries and userspace caches trying to track them.
      
         Further, this contradicts the description of gc_thresh2 which says
         "Entries older than 5 seconds will be cleared".
      
         One workaround is to make gc_thresh2 == gc_thresh3 but that negates the
         whole point of having separate thresholds.
      
      3. Clearing *all* neigh non-PERMANENT/REACHABLE/externally learned entries
         when gc_thresh2 is exceeded is over kill and contributes to trashing
         especially during startup.
      
      This patch addresses these problems as follows:
      
      1. Use of a separate list_head to track entries that can be garbage
         collected along with a separate counter. PERMANENT entries are not
         added to this list.
      
         The gc_thresh parameters are only compared to the new counter, not the
         total entries in the table. The forced_gc function is updated to only
         walk this new gc_list looking for entries to evict.
      
      2. Entries are added to the list head at the tail and removed from the
         front.
      
      3. Entries are only evicted if they were last updated more than 5 seconds
         ago, adhering to the original intent of gc_thresh2.
      
      4. Forced gc is stopped once the number of gc_entries drops below
         gc_thresh2.
      
      5. Since gc checks do not apply to PERMANENT entries, gc levels are skipped
         when allocating a new neighbor for a PERMANENT entry. By extension this
         means there are no explicit limits on the number of PERMANENT entries
         that can be created, but this is no different than FIB entries or FDB
         entries.
      Signed-off-by: default avatarDavid Ahern <dsahern@gmail.com>
      Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      58956317
  2. 07 Dec, 2018 33 commits
  3. 06 Dec, 2018 6 commits