linux-kernel - Re: Dropping NETIF_F_SG since no checksum feature.

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Message-ID: <20061011001338.GA30093@mellanox.co.il>
Date:	Wed, 11 Oct 2006 02:13:38 +0200
From:	"Michael S. Tsirkin" <mst@...lanox.co.il>
To:	Stephen Hemminger <shemminger@...l.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org, openib-general@...nib.org,
	Roland Dreier <rolandd@...co.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Dropping NETIF_F_SG since no checksum feature.

Quoting r. Stephen Hemminger <shemminger@...l.org>:
> > > > I'm trying to build a network device driver supporting a very large MTU
> > > > (around 64K) on top of an infiniband connection, and I've hit a couple
> > > > of issues I'd appreciate some feedback on:
> > > > 
> > > > 1. On the send side,
> > > >    I've set NETIF_F_SG, but hardware does not support checksum
> > > >    offloading, and I see "dropping NETIF_F_SG since no checksum feature"
> > > >    warning, and I seem to be getting large packets all in one chunk.
> > > >    The reason I've set NETIF_F_SG, is because I'm concerned that under
> > > >    real life stress Linux won't be able to allocate 64K of continuous
> > > >    memory.
> > > > 
> > > >    Is this concern of mine valid? I saw in-tree drivers allocating at
> > > >    least 8K.  What's the best way to enable S/G on send side?  Is
> > > >    checksum offloading really required for S/G?
> > > 
> > > Yes, in the current implementation, Linux needs checksum offload. But
> > > there is no reason, your driver can't compute the checksum in software.
> > >
> > I'm worried whether an extra pass over data won't eat up all of
> > the performance gains I get from the large MTU ...
> 
> Yup, the cost is in touching the data, not in the copy.

Maybe I can patch linux to allow SG without checksum?
Dave, maybe you could drop a hint or two on whether this is worthwhile
and what are the issues that need addressing to make this work?

I imagine it's not just the matter of changing net/core/dev.c :).

-- 
MST
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