Re: [dpdk-dev] [PATCH 1/2] bitmap: add create bitmap with all bits set

[Suanming Mou <suanmingm@mellanox.com>]

On 4/3/2020 10:49 PM, Andrzej Ostruszka wrote:
> Hello Suanming
>
> Please find my comments below.  However please note that so far I have
> never used DPDK bitmaps so I might not be the best person to comment -
> this patch needs some attention so I spent some time on it.
>
> Overall I'm fine with the changes however since this is a performance
> enhancement I've added some remarks/comments.
Hi Andrzej , thanks for your suggestions.
>
> On 3/10/20 9:21 AM, Suanming Mou wrote:
>> Currently, in the case to use bitmap as resource allocator, after
>> bitmap creation, all the bitmap bits should be set to indicate the
>> bit available. Every time when allocate one bit, search for the set
>> bits and clear it to make it in use.
>>
>> Add a new rte_bitmap_init_with_all_set() function to have a quick
>> fill up the bitmap bits.
>>
>> Comparing with the case create the bitmap as empty and set the bitmap
>> one by one, the new function costs less cycles.
>>
>> Signed-off-by: Suanming Mou <suanmingm@mellanox.com>
>> ---
>>   lib/librte_eal/common/include/rte_bitmap.h | 32 ++++++++++++++++++++++++++++++
>>   1 file changed, 32 insertions(+)
>>
>> diff --git a/lib/librte_eal/common/include/rte_bitmap.h b/lib/librte_eal/common/include/rte_bitmap.h
>> index 6b846f2..36b32e4 100644
>> --- a/lib/librte_eal/common/include/rte_bitmap.h
>> +++ b/lib/librte_eal/common/include/rte_bitmap.h
>> @@ -483,6 +483,38 @@ struct rte_bitmap {
>>   	return 0;
>>   }
>>   
>> +/**
>> + * Bitmap initialization with all bits set
>> + *
>> + * @param n_bits
>> + *   Number of pre-allocated bits in array2.
>> + * @param mem
>> + *   Base address of array1 and array2.
>> + * @param mem_size
>> + *   Minimum expected size of bitmap.
>> + * @return
>> + *   Handle to bitmap instance.
>> + */
>> +static inline struct rte_bitmap *
>> +rte_bitmap_init_with_all_set(uint32_t n_bits, uint8_t *mem, uint32_t mem_size)
>> +{
>> +	uint32_t i;
>> +	uint32_t slabs = n_bits / RTE_BITMAP_SLAB_BIT_SIZE;
>> +	struct rte_bitmap *bmp = rte_bitmap_init(n_bits, mem, mem_size);
>> +
>> +	if (!bmp)
>> +		return NULL;
>> +	/* Fill the arry2 byte aligned bits. */
>> +	memset(bmp->array2, 0xff, slabs * sizeof(bmp->array2[0]));
> In rte_bitmap_init() we clear memory with 0 and now we set it with 1s.
> Maybe separating the configuration from the actual initialization would
> be better?  So that you call __rte_bitmap_init() and later zero in
> rte_bitmap_init() and set to 1s here.

Good idea. In fact, the first proposal was to add a new function which 
can set all the n_bits.

Since currently, the bitmap struct does not contain n_bits, the 
rte_bitmap_init_with_all_set() was introduced.

>
>> +	/* Fill the arry1 bits. */
>> +	for (i = 0; i < n_bits; i += RTE_BITMAP_CL_BIT_SIZE)
>> +		rte_bitmap_set(bmp, i);
> Maybe you could here also compute the number of array1 bytes that can be
> set to FF and use memset() and for the remaining user rte_bitmap_set()?
> Right now you are also touching array2 memory which was already set above.

The RTE_BITMAP_CL_BIT_SIZE is 512 with cache_line size 64. Maybe for 
most of the cases which creates the bitmap less than 4K bits will not 
have chance with the memset.

Anyway, will add it.

>
>> +	/* Fill the arry2 left not byte aligned bits. */
>> +	for (i = slabs * RTE_BITMAP_SLAB_BIT_SIZE; i < n_bits; i++)
>> +		rte_bitmap_set(bmp, i);
>> +	return bmp;
>> +}
>> +
> With regards
> Andrzej Ostruszka
>