Re: [EXTERNAL] [PATCH dpdk] graph: replace circular buffer with priority-based bitmap scheduling

["Robin Jarry" <rjarry@redhat.com>]

Hi Jerin,

Jerin Jacob, May 19, 2026 at 12:25:
> Some high-level comments:
>
> 1)What will be the performance overhead for graph walk.  Try
>   app/test/test_graph_perf.c and l3fwd_graph

I have made some measurements. There was a regression caused by
systematic access to a cache line. This will be fixed in v2.

> 2)If priorities are same, Does it have similar performance with
>   existing code?

Yes with v2. I have added a topo order as a fallback when all priorities
are equal so that it behaves as previously.

> 3)Does it have any effect on packet ordering on egress.i.e for a given
>   flow ingress order != egress order

Some independent flows "may" be reordered but that depends on the
topology, not the graph walk algorithm.

For example, the following graph:

        +--------+
        | eth_in |
        +--------+
        /        \
       v          v
   +-------+     +---------+
   | ip_in | <-- | mpls_in |
   +-------+     +---------+
       |
       |  ,------------.
       v  v            |
   +--------+     +----------+
   | ip_out | --> | ipip_out |
   +--------+     +----------+
       |
       v
   +---------+
   | eth_out |
   +---------+

If you receive the following stream of packets in "eth_in":

   1. Eth/IP
   2. Eth/IP
   3. Eth/MPLS/IP
   4. Eth/MPLS/IP
   5. Eth/MPLS/IP
   6. Eth/IP (needs IPIP encap)
   7. Eth/IP (needs IPIP encap)
   8. Eth/IP

The "eth_out" node will see:

   1. Eth/IP
   2. Eth/IP
   3. Eth/IP
   4. Eth/IP
   5. Eth/IP
 * 8. Eth/IP <----------- reordered before 6 and 7
   6. Eth/IP/IP
   7. Eth/IP/IP

With the FIFO circular buffer and with the new bitmap priority scheduler.

The only difference with the bitmap system is that If you set "mpls_in"
and "ip_out" higher priorities, the "ip_in" and "eth_out" nodes will be
visited only once.

> If there is performance regression we may need to consider new
> RTE_GRAPH_MODEL_XXXXX to enable this feature


-- 
Robin

> Times approximate.