There is a noticeable performance drop with more scattering of the huge pages.
I did not measure any difference accurately but I ended up rebooting my DUT between each performance test because the pages get scattered with time and re-launch of the DPDK application instead of the whole machine, because the tests showed higher memory access cost each time I re-launched the application. Tom ----- Mail original ----- De: "Andriy Berestovskyy" <aber at semihalf.com> ?: "Renata Saiakhova" <Renata.Saiakhova at oneaccess-net.com> Cc: "Sergio Gonzalez Monroy" <sergio.gonzalez.monroy at intel.com>, "users" <users at dpdk.org> Envoy?: Mardi 4 Octobre 2016 13:27:23 Objet: Re: [dpdk-users] rte_segments: hugepages are not in contiguous memory Renata, In theory 512 contiguous 2MB huge pages might get transparently promoted to a single 1GB "superpage" and single TLB entry, but I am not even sure if it is implemented in Linux... So, I do not think there will be any noticeable performance difference between contiguous and non-contiguous 2MB huge pages. But you better measure it to make sure ;) Regards, Andriy On Tue, Oct 4, 2016 at 12:48 PM, Renata Saiakhova <Renata.Saiakhova at oneaccess-net.com> wrote: > Hi Andriy, > > thanks for your reply. I guess that contiguous memory is requested because > of the performance reasons. Do you know if I can expect a noticeable > performance drop using non-contiguous memory? > > Renata > > > On 10/04/2016 12:13 PM, Andriy Berestovskyy wrote: >> >> Hi Renata, >> DPDK supports non-contiguous memory pools, but >> rte_pktmbuf_pool_create() uses rte_mempool_create_empty() with flags >> set to zero, i.e. requests contiguous memory. >> >> As a workaround, in rte_pktmbuf_pool_create() try to pass >> MEMPOOL_F_NO_PHYS_CONTIG flag as the last argument to >> rte_mempool_create_empty(). >> >> Note that KNI and some PMDs in 16.07 still require contiguous memory >> pools, so the trick might not work for your setup. For the KNI try the >> DPDK's master branch which includes the commit by Ferruh Yigit: >> >> 8451269 kni: remove continuous memory restriction >> >> Regards, >> Andriy >> >> >> On Tue, Oct 4, 2016 at 11:38 AM, Renata Saiakhova >> <Renata.Saiakhova at oneaccess-net.com> wrote: >>> >>> Hi Sergio, >>> >>> thank you for your quick answer. I also tried to allocate 1GB hugepage, >>> but >>> seems kernel fails to allocate it: previously I've seen that >>> HugePages_Total >>> in /proc/meminfo is set to 0, now - kernel hangs at boot time (don't know >>> why). >>> But anyway, if there is no way to control hugepage allocation in the >>> sense >>> they are in contiguous memory there is only way to accept it and adapt >>> the >>> code that it creates several pools which in total satisfy the requested >>> size. >>> >>> Renata >>> >>> >>> On 10/04/2016 10:27 AM, Sergio Gonzalez Monroy wrote: >>>> >>>> On 04/10/2016 09:00, Renata Saiakhova wrote: >>>>> >>>>> Hi all, >>>>> >>>>> I'm using dpdk 16.04 (I tried 16.07 with the same results) and linux >>>>> kernel 4.4.20 in a virtual machine (I'm using libvirt framework). I >>>>> pass a >>>>> parameter in kernel command line to allocate 512 hugepages of 2 MB at >>>>> boot >>>>> time. They are successfully allocated. When an application with dpdk >>>>> starts >>>>> it calls rte_pktmbuf_pool_create() which in turns requests internally >>>>> 649363712 bytes. Those bytes should be allocated from one of >>>>> rte_memseg. >>>>> rte_memsegs describes contiguous portions of memory (both physical and >>>>> virtual) built on hugepages. This allocation fails, because there are >>>>> no >>>>> rte_memsegs of this size (or bigger). Further debugging shows that >>>>> hugepages >>>>> are allocated in non-contiguous physical memory and therefore >>>>> rte_memsegs >>>>> are built respecting gaps in physical memory. >>>>> Below are the sizes of segments built on hugepages (in bytes) >>>>> 2097152 >>>>> 6291456 >>>>> 2097152 >>>>> 524288000 >>>>> 2097152 >>>>> 532676608 >>>>> 2097152 >>>>> 2097152 >>>>> So there are 5 segments which includes only one hugepage! >>>>> This behavior is completely different to what I observe with linux >>>>> kernel >>>>> 3.8 (used with the same application with dpdk) - where all hugepages >>>>> are >>>>> allocated in contiguous memory. >>>>> Does anyone experience the same issue? Could it be some kernel option >>>>> which can do the magic? If not, and kernel can allocated hugepages in >>>>> non-contiguous memory how dpdk is going to resolve it? >>>>> >>>> I don't think there is anything we can do to force the kernel to >>>> pre-allocate contig hugepages on boot. If there was, we wouldn't need to >>>> do >>>> all this mapping sorting and grouping we do on DPDK >>>> as we would rely on the kernel giving us pre-allocated contig hugepages. >>>> >>>> If you have plenty of memory one possible work around would be to >>>> increase >>>> the number of default hugepages so we are likely to find more contiguous >>>> ones. >>>> >>>> Is using 1GB hugepages a possibility in your case? >>>> >>>> Sergio >>>> >>>>> Thanks in advance, >>>>> Renata >>>>> >>>> . >>>> >> >> > -- Andriy Berestovskyy
