Another reason why the IPsec example uses atomic queues is that linux-generic and other ODP implementations thatt use SW scheduling do not support ordered queues... So atomic queues are used instead. I think the example still uses an atomic operation for the sequence number allocation which isn't strictly necessary when using atomic queues.
Ideally SW scheduling should support ordered queues as well (even if they degenerate into atomic scheduling) so that examples could use the best design (in this case probably ordered queues with an atomic update for allocating the seqno). ODP implementation limitations should not shine through the API and affect examples and other user code. -- Ola On 7 May 2015 at 17:36, Ola Liljedahl <[email protected]> wrote: > Using atomic queues will preserve the ingress order when allocating and > assigning the sequence number. Also you don't need to use an expensive > atomic operation for updating the sequence number as the atomic queue and > scheduling will provide mutual exclusion. > > If the packets that require a sequence number came from parallel or > ordered queues, there would be no guarantee that the sequence numbers would > be allocated in packet (ingress) order. Just using an atomic operation > (e.g. fetch_and_add or similar) only guarantees proper update of the > sequence number variable, not any specific ordering. > > If you are ready to trade absolute "correctness" for performance, you > could use ordered or may even parallel (questionable for other reasons) > queues and then allocate the sequence number using an atomic fetch_and_add. > Sometimes packets egress order will then not match the sequence number > order (for a flow/SA). For IPSec, this might affect the replay window check > & update at the receiving end but as the replay protection uses a sliding > window of sequence numbers (to handle misordered packets), there might not > be any adverse effects in practice. The most important aspect is probably > to preserve original packet order. > > -- Ola > > On 6 May 2015 at 11:29, [email protected] < > [email protected]> wrote: > >> Hi, >> >> >> >> In IPSEC example application, queues are used to update the sequence >> number. I was wondering why we have used queues to update sequence number >> which will add to scheduling delays and adversely hit the performance >> throughput. Is there any specific advantage of using queues over atomic >> variables. >> >> >> >> Thanks in advance >> >> Nikhil >> >> >> >> _______________________________________________ >> lng-odp mailing list >> [email protected] >> https://lists.linaro.org/mailman/listinfo/lng-odp >> >> >
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