> On Apr 19, 2018, at 9:30 AM, Shailja Pandey <csz168...@iitd.ac.in> wrote: > >> 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.
In this case you can not do a prefetch on other cache lines far enough in advance to not get a CPU stall for a cacheline. >> >> Why did you use memcpy and not rte_memcpy here as rte_memcpy should be >> faster? Still did not answer this question. >> >> 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(). Then please show the real code fragment as your example was confusing. > > # 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 | > Refcnt also needs to adjust the value using a atomic update and you still have not told me the type of system you are on x86 or ??? Please describe your total system Host OS, DPDK version, NICs used, … a number of people have performance similar test and do not see the problem you are suggesting. Maybe modify say L3fwd (which does some thing similar to your example code) and see if you still see the difference. They you can post the patch to that example app and we can try to figure it out. > Throughput is in MPPS. > > -- > > Thanks, > Shailja > Regards, Keith
