Petri Savolainen(psavol) replied on github web page:
platform/linux-generic/include/odp_ring_st_internal.h
line 78
@@ -0,0 +1,118 @@
+/* Copyright (c) 2018, Linaro Limited
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef ODP_RING_ST_INTERNAL_H_
+#define ODP_RING_ST_INTERNAL_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <odp/api/hints.h>
+#include <odp_align_internal.h>
+
+/* Basic ring for single thread usage. Operations must be synchronized by using
+ * locks (or other means), when multiple threads use the same ring. */
+typedef struct {
+ uint32_t head;
+ uint32_t tail;
+ uint32_t mask;
+ uint32_t *data;
+
+} ring_st_t;
+
+/* 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)
+{
+ ring->head = 0;
+ ring->tail = 0;
+ ring->mask = size - 1;
+ ring->data = data;
+}
+
+/* Dequeue data from the ring head. Max_num is smaller than ring size.*/
+static inline uint32_t ring_st_deq_multi(ring_st_t *ring, uint32_t data[],
+ uint32_t max_num)
+{
+ uint32_t head, tail, mask, idx;
+ uint32_t num, i;
+
+ head = ring->head;
+ tail = ring->tail;
+ mask = ring->mask;
+ num = tail - head;
+
+ /* Empty */
+ if (num == 0)
+ return 0;
+
+ if (num > max_num)
+ num = max_num;
+
+ idx = head & mask;
+
+ for (i = 0; i < num; i++) {
+ data[i] = ring->data[idx];
+ idx = (idx + 1) & mask;
+ }
+
+ ring->head = head + num;
+
+ return num;
+}
+
+/* Enqueue data into the ring tail. Num_data is smaller than ring size. */
+static inline uint32_t ring_st_enq_multi(ring_st_t *ring, const uint32_t
data[],
+ uint32_t num_data)
+{
+ uint32_t head, tail, mask, size, idx;
+ uint32_t num, i;
+
+ head = ring->head;
+ tail = ring->tail;
+ mask = ring->mask;
+ size = mask + 1;
+ num = size - (tail - head);
Comment:
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_r169884461
updated_at 2018-02-22 09:32:40
