From: Pravin Shelar <pshelar@nicira.com>
To: Cong Wang <amwang@redhat.com>
Cc: Jesse Gross <jesse@nicira.com>,
netdev@vger.kernel.org, Thomas Graf <tgraf@suug.ch>
Subject: Re: A question on the design of OVS GRE tunnel
Date: Mon, 8 Jul 2013 09:28:00 -0700 [thread overview]
Message-ID: <CALnjE+q4L9Q8Ebj+O-D8xQHyzSo=FZrb7A-XVEGERzeN-MZMGQ@mail.gmail.com> (raw)
In-Reply-To: <1373277065.8227.26.camel@cr0>
On Mon, Jul 8, 2013 at 2:51 AM, Cong Wang <amwang@redhat.com> wrote:
> Hi, Jesse, Pravin
>
> I have a question on the design of OVS GRE tunnel. Why OVS GRE tunnel
> doesn't register a netdev? I understand it is enough for GRE to function
> without registering a netdev, just a GRE vport is sufficient and
> probably even simpler.
>
kernel-gre device has gre-parameters/state associated with it and
ovs-gre-vport is completely stateless. ovs-gre state is in user-space
which make kernel module alot simpler. Therefore I doubt it will be
easy or simpler to use netdev at this point.
> However, I noticed there is some problem with such design:
>
> I saw very bad performance with the _default_ setup with OVS GRE. After
> digging it a little bit, clearly the cause is that OVS GRE tunnel adds
> an outer IP header and a GRE header for every packet that passed to it,
> which could result in a packet whose length is larger than the MTU of
> the uplink, therefore after the packet goes through OVS, it has to be
> fragmented by IP before going to the wire.
>
I do not understand what do you mean, gre packets greater than MTU
must be fragmented before sent on wire and it is done by GRE-GSO code.
> Of course we can workaround this problem by:
>
> 1) lower down the MTU of the first net device to reserve some room for
> GRE header
>
> 2) pass vnet_hdr=on to KVM guests so that packets going out there are
> still GSO packets even on hosts (I never try this, just analysis it by
> reading code)
>
> Do we have to live with this? Can we solve this problem at OVS layer? So
> that no matter how large the packets are we can avoid IP fragmentation?
> One solution in my mind is registering a netdev for OVS GRE tunnel too,
> so that we could probably reuse GRO cells, thus packets can be merged
> before OVS processing them, and it will be segmented again before going
> to the wire. But I could easily miss something here. ;)
>
> What do you think? Any better idea to fix this?
>
> Thanks!
>
next prev parent reply other threads:[~2013-07-08 16:28 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-08 9:51 A question on the design of OVS GRE tunnel Cong Wang
2013-07-08 16:28 ` Pravin Shelar [this message]
2013-07-09 2:41 ` Cong Wang
2013-07-09 6:26 ` Jesse Gross
2013-07-10 3:34 ` Cong Wang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CALnjE+q4L9Q8Ebj+O-D8xQHyzSo=FZrb7A-XVEGERzeN-MZMGQ@mail.gmail.com' \
--to=pshelar@nicira.com \
--cc=amwang@redhat.com \
--cc=jesse@nicira.com \
--cc=netdev@vger.kernel.org \
--cc=tgraf@suug.ch \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).