[ovs-dev] [PATCH ovn 2/2] ovn-controller: Fix flow installation latency caused by recompute.

Numan Siddique nusiddiq at redhat.com
Tue Jul 30 04:09:57 UTC 2019


On Tue, Jul 30, 2019 at 7:10 AM Han Zhou <zhouhan at gmail.com> wrote:

> From: Han Zhou <hzhou8 at ebay.com>
>
> When there are in-flight flow-installations pending to ovs-vswitchd,
> current incremental processing logic prioritizes new change handling.
> However, in scenarios where frequent recomputes are triggered, the
> later recompute would block the flow-installation for previously
> computed flows because recompute usually takes long time, especially
> when there are large number of flows. This results in worse latency
> than the version without incremental processing in specific scale
> test scenarios.
>
> While we can simply fix the problem by prioritizing flow installation
> rather than new change handling, it can cause the incremental
> processing to degrade to always recompute in certain scenarios when
> there are some changes triggering recomputes, followed by a lot of
> continously coming changes that can be handled incrementally. Because
> OVSDB change tracker cannot preserve changes across iterations, once
> the recompute is triggered and resulted in a lot of pending messages
> to ovs-vswitchd, and if we choose to skip the engine_run()
> in the next iteration when a incrementally processible change comes,
> we miss the opportunity to handle that tracked change and will have
> to trigger recompute again in the next next iteration, and so on, if
> such changes come continously.
>
> This patch solves the problem by introducing engine_set_abort_recompute(),
> so that we can prioritize new change handling if the change can be
> incrementally processed, but if the change triggers recompute, we
> abort there without spending CPU on the recompute to avoid blocking
> the previous computed flow installation.
>
> Reported-by: Daniel Alvarez Sanchez <dalvarez at redhat.com>
> Reported-by: Numan Siddique <nusiddiq at redhat.com>
> Reported-at:
> https://mail.openvswitch.org/pipermail/ovs-discuss/2019-June/048822.html
> Tested-by: Numan Siddique <nusiddiq at redhat.com>
> Acked-by: Numan Siddique <nusiddiq at redhat.com>
> Acked-by: Mark Michelson <mmichels at redhat.com>
> Signed-off-by: Han Zhou <hzhou8 at ebay.com>
> ---
>

Thanks. I applied this series to master.

Numan


>  controller/ofctrl.c         |  2 +-
>  controller/ofctrl.h         |  1 +
>  controller/ovn-controller.c | 30 +++++++++++++++++++++++++++---
>  lib/inc-proc-eng.c          | 26 ++++++++++++++++++++++----
>  lib/inc-proc-eng.h          |  9 +++++++--
>  5 files changed, 58 insertions(+), 10 deletions(-)
>
> diff --git a/controller/ofctrl.c b/controller/ofctrl.c
> index 4d24a8b..8928205 100644
> --- a/controller/ofctrl.c
> +++ b/controller/ofctrl.c
> @@ -985,7 +985,7 @@ add_meter(struct ovn_extend_table_info *m_desired,
>   * in the correct state and not backlogged with existing flow_mods.  (Our
>   * criteria for being backlogged appear very conservative, but the socket
>   * between ovn-controller and OVS provides some buffering.) */
> -static bool
> +bool
>  ofctrl_can_put(void)
>  {
>      if (state != S_UPDATE_FLOWS
> diff --git a/controller/ofctrl.h b/controller/ofctrl.h
> index 114c9ef..1e9ac16 100644
> --- a/controller/ofctrl.h
> +++ b/controller/ofctrl.h
> @@ -51,6 +51,7 @@ void ofctrl_put(struct ovn_desired_flow_table *,
>                  const struct sbrec_meter_table *,
>                  int64_t nb_cfg,
>                  bool flow_changed);
> +bool ofctrl_can_put(void);
>  void ofctrl_wait(void);
>  void ofctrl_destroy(void);
>  int64_t ofctrl_get_cur_cfg(void);
> diff --git a/controller/ovn-controller.c b/controller/ovn-controller.c
> index d3b28b9..ad33d17 100644
> --- a/controller/ovn-controller.c
> +++ b/controller/ovn-controller.c
> @@ -1896,6 +1896,7 @@ main(int argc, char *argv[])
>
>      uint64_t engine_run_id = 0;
>      uint64_t old_engine_run_id = 0;
> +    bool engine_aborted = false;
>
>      unsigned int ovs_cond_seqno = UINT_MAX;
>      unsigned int ovnsb_cond_seqno = UINT_MAX;
> @@ -1982,7 +1983,30 @@ main(int argc, char *argv[])
>                      stopwatch_start(CONTROLLER_LOOP_STOPWATCH_NAME,
>                                      time_msec());
>                      if (ovnsb_idl_txn) {
> -                        engine_run(&en_flow_output, ++engine_run_id);
> +                        if (!ofctrl_can_put()) {
> +                            /* When there are in-flight messages pending
> to
> +                             * ovs-vswitchd, we should hold on
> recomputing so
> +                             * that the previous flow installations won't
> be
> +                             * delayed.  However, we still want to try if
> +                             * recompute is not needed and we can quickly
> +                             * incrementally process the new changes, to
> avoid
> +                             * unnecessarily forced recomputes later on.
> This
> +                             * is because the OVSDB change tracker cannot
> +                             * preserve tracked changes across
> iterations.  If
> +                             * change tracking is improved, we can simply
> skip
> +                             * this round of engine_run and continue
> processing
> +                             * acculated changes incrementally later when
> +                             * ofctrl_can_put() returns true. */
> +                            if (!engine_aborted) {
> +                                engine_set_abort_recompute(true);
> +                                engine_aborted =
> engine_run(&en_flow_output,
> +
> ++engine_run_id);
> +                            }
> +                        } else {
> +                            engine_set_abort_recompute(false);
> +                            engine_aborted = false;
> +                            engine_run(&en_flow_output, ++engine_run_id);
> +                        }
>                      }
>                      stopwatch_stop(CONTROLLER_LOOP_STOPWATCH_NAME,
>                                     time_msec());
> @@ -2024,8 +2048,8 @@ main(int argc, char *argv[])
>                  }
>
>              }
> -            if (old_engine_run_id == engine_run_id) {
> -                if (engine_need_run(&en_flow_output)) {
> +            if (old_engine_run_id == engine_run_id || engine_aborted) {
> +                if (engine_aborted || engine_need_run(&en_flow_output)) {
>                      VLOG_DBG("engine did not run, force recompute next
> time: "
>                               "br_int %p, chassis %p", br_int, chassis);
>                      engine_set_force_recompute(true);
> diff --git a/lib/inc-proc-eng.c b/lib/inc-proc-eng.c
> index 1ddea1a..1064a08 100644
> --- a/lib/inc-proc-eng.c
> +++ b/lib/inc-proc-eng.c
> @@ -31,6 +31,7 @@
>  VLOG_DEFINE_THIS_MODULE(inc_proc_eng);
>
>  static bool engine_force_recompute = false;
> +static bool engine_abort_recompute = false;
>  static const struct engine_context *engine_context;
>
>  void
> @@ -39,6 +40,12 @@ engine_set_force_recompute(bool val)
>      engine_force_recompute = val;
>  }
>
> +void
> +engine_set_abort_recompute(bool val)
> +{
> +    engine_abort_recompute = val;
> +}
> +
>  const struct engine_context *
>  engine_get_context(void)
>  {
> @@ -121,11 +128,11 @@ engine_ovsdb_node_add_index(struct engine_node
> *node, const char *name,
>      ed->n_indexes ++;
>  }
>
> -void
> +bool
>  engine_run(struct engine_node *node, uint64_t run_id)
>  {
>      if (node->run_id == run_id) {
> -        return;
> +        return true;
>      }
>      node->run_id = run_id;
>
> @@ -133,11 +140,13 @@ engine_run(struct engine_node *node, uint64_t run_id)
>      if (!node->n_inputs) {
>          node->run(node);
>          VLOG_DBG("node: %s, changed: %d", node->name, node->changed);
> -        return;
> +        return true;
>      }
>
>      for (size_t i = 0; i < node->n_inputs; i++) {
> -        engine_run(node->inputs[i].node, run_id);
> +        if (!engine_run(node->inputs[i].node, run_id)) {
> +            return false;
> +        }
>      }
>
>      bool need_compute = false;
> @@ -160,6 +169,10 @@ engine_run(struct engine_node *node, uint64_t run_id)
>      if (need_recompute) {
>          VLOG_DBG("node: %s, recompute (%s)", node->name,
>                   engine_force_recompute ? "forced" : "triggered");
> +        if (engine_abort_recompute) {
> +            VLOG_DBG("node: %s, recompute aborted", node->name);
> +            return false;
> +        }
>          node->run(node);
>      } else if (need_compute) {
>          for (size_t i = 0; i < node->n_inputs; i++) {
> @@ -170,6 +183,10 @@ engine_run(struct engine_node *node, uint64_t run_id)
>                      VLOG_DBG("node: %s, can't handle change for input %s,
> "
>                               "fall back to recompute",
>                               node->name, node->inputs[i].node->name);
> +                    if (engine_abort_recompute) {
> +                        VLOG_DBG("node: %s, recompute aborted",
> node->name);
> +                        return false;
> +                    }
>                      node->run(node);
>                      break;
>                  }
> @@ -178,6 +195,7 @@ engine_run(struct engine_node *node, uint64_t run_id)
>      }
>
>      VLOG_DBG("node: %s, changed: %d", node->name, node->changed);
> +    return true;
>  }
>
>  bool
> diff --git a/lib/inc-proc-eng.h b/lib/inc-proc-eng.h
> index 6f0d08d..3a69dc2 100644
> --- a/lib/inc-proc-eng.h
> +++ b/lib/inc-proc-eng.h
> @@ -121,8 +121,9 @@ struct engine_node {
>  void engine_init(struct engine_node *);
>
>  /* Execute the processing recursively, which should be called in the main
> - * loop. */
> -void engine_run(struct engine_node *, uint64_t run_id);
> + * loop. Returns true if the execution is compelte, false if it is
> aborted,
> + * which could happen when engine_abort_recompute is set. */
> +bool engine_run(struct engine_node *, uint64_t run_id);
>
>  /* Clean up the data for the engine nodes recursively. It calls each
> node's
>   * cleanup() method if not NULL. It should be called before the program
> @@ -150,6 +151,10 @@ void engine_add_input(struct engine_node *node,
> struct engine_node *input,
>   * iteration, and the change can't be tracked across iterations */
>  void engine_set_force_recompute(bool val);
>
> +/* Set the flag to cause engine execution to be aborted when there
> + * is any recompute to be triggered in any node. */
> +void engine_set_abort_recompute(bool val);
> +
>  const struct engine_context * engine_get_context(void);
>
>  void engine_set_context(const struct engine_context *);
> --
> 2.1.0
>
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