> -----Original Message----- > From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ouyang, Changchun > Sent: Monday, October 27, 2014 6:56 AM > To: Thomas Monjalon > Cc: dev at dpdk.org > Subject: Re: [dpdk-dev] [PATCH] vhost: Check descriptor number for vector Rx > > Hi Thomas, > > > -----Original Message----- > > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > Sent: Monday, October 27, 2014 4:46 PM > > To: Ouyang, Changchun > > Cc: dev at dpdk.org > > Subject: Re: [dpdk-dev] [PATCH] vhost: Check descriptor number for vector > > Rx > > > > 2014-10-25 00:48, Ouyang, Changchun: > > > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > > > 2014-10-24 16:38, Ouyang Changchun: > > > > > For zero copy, it need check whether RX descriptor num meets the > > > > > least requirement when using vector PMD Rx function, and give user > > > > > more hints if it fails to meet the least requirement. > > > > [...] > > > > > --- a/examples/vhost/main.c > > > > > +++ b/examples/vhost/main.c > > > > > @@ -131,6 +131,10 @@ > > > > > #define RTE_TEST_RX_DESC_DEFAULT_ZCP 32 /* legacy: 32, DPDK virt > > FE: 128. */ > > > > > #define RTE_TEST_TX_DESC_DEFAULT_ZCP 64 /* legacy: 64, DPDK virt > > FE: 64. */ > > > > > > > > > > +#ifdef RTE_IXGBE_INC_VECTOR > > > > > +#define VPMD_RX_BURST 32 > > > > > +#endif > > > > > + > > > > > /* Get first 4 bytes in mbuf headroom. */ #define > > > > > MBUF_HEADROOM_UINT32(mbuf) (*(uint32_t *)((uint8_t *)(mbuf) \ > > > > > + sizeof(struct rte_mbuf))) > > > > > @@ -792,6 +796,19 @@ us_vhost_parse_args(int argc, char **argv) > > > > > return -1; > > > > > } > > > > > > > > > > +#ifdef RTE_IXGBE_INC_VECTOR > > > > > + if ((zero_copy == 1) && (num_rx_descriptor <= > VPMD_RX_BURST)) { > > > > > + RTE_LOG(INFO, VHOST_PORT, > > > > > + "The RX desc num: %d is too small for PMD to > > work\n" > > > > > + "properly, please enlarge it to bigger than %d > if\n" > > > > > + "possible by the option: '--rx-desc-num > > <number>'\n" > > > > > + "One alternative is disabling > > RTE_IXGBE_INC_VECTOR\n" > > > > > + "in config file and rebuild the libraries.\n", > > > > > + num_rx_descriptor, VPMD_RX_BURST); > > > > > + return -1; > > > > > + } > > > > > +#endif > > > > > + > > > > > return 0; > > > > > } > > > > > > > > I feel there is a design problem here. > > > > An application shouldn't have to care about the underlying driver. > > > > > > For most of other applications, as their descriptor numbers are set as > > > big enough(1024 or so) , So there is no need to check the descriptor > > number at the early stage of running. > > > > > > But for vhost zero copy(note vhost one copy also has 1024 descriptor > > > number) has the default descriptor number of 32. > > > Why use 32? > > > because vhost zero copy implementation (working as backend) need > > > support dpdk based app which use pmd virtio-net driver, And also need > > support linux legacy virtio-net based application. > > > When it is the linux legacy virtio-net case, on one side the qemu has > > > hard code to confine the total virtio descriptor size to 256, On other > > > side, > > legacy virtio use half of them as virtio header, and then only another half > > i.e. > > 128 descriptors are available to use as real buffer. > > > > > > In PMD mode, all HW descriptors need to be filled DMA address in the rx > > initial stage, otherwise there is probably exceptional in rx process. > > > Based on that, we need use really limited virtio buffer to fully fill > > > all hw descriptor DMA address, Or in other word, the available virtio > > > descriptor size will determine the total mbuf size and hw descriptor > > > size in the case of zero copy, > > > > > > Tune and find that 32 is the suitable value for vhost zero copy to work > > properly when it legacy linux virtio case. > > > Another factor to reduce the value to 32, is that mempool use ring to > > > accommodate the mbuf, it cost one to flag the ring head/tail, And there > > > are > > some other overheads like temporary mbufs(size as RX_BURST) when rx. > > > Note that number descriptor should need power 2. > > > > > > Why the change occur at this moment? > > > Recently the default rx function is modified into vector RX function, > > > while it use non-vector mode (scalar mode) Rx previously, Vector RX > > function need more than 32 descriptor to work properly, but scalar mode RX > > hasn't this limitation. > > > > > > As the RX function is changeable(you can use vector mode or non-vector), > > and descriptor number can also be changed. > > > So here in the vhost app, check if they match to make sure all things > > > could > > work normally, and give some hints if they don't match. > > > > > > Hope the above could make it a bit clearer. :-) > > > > Thank you for your explanation. > > Your fix shows that driver and application are tightly linked. > > It's a design issue. As I said: > > "An application shouldn't have to care about the underlying driver." > > I didn't dig enough in vhost to suggest a good fix but I'm sure someone > > could > > have an idea. > > > Agree with you, there is something linked between app and driver, but that's > due > to a few things: > 1.Qume has hard code to confine the total vring size; > 2.PMD driver need fully fill the dma address of descriptor at setup stage;
Can PMD driver fill its desc ring until all has been filled or there is no mbuf in pool? > 3.PMD driver use vector PMD as default path which require more than 32 in the > burst RX; > 4.Zero copy need use external buffer directly and set to HW descriptor dma > address; > > Except for item 3, everything could not be removed or ignored easily. > As for item 3, I don't know why we set vector PMD as default path, while > perusing performance, but at the cost of flexibility, > Here is an example, vhost zero copy need check it and descriptor number. I am > not sure if the default vector pmd path will affect other app or not. > > On the other hand, without this check, and hint to user, > User will suffer from this, vhost can't receive packets any more after > forwarding > 31 packets or so, > and user don't know what really happen behind, also can't easily get the > information of > their descriptor is not enough for vector pmd RX. > Then this is followed by painfully debugging and figuring out the issue. > > While with this check and hint to user, things go smoothly. > > Meanwhile we can waiting for other people's viewpoints. > > Thanks > Changchun
