My alternative approach that should achieve that same goals as Petri
but give more freedom to implementations. You don't have to approve of
it, I just want to show that given a defined and understood problem,
many potential solutions exist and the first alternative might not be
the best. Let's work together.
* init_seg_len:
On input: user's required (desired) minimal length of initial
segment (including headroom)
On output: implementation's best effort to match user's request
Purpose: ensure that those packet headers the application
normally will process are stored in a consecutive memory area.
Applications do not have to check any configuration in order
to initialize a configuration which the implementation anyway has to
check if it can support.
Applications should check output values to see if its desired
values were matched. The application decides whether a failure to
match is a fatal error or the application can handle the situation
anyway (e.g. with degraded performance because it has to do some
workarounds in SW).
* seg_len:
On input: user's desired length of other segments
On output: implementation's best effort to match user's request
Purpose: a hint from the user how to partition to pool into
segments for best trade-off between memory utilization and SW
processing performance.
Note: I know some HW can support multiple segment sizes so I
think it is useful for the API to allow for this. Targets which only
support one segment size (per packet pool) could use e.g.
max(int_seg_len, seg_len). Some targets may not allow user-defined
segment sizes at all, the ODP implementation will just return the
actual values and the application can check whether those are
acceptable.
* init_seg_num:
On input: Number of initial segments.
On output: Updated with actual number of segments if a shared
memory region was specified?
* seg_num:
On input: Number of other segments.
On output: Updated with actual number of segments if a shared
memory region was specified?
I dislike the defines because they will make a future portable ABI
(binary interface) definition impossible. We will need a portable ABI
to support e.g. shared library implementations. So all ODP_CONFIG's
should only be used internally (by the implementation in question) and
not be accessible as compile time constants to applications, i.e. they
should not be part of the ODP API. Are there any other instances where
the application is supposed to use these constants?
-- Ola
On 3 February 2015 at 14:31, Ola Liljedahl <[email protected]> wrote:
> On 3 February 2015 at 13:59, Petri Savolainen
> <[email protected]> wrote:
>> Completed odp_pool_param_t definition with packet pool parameters.
>> Parameter definition is close to what we are using already.
>>
>> * seg_len: Defines minimum segment buffer length.
>> With this parameter user can:
>> * trade-off between pool memory usage and SW performance (linear
>> memory access)
>> * avoid segmentation in packet head (e.g. if legacy app cannot
>> handle
>> segmentation in the middle of the packet headers)
> We already had defined a minimum segment size for conforming ODP
> implementations. Isn't that enough?
>
> I can see value in specifying the minimum size of the first segment of
> a packet (which would contain all headers the application is going to
> process). But this proposal goes much further than that.
>
>
>> * seg_len < ODP_CONFIG_PACKET_SEG_LEN_MIN is rounded up to
>> ODP_CONFIG_PACKET_SEG_LEN_MIN
>> * seg_len > ODP_CONFIG_PACKET_SEG_LEN_MAX is not valid
>>
>> * seg_align: Defines minimum segment buffer alignment. With this parameter,
>> user can force buffer alignment to match e.g. aligment
>> requirements
>> of data structures stored in or algorithms accessing the packet
> Can you give a practical example of when this configuration is useful?
> To my knowledge, most data structures have quite small alignment
> requirements, e.g. based on alignment requirements of individual
> fields. But here I assume that we would specify alignment in multiples
> of cache lines here (because the minimum segment alignment would be
> the cache line size).
>
>> headroom. When user don't have specific alignment requirement 0
>> should be used for default.
>>
>> * seg_num: Number of segments. This is also the maximum number of packets.
> I think these configurations could be hints but not strict
> requirements. They do not change the *functionality* so an application
> should not fail if these configurations can not be obeyed (except for
> that legacy situation you describe above). The hints enable more
> optimal utilization of e.g. packet memory and may decrease SW overhead
> during packet processing but do not change the functionality.
>
> To enable different hardware implementations, ODP apps should not
> enforce unnecessary (non-functional) requirements on the ODP
> implementations and limit the number of targets ODP can be implemented
> on. ODP is not DPDK.
>
> Applications should also not have to first check the limits of the
> specific ODP implementation (as you suggested yesterday), adapts its
> configuration to that and then send back those requirements to the ODP
> implementation (which still has to check the parameters to verify that
> they are valid). This is too complicated and will likely lead to code
> that cheats and thus is not portable. Better for applications just to
> specify its requested configuration to ODP and then get back the
> results (i.e. actual values that will be used). The application can
> then if necessary check that the configuration was honored. This
> follows the normal programming flow.
>
>>
>> Signed-off-by: Petri Savolainen <[email protected]>
>> ---
>> include/odp/api/pool.h | 26 +++++++++++++++++++++-----
>> 1 file changed, 21 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/odp/api/pool.h b/include/odp/api/pool.h
>> index d09d92e..a1d7494 100644
>> --- a/include/odp/api/pool.h
>> +++ b/include/odp/api/pool.h
>> @@ -61,13 +61,29 @@ typedef struct odp_pool_param_t {
>> of 8. */
>> uint32_t num; /**< Number of buffers in the pool */
>> } buf;
>> -/* Reserved for packet and timeout specific params
>> struct {
>> - uint32_t seg_size;
>> - uint32_t seg_align;
>> - uint32_t num;
>> + uint32_t seg_len; /**< Minimum packet segment
>> buffer
>> + length in bytes. It includes
>> + possible head-/tailroom
>> bytes.
>> + The maximum value is
>> defined by
>> +
>> ODP_CONFIG_PACKET_SEG_LEN_MAX.
>> + Use 0 for default length. */
>> + uint32_t seg_align; /**< Minimum packet segment
>> buffer
>> + alignment in bytes. Valid
>> + values are powers of two.
>> The
>> + maximum value is defined by
>> +
>> ODP_CONFIG_PACKET_SEG_ALIGN_MAX
>> + . Use 0 for default
>> alignment.
>> + Default will always be a
>> + multiple of 8.
>> + */
>> + uint32_t seg_num; /**< Number of packet segments in
>> + the pool. This is also the
>> + maximum number of packets,
>> + since each packet consist of
>> + at least one segment.
> What if both seg_num and a shared memory region is specified in the
> odp_pool_create call? Which takes precedence?
>
>> + */
>> } pkt;
>> -*/
>> struct {
>> uint32_t __res1; /* Keep struct identical to buf, */
>> uint32_t __res2; /* until pool implementation is
>> fixed*/
>> --
>> 2.2.2
>>
>>
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