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From: Shahaf Shuler <shahafs@mellanox.com>
To: Flavio Leitner <fbl@sysclose.org>
Cc: David Marchand <david.marchand@redhat.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	 Maxime Coquelin <maxime.coquelin@redhat.com>,
	Tiwei Bie <tiwei.bie@intel.com>,
	Zhihong Wang <zhihong.wang@intel.com>,
	Obrembski MichalX <michalx.obrembski@intel.com>,
	Stokes Ian <ian.stokes@intel.com>
Subject: Re: [dpdk-dev] [PATCH] vhost: add support to large linear mbufs
Date: Wed, 2 Oct 2019 17:50:41 +0000	[thread overview]
Message-ID: <AM0PR0502MB379502A7DDF21B6DE6493138C39C0@AM0PR0502MB3795.eurprd05.prod.outlook.com> (raw)
In-Reply-To: <20191002095831.5927af93@p50.lan>

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. 

> 
> 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? 

> 
> fbl
> 
> >
> > >
> > > If it is from an additional mempool passed to the vhost pmd, I can't
> > > see the difference with Flavio proposal.
> > >
> > >
> > > > The pros of this approach is that you have full flexibility on the
> > > > memory
> > > allocation, and therefore a lower footprint.
> > > > The cons is the OVS will need to know how to handle mbuf w/
> > > > external
> > > buffers (not too complex IMO).
> > >
> > >
> > > --
> > > David Marchand


  reply	other threads:[~2019-10-02 17:50 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-01 22:19 Flavio Leitner
2019-10-01 23:10 ` Flavio Leitner
2019-10-02  4:45 ` Shahaf Shuler
2019-10-02  8:04   ` David Marchand
2019-10-02  9:00     ` Shahaf Shuler
2019-10-02 12:58       ` Flavio Leitner
2019-10-02 17:50         ` Shahaf Shuler [this message]
2019-10-02 18:15           ` Flavio Leitner
2019-10-03 16:57             ` Ilya Maximets
2019-10-03 21:25               ` Flavio Leitner
2019-10-02  7:51 ` Maxime Coquelin
2019-10-04 20:10 ` [dpdk-dev] [PATCH v2] vhost: add support for large buffers Flavio Leitner
2019-10-06  4:47   ` Shahaf Shuler
2019-10-10  5:12   ` Tiwei Bie
2019-10-10 12:12     ` Flavio Leitner
2019-10-11 17:09   ` [dpdk-dev] [PATCH v3] " Flavio Leitner
2019-10-14  2:44     ` Tiwei Bie
2019-10-15 16:17     ` [dpdk-dev] [PATCH v4] " Flavio Leitner
2019-10-15 17:41       ` Ilya Maximets
2019-10-15 18:44         ` Flavio Leitner
2019-10-15 18:59       ` [dpdk-dev] [PATCH v5] " Flavio Leitner
2019-10-16 10:02         ` Maxime Coquelin
2019-10-16 11:13         ` Maxime Coquelin
2019-10-16 13:32           ` Ilya Maximets
2019-10-16 13:46             ` Maxime Coquelin
2019-10-16 14:02               ` Flavio Leitner
2019-10-16 14:08                 ` Ilya Maximets
2019-10-16 14:14                   ` Flavio Leitner
2019-10-16 14:05               ` Ilya Maximets
2019-10-29  9:02         ` David Marchand
2019-10-29 12:21           ` Flavio Leitner
2019-10-29 16:19             ` David Marchand

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