Re: [Xen-devel] Problem with MTU > 1500, ifconfig segmentation fault

["Satish Raghunath" <satish_raghunath@xxxxxxxxxxx>]

Hi again,

I am trying to test the large-MTU fix back-ported to 2.0 from the 3.0 code. 
Upon testing with "iperf", I am seeing a lot of drops on the RX side. This 
is happening even when I have MTU set at 1500. Consequently, the TCP 
throughput reduces drastically. For example here is how the ifconfig 
counters look like:
On the "iperf -s" host:
         RX packets:4406226 errors:151086 dropped:151086 overruns:0 frame:0
         TX packets:5216687 errors:0 dropped:0 overruns:0 carrier:0

On the "iperf -c" host:
         RX packets:5202875 errors:1171787 dropped:1171787 overruns:13775 
frame:0
         TX packets:4406204 errors:0 dropped:0 overruns:0 carrier:0

Is there some other dependency between dev_alloc_skb and the network card 
driver? If there are some places in the code I should be first looking to 
handle this problem, it would be great to know.
Thank you,
Satish


> From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
> To: <satish_raghunath@xxxxxxxxxxx>
> CC: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Problem with MTU > 1500, ifconfig segmentation 
> fault
> Date: Tue, 13 Sep 2005 09:43:18 +0100
>
> On 12 Sep 2005, at 22:48, Satish Raghunath wrote:
>
> > I modified the
> > xen-2.0.7/linux-2.6.11-xen-sparse/arch/xen/kernel/skbuff.c to borrow
> > changes from the same file in the 3.0 branch. I omitted calls to
> > "xen_create_contiguous_region" and "xen_destroy_contiguous_region" since
> > they seemed to call newer hypercalls. The resulting patch is pasted
> > below. I am able to ping with large frames (up to 8174 bytes). I have
> > not tested this beyond "ping -s", so I am not sure if this is the right
> > way to do it.
> >
> > Is the call to "xen_create_contiguous_region" necessary or is it a
> > performance enhancement?
> The call to xen_create_contiguous_memory() is important, but it's called 
> something else in 2.0 (xen_contig_memory()). It's used in the same way 
> though. The call to xen_destroy_contiguous_memory() can be safely left out.
> Without using xen_contig_memory, you'll find that jumbo frames will fail if 
> you do a debug build of Xen.
> If you add that in then we'll take the patch. Thanks!
>
>  -- Keir
>
>
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