Date: Wed, 18 Jun 2008 18:22:16 +0530
From: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
To: linux-kernel@vger.kernel.org, e1000-devel@lists.sourceforge.net
Cc: varunc@linux.vnet.ibm.com, jbarnes@virtuousgeek.org, greg@kroah.com
Subject: Strange problem with e1000 driver - ping packet loss
Message-ID: <20080618125215.GC3988@linux.vnet.ibm.com>
Reply-To: vatsa@linux.vnet.ibm.com
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Hi,
	I happened to look at a system which was exhibiting poor ping
performance with e1000 driver (in 2.6.25) and had some questions regarding that.

Ping test was done between the system and a laptop, which were connected
using a straight ethernet cable. Ping reported round trip times running
into seconds (!) and also packet loss.

Upon some investigation, I found that the interrupt count field in
/proc/interrupts (associated with eth1) is not incrementing as fast as
it should. Moreover eth1 interrupt line is shared with the hard disk
interrupt (ata_piix) as below:

# cat /proc/interrupts

..

 10:       2296    XT-PIC-XT        ata_piix, eth0, eth1

..

IRQ10 is thus being shared by both the hard disk and eth0/eth1.

Here's the strange observation I made:

When I initiate some disk activity (ex: dd if=/dev/zero of=/tmp/file), ping 
performance suddently shot up (round trip time in double digits ms, 0% packet 
loss)! I presume this is because that e1000 intr handler is called
whenever there was a interrupt from hard disk on IRQ10, which polled
NIC and processed packets immediately.

As soon as I kill the background disk-write intensive job, ping
performance again dropped.

This meant that e1000 NIC is having trouble interrupting the OS.

Before I could jump up and say this is a hardware issue, I was told
that Windows works just fine on the server (and as well as 2.4 kernel,
which I couldnt verify) :(


Some more observations:

1. I tried setting e1000 parameters (RxIntDelay=0, RxAbsIntDelay=0,
   TxIntDelay=0, TxAbsIntDelay=0, InterruptThrottleRate=0). None of
   them helped.

2. When ping performance was poor, readprofile showed that system
   is mostly idle. This confirms that OS is not getting very
   frequenty interrupts from eth1 and hence idling.

3. When ping performance was poor, ethtool -S eth1 showed that
   rx_bytes was incrementing at a good pace, showing that the 
   NIC was receiving ping responses back, but not handing them over
   to OS for further processing

4. e1000 chipset is 82546GB

5. e1000e driver didnt work at all (it doesnt recognize the cards).


Any advice on how to fix this problem?


-- 
Regards,
vatsa
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