Re: [dpdk-dev] [PATCH v2] mempool: replace c memcpy code semantics with optimized rte_memcpy

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

On Tue, May 31, 2016 at 11:05:30PM +0200, Olivier MATZ wrote:
> Hi Jerin,

Hi Olivier,

> 
> >>>  	/* Add elements back into the cache */
> >>> -	for (index = 0; index < n; ++index, obj_table++)
> >>> -		cache_objs[index] = *obj_table;
> >>> +	rte_memcpy(&cache_objs[0], obj_table, sizeof(void *) * n);
> >>>  
> >>>  	cache->len += n;
> >>>  
> >>>
> >>
> >> I also checked in the get_bulk() function, which looks like that:
> >>
> >> 	/* Now fill in the response ... */
> >> 	for (index = 0, len = cache->len - 1;
> >> 			index < n;
> >> 			++index, len--, obj_table++)
> >> 		*obj_table = cache_objs[len];
> >>
> >> I think we could replace it by something like:
> >>
> >> 	rte_memcpy(obj_table, &cache_objs[len - n], sizeof(void *) * n);
> >>
> >> The only difference is that it won't reverse the pointers in the
> >> table, but I don't see any problem with that.
> >>
> >> What do you think?
> > 
> > In true sense, it will _not_ be LIFO. Not sure about cache usage implications
> > on the specific use cases.
> 
> Today, the objects pointers are reversed only in the get(). It means
> that this code:
> 
> 	rte_mempool_get_bulk(mp, table, 4);
> 	for (i = 0; i < 4; i++)
> 		printf("obj = %p\n", t[i]);
> 	rte_mempool_put_bulk(mp, table, 4);
> 
> 
> 	printf("-----\n");
> 	rte_mempool_get_bulk(mp, table, 4);
> 	for (i = 0; i < 4; i++)
> 		printf("obj = %p\n", t[i]);
> 	rte_mempool_put_bulk(mp, table, 4);
> 
> prints:
> 
> 	addr1
> 	addr2
> 	addr3
> 	addr4
> 	-----
> 	addr4
> 	addr3
> 	addr2
> 	addr1
> 
> Which is quite strange.

IMO, It is the expected LIFO behavior. Right ?

What is not expected is the following, which is the case after change. Or Am I
missing something here?

addr1
addr2
addr3
addr4
-----
addr1
addr2
addr3
addr4

> 
> I don't think it would be an issue to replace the loop by a
> rte_memcpy(), it may increase the copy speed and it will be
> more coherent with the put().
> 
> 
> Olivier