[Devel] atl1 (Attansic L1 Gigabit Ethernet) driver & TCP weirdness

[Vasily Averin]

On 01/29/2011 06:52 PM, Solar Designer wrote:
> Hi,
>
> It appears that linux-2.6.18-atl1-1.0.41.0.patch in RHEL5 branch OpenVZ
> kernels is your addition, not Red Hat's, correct?

yes, it's our patch, prepared 4 years ago, sources were taken from 
http://atl1.sourceforge.net/

> I have a machine with this NIC chip (onboard).  It uses this driver,
> which mostly works fine.  However, there's an OpenVZ-specific bug where
> TCP data transfer throughput from the host system is extremely poor
> (like 30 KB/sec), whereas the throughput from OpenVZ containers on the
> same system is OK (11 MB/sec, which is just right for 100 Mbps).
>
> Investigating this with tcpdump, I see that when I transfer stuff from
> the host system, the sending machine (the one with atl1) sometimes skips
> a 4-byte block inbetween TCP segments.  Specifically, it sends a 1456
> byte segment followed by a 1460 byte one, with 4 bytes skipped.  The
> connection gets stalled for 200 ms.  Then the buggy system retransmits
> the segment (or maybe both, I don't recall) at full 1460 bytes, and the
> transfer goes further, until this happens again after just a few packets.
>
> tcpdump on transfers from OpenVZ containers on the same system shows all
> 1460-byte segments.
>
> This happens with at least 2.6.18-194.26.1.el5.028stab079.1.owl2 (the
> kernel build we had in Owl 3.0 release).  The exact same kernel build
> works just fine on plenty of other machines.  This is the only machine
> exhibiting the problem, and is also the only one with an atl1 NIC.
>
> Any ideas?  I did not try matching this against the code yet.
>
> Thanks,
>
> Alexander
>
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