On Wed, 2 Oct 2019 17:50:41 +0000 Shahaf Shuler <shah...@mellanox.com> wrote:
> Wednesday, October 2, 2019 3:59 PM, Flavio Leitner: > > Obrembski MichalX <michalx.obremb...@intel.com>; Stokes Ian > > <ian.sto...@intel.com> > > Subject: Re: [dpdk-dev] [PATCH] vhost: add support to large linear > > mbufs > > > > > > Hi Shahaf, > > > > Thanks for looking into this, see my inline comments. > > > > On Wed, 2 Oct 2019 09:00:11 +0000 > > Shahaf Shuler <shah...@mellanox.com> wrote: > > > > > Wednesday, October 2, 2019 11:05 AM, David Marchand: > > > > Subject: Re: [dpdk-dev] [PATCH] vhost: add support to large > > > > linear mbufs > > > > > > > > Hello Shahaf, > > > > > > > > On Wed, Oct 2, 2019 at 6:46 AM Shahaf Shuler > > > > <shah...@mellanox.com> wrote: > > > > > > > [...] > > > > > > > > > I am missing some piece here. > > > > Which pool would the PMD take those external buffers from? > > > > > > The mbuf is always taken from the single mempool associated w/ the > > > rxq. The buffer for the mbuf may be allocated (in case virtio > > > payload is bigger than current mbuf size) from DPDK hugepages or > > > any other system memory and be attached to the mbuf. > > > > > > You can see example implementation of it in mlx5 PMD (checkout > > > rte_pktmbuf_attach_extbuf call) > > > > Thanks, I wasn't aware of external buffers. > > > > I see that attaching external buffers of the correct size would be > > more efficient in terms of saving memory/avoiding sparsing. > > > > However, we still need to be prepared to the worse case scenario > > (all packets 64K), so that doesn't help with the total memory > > required. > > Am not sure why. > The allocation can be per demand. That is - only when you encounter a > large buffer. > > Having buffer allocated in advance will benefit only from removing > the cost of the rte_*malloc. However on such big buffers, and further > more w/ device offloads like TSO, am not sure that is an issue. Now I see what you're saying. I was thinking we had to reserve the memory before, like mempool does, then get the buffers as needed. OK, I can give a try with rte_*malloc and see how it goes. > > The current patch pushes the decision to the application which > > knows better the workload. If more memory is available, it can > > optionally use large buffers, otherwise just don't pass that. Or > > even decide whether to share the same 64K mempool between multiple > > vhost ports or use one mempool per port. > > > > Perhaps I missed something, but managing memory with mempool still > > require us to have buffers of 64K regardless if the data consumes > > less space. Otherwise the application or the PMD will have to > > manage memory itself. > > > > If we let the PMD manages the memory, what happens if a port/queue > > is closed and one or more buffers are still in use (switching)? I > > don't see how to solve this cleanly. > > Closing of the dev should return EBUSY till all buffers are free. > What is the use case of closing a port while still having packet > pending on other port of the switch? And why we cannot wait for them > to complete transmission? The vswitch gets the request from outside and the assumption is that the command will succeed. AFAIK, there is no retry mechanism. Thanks Shahaf! fbl
