Hi, We have identified a problem when using select type groups with the selection_method set to dp_hash in Open vSwitch. Specifically, when using the insert-bucket command to add a new bucket to the group, traffic disproportionately favors the newly inserted bucket, resulting in an imbalance. This issue has been observed in OVS versions 2.15.2, 2.17.6, and persists in version 3.15.9.
To reproduce the issue, consider the following example: ovs-ofctl add-group br-int 'group_id=123,type=select' ovs-ofctl insert-buckets br-int 'group_id=123,command_bucket_id=last,bucket=bucket_id:1,actions=output:eth1' ovs-ofctl insert-buckets br-int 'group_id=123,command_bucket_id=last,bucket=bucket_id:2,actions=output:eth2' ovs-ofctl insert-buckets br-int 'group_id=123,command_bucket_id=last,bucket=bucket_id:3,actions=output:eth3’ Upon investigation, we found that OVS clones the buckets from the group modification to the new group, specifically the single inserted bucket. As in the following code segment: https://github.com/openvswitch/ovs/blob/365c2e504b41d23904611146ff26cfc50ce77aa5/ofproto/ofproto.c#L7589 In the group_setup_dp_hash_table function, OVS constructs the dp_hash hash table based on this single inserted bucket, causing the hash table to contain only that bucket: https://github.com/openvswitch/ovs/blob/365c2e504b41d23904611146ff26cfc50ce77aa5/ofproto/ofproto-dpif.c#L5303 This results in all traffic being disproportionately routed to the most recently inserted bucket, leading to the observed imbalance.
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