Petri Savolainen(psavol) replied on github web page:
platform/linux-generic/odp_queue.c
line 103
@@ -192,6 +201,9 @@ static odp_queue_t queue_create(const char *name,
param = &default_param;
}
+ if (param->size > CONFIG_QUEUE_SIZE)
Comment:
One entry is not lost. User provided size if not (currently) used. Queue size
is always 4k.
> Petri Savolainen(psavol) wrote:
> One entry is not lost.
>> Petri Savolainen(psavol) wrote:
>> One entry is not lost.
>>> Petri Savolainen(psavol) wrote:
>>> OK, added checks in v2.
>>>> Petri Savolainen(psavol) wrote:
>>>> OK. Compiler probably did that already, but changed in v2.
>>>>> Petri Savolainen(psavol) wrote:
>>>>> Tail and head indexes are (masked from) uint32_t and do not wrap around
>>>>> when the ring is full. I think you assume that the store index is
>>>>> 0...size-1, while it's full uint32_t which is then masked to get the
>>>>> actual index.
>>>>>
>>>>> For example:
>>>>> size = 100;
>>>>>
>>>>> Empty:
>>>>> head = 100
>>>>> tail = 100
>>>>> num = 100 - 100 = 0
>>>>>
>>>>> Full:
>>>>> head = 100
>>>>> tail = 200
>>>>> num = 200 - 100 = 100
>>>>>
>>>>> Wrap uint32_t + full:
>>>>> head = 0xFFFFFF9C
>>>>> tail = 0
>>>>> num = 0 - 0xFFFFFF9C = 0x64 = 100
>>>>>
>>>>> So, no abs() needed. Ring size can be 4096, instead of 4095.
>>>>>> Petri Savolainen(psavol) wrote:
>>>>>> It's already documented 5 lines above:
>>>>>>
>>>>>> /* Initialize ring. Ring size must be a power of two. */
>>>>>> static inline void ring_st_init(ring_st_t *ring, uint32_t *data,
>>>>>> uint32_t size)
>>>>>> {
>>>>>>> Petri Savolainen(psavol) wrote:
>>>>>>> This function converts 32 bit buffer indexes to buffer header pointers.
>>>>>>> The counter operation is buffer_index_from_buf(). The prefetch is a
>>>>>>> side effect of the function, which may be changed/moved any time if
>>>>>>> it's found out that there's a place for prefetching. I actually plan to
>>>>>>> test if number of prefetches should be limited as e.g. 32 consecutive
>>>>>>> prefetches may be too much for some CPU architectures.
>>>>>>>> Petri Savolainen(psavol) wrote:
>>>>>>>> I prefer style where '== 0' is used instead of '!'. Especially, when
>>>>>>>> the if clause is as complex as this and there's danger for reader to
>>>>>>>> miss the '!' sign.
>>>>>>>>> Petri Savolainen(psavol) wrote:
>>>>>>>>> It's there to ensure that all bits are zero also when someone would
>>>>>>>>> modify the bitfield from two to three fields later on. Similarly to
>>>>>>>>> memset() zero is used for struct inits.
>>>>>>>>>> Petri Savolainen(psavol) wrote:
>>>>>>>>>> There's no need for abs(). Since it's all uint32_t variables, wrap a
>>>>>>>>>> round is handled already.
>>>>>>>>>> An example in 8bits:
>>>>>>>>>> 0xff - 0xfd = 0x02
>>>>>>>>>> 0x00 - 0xfe = 0x02
>>>>>>>>>> 0x01 - 0xff = 0x02
>>>>>>>>>> 0x02 - 0x00 = 0x02
>>>>>>>>>>
>>>>>>>>>> This passes both gcc and clang, and is used already in the other
>>>>>>>>>> ring implementation see ring_deq_multi().
>>>>>>>>>>> Petri Savolainen(psavol) wrote:
>>>>>>>>>>> I prefer style with blank line in the end of a typedef, since it's
>>>>>>>>>>> easier to spot the type name (as it's not mixed into struct field
>>>>>>>>>>> names). Checkpatch passes so this style should be OK.
>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>> Does this mean that sizes larger than 32 have no added performance
>>>>>>>>>>>> benefit?
>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>> Must use `CONFIG_QUEUE_SIZE - 1` here, as noted earlier, if we're
>>>>>>>>>>>>> not going to use the user-supplied queue size.
>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>> Given its name, this looks like an extraneous statement that
>>>>>>>>>>>>>> should be deleted. Renaming this to something like
>>>>>>>>>>>>>> `prefetch_dequeued_bufs()` would make the intent clearer here.
>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>> `if (!ring_st_is_empty(&queue->s.ring_st))` seems more natural
>>>>>>>>>>>>>>> here.
>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>> Change to `if (param->size >= CONFIG_QUEUE_SIZE)` to handle
>>>>>>>>>>>>>>>> the effective queue capacity. The user-supplied `size` should
>>>>>>>>>>>>>>>> then be set to `ROUNDUP_POWER2_U32(size) - 1` for the masking
>>>>>>>>>>>>>>>> to work properly.
>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>> Same comment here as for plain queues.
>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>> As noted earlier, due to "losing" one entry to distinguish
>>>>>>>>>>>>>>>>>> queue empty/full, this should be returned as
>>>>>>>>>>>>>>>>>> `CONFIG_QUEUE_SIZE - 1`, and we also need to ensure that
>>>>>>>>>>>>>>>>>> `CONFIG_QUEUE_SIZE` is itself a power of 2.
>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>> Since you're initializing `index.pool` and `index.buffer`
>>>>>>>>>>>>>>>>>>> there's no need to set `index.u32` here.
>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>> We originally had this index partitioning based on
>>>>>>>>>>>>>>>>>>>> `ODP_CONFIG_POOLS`. Do we want to return to that here?
>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>> If not, we at least need an `ODP_STATIC_ASSERT()` to
>>>>>>>>>>>>>>>>>>>> ensure that `ODP_CONFIG_POOLS < 256` or else bad things
>>>>>>>>>>>>>>>>>>>> will happen here.
>>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>>> This routine can be optimized to:
>>>>>>>>>>>>>>>>>>>>> ```
>>>>>>>>>>>>>>>>>>>>> return ring->head == ring->tail;
>>>>>>>>>>>>>>>>>>>>> ```
>>>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>>>> Your invariant is the queue is empty when `head == tail`
>>>>>>>>>>>>>>>>>>>>>> therefore the queue is full when `abs(tail - head) ==
>>>>>>>>>>>>>>>>>>>>>> mask`, so the correct calculation here is:
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> `num = mask - abs(tail - head);`
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> The effect is that a queue can only hold `size - 1`
>>>>>>>>>>>>>>>>>>>>>> elements, otherwise you cannot distinguish between a
>>>>>>>>>>>>>>>>>>>>>> full and an empty queue without another bit of metadata,
>>>>>>>>>>>>>>>>>>>>>> which is a cost you're trying to avoid.
>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>> This is somewhat problematic if the caller is trying to
>>>>>>>>>>>>>>>>>>>>>> be "optimal" by specifying a power of two in the `size`
>>>>>>>>>>>>>>>>>>>>>> parameter of the `odp_queue_param_t` passed to
>>>>>>>>>>>>>>>>>>>>>> `odp_queue_create()`. For this reason we may wish to
>>>>>>>>>>>>>>>>>>>>>> return a `max_size` of a power of 2 - 1 in
>>>>>>>>>>>>>>>>>>>>>> `odp_queue_capability()` as part of this patch series.
>>>>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>>>>> This only works if `size` is a power of 2. Should be
>>>>>>>>>>>>>>>>>>>>>>> documented as such, since this is an internal routine.
>>>>>>>>>>>>>>>>>>>>>>> In this case an `ODP_ASSERT(size ==
>>>>>>>>>>>>>>>>>>>>>>> ROUNDUP_POWER2_U32(size))` for this requirement would
>>>>>>>>>>>>>>>>>>>>>>> be a useful debugging aid.
>>>>>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>>>>>> should be `num = abs(tail - head);` to deal with wrap
>>>>>>>>>>>>>>>>>>>>>>>> arounds, otherwise may be misinterpreted as overly
>>>>>>>>>>>>>>>>>>>>>>>> large since it's `uint32_t`. Note that GCC and clang
>>>>>>>>>>>>>>>>>>>>>>>> recognize `abs()` and treat it as a builtin, so
>>>>>>>>>>>>>>>>>>>>>>>> there's no actual `stdlib.h` call here.
>>>>>>>>>>>>>>>>>>>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>>>>>>>>>>>>>>>>>>>> Extra blank line should be removed (nit).
https://github.com/Linaro/odp/pull/492#discussion_r169891968
updated_at 2018-02-22 09:32:40
