Re: [dpdk-dev] [PATCH] vhost: add support to large linear mbufs

[Flavio Leitner <fbl@sysclose.org>]

On Wed, 2 Oct 2019 17:50:41 +0000
Shahaf Shuler <shahafs@mellanox.com> wrote:

> Wednesday, October 2, 2019 3:59 PM, Flavio Leitner:
> > Obrembski MichalX <michalx.obrembski@intel.com>; Stokes Ian
> > <ian.stokes@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 <shahafs@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
> > > > <shahafs@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