Petri Savolainen(psavol) replied on github web page:
platform/linux-generic/include/odp_ring_st_internal.h
line 32
@@ -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;
Comment:
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_r169880519
updated_at 2018-02-22 09:32:40
