[dpdk-dev] Why packet replication is more efficient when done using memcpy( ) as compared to rte_mbuf_refcnt_update() function?

[Shailja Pandey]

> The two code fragments are doing two different ways the first is using a loop to create possible more then one replication and the second one is not, correct? The loop can cause performance hits, but should be small.
Sorry for the confusion, for memcpy version also we are using a loop 
outside of this function. Essentially, we are making same number of 
copies in both the cases.
> The first one is using the hdr->next pointer which is in the second cacheline of the mbuf header, this can and will cause a cacheline miss and degrade your performance. The second code does not touch hdr->next and will not cause a cacheline miss. When the packet goes beyond 64bytes then you hit the second cacheline, are you starting to see the problem here.
We also performed same experiment for different packet sizes(64B, 128B, 
256B, 512B, 1024B, 1518B), the sharp drop in throughput is observed only 
when the packet size increases from 64B to 128B and not after that. So, 
cacheline miss should happen for other packet sizes also. I am not sure 
why this is the case. Why the drop is not sharp after 128 B packets when 
replicated using rte_pktmbuf_refcnt_update().

>   Every time you touch a new cache line performance will drop unless the cacheline is prefetched into memory first, but in this case it really can not be done easily. Count the cachelines you are touching and make sure they are the same number in each case.
I don't understand the complexity here, could you please explain it in 
detail.
>
> Why did you use memcpy and not rte_memcpy here as rte_memcpy should be faster?
>
> I believe now DPDK has a rte_pktmbuf_alloc_bulk() function to reduce the number of rte_pktmbuf_alloc() calls, which should help if you know the number of packets you need to replicate up front.
We are already using both of these functions, just to simplify the 
pseudo-code I used memcpy and rte_pktmbuf_alloc().

# pktsz 1(64 bytes)    |   pktsz 2(128 bytes)     |  pktsz 3(256 
bytes)    |  pktsz 4(512 bytes)   | pktsz 4(1024 bytes)    |
# memcpy    refcnt    |   memcpy    refcnt      | memcpy refcnt       |  
memcpy  refcnt       | memcpy   refcnt         |
    5949888    5806720|   5831360    2890816  |  5640379    2886016 |  
5107840   2863264  | 4510121   2692876    |

Throughput is in MPPS.

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Thanks,
Shailja