[ovs-dev] [OVN Patch 2/2] northd: Alter initial hash sizing for dp_groups+parallel

anton.ivanov at cambridgegreys.com anton.ivanov at cambridgegreys.com
Wed Aug 25 07:35:00 UTC 2021


From: Anton Ivanov <anton.ivanov at cambridgegreys.com>

When running with dp_groups and parallelization enabled we have
a possible worse case scenario where northd connects for the
first time to a pre-populated database.
If we do not size the hash to a sufficiently big size to
accommodate for this, we end up with severe lock contention
and very slow lookups.
If both dp_groups and parallelization are enabled we have
no choice, but to allocate for a worst case scenario on the
first run and adjust later.

Signed-off-by: Anton Ivanov <anton.ivanov at cambridgegreys.com>
---
 northd/ovn-northd.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/northd/ovn-northd.c b/northd/ovn-northd.c
index 14659c407..3a5ab1d9f 100644
--- a/northd/ovn-northd.c
+++ b/northd/ovn-northd.c
@@ -13034,7 +13034,7 @@ ovn_sb_set_lflow_logical_dp_group(
     sbrec_logical_flow_set_logical_dp_group(sbflow, dpg->dp_group);
 }
 
-static ssize_t max_seen_lflow_size = 128;
+static ssize_t max_seen_lflow_size = 0;
 
 /* Updates the Logical_Flow and Multicast_Group tables in the OVN_SB database,
  * constructing their contents based on the OVN_NB database. */
@@ -13048,6 +13048,25 @@ build_lflows(struct northd_context *ctx, struct hmap *datapaths,
 {
     struct hmap lflows;
 
+    if (!max_seen_lflow_size) {
+        if (use_logical_dp_groups && use_parallel_build) {
+            /* We are running for the first time. The user has
+             * requested both dp_groups and parallelisation. We
+             * may encounter a very large amount of flows on first
+             * run and we have no way to guess the flow hash size.
+             * We allocate for worst a case scenario on the first
+             * run. This will be resized to a sane size later. */
+            max_seen_lflow_size = INT_MAX;
+        } else {
+            /* If the build is not parallel, this will be resized
+             * to a correct size. If it is parallel, but without
+             * dp_groups, the sizing is irrelevant as the hash is
+             * not used for lookups during build. We resize it to
+             * a correct size after that. */
+            max_seen_lflow_size = 128;
+        }
+    }
+
     fast_hmap_size_for(&lflows, max_seen_lflow_size);
     if (use_parallel_build) {
         update_hashrow_locks(&lflows, &lflow_locks);
-- 
2.20.1



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