Re: RFC: NAPI packet weighting patch

[Jesse Brandeburg <jesse.brandeburg@intel.com>]

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On Tue, 7 Jun 2005, David S. Miller wrote:
> > There also seems to be some misconceptions about changing the weight
> > value.  It actually improves the performance of other drivers as well.
> > Not as much as it improves the e1000 performance but it does seem to
> > help others as well.
> 
> One reason it helps e1000 more, which Robert Olsson mentioned, could
> be the HW irq mitigation settings used by the e1000 driver.  Lowering
> these would be a good test.

Well, first a little more data.  The machine in question is a dual xeon 
running 2.6.12-rc5 or 2.6.12-rc4-supertso

with the 2.6.12-rc5 kernel (the old) tso promptly shuts down after a SACK, 
and after that point the machine is CPU bound at 100%.  This is the point 
that we start to drop packets at the hardware level.

I tried the experiment today where I replenish buffers to hardware every 
16 packets or so.  This appears to mitigate all drops at the hardware 
level (no drops).  We're still at 100% with the rc5 kernel, however.

even with this replenish fix, the addition of dropping the weight to 16 
helped increase our throughput, although only about 1%.

On the other hand, taking our driver as is with no changes and running the 
supertso (not the split out version, yet) kernel, we show no dropped 
packets and 60% cpu use.  This combines with a 6% increase in throughput, 
and the data pattern on the wire is much more constant (i have tcpdumps, 
do you want to see them Dave?)

I'm looking forward to trying the split out patches tomorrow.

here is my (compile tested) patch, for e1000

diff -rup e1000-6.0.60.orig/src/e1000_main.c e1000-6.0.60/src/e1000_main.c
--- e1000-6.0.60.orig/src/e1000_main.c	2005-06-07 19:07:37.000000000 -0700
+++ e1000-6.0.60/src/e1000_main.c	2005-06-07 19:15:05.000000000 -0700
@@ -3074,11 +3074,14 @@ e1000_clean_rx_irq(struct e1000_adapter
  next_desc:
  		rx_desc->status = 0;
  		buffer_info->skb = NULL;
+		if(unlikely((i & ~(E1000_RX_BUFFER_WRITE - 1)) == i))
+			adapter->alloc_rx_buf(adapter);
  		if(unlikely(++i == rx_ring->count)) i = 0;

  		rx_desc = E1000_RX_DESC(*rx_ring, i);
  	}
  	rx_ring->next_to_clean = i;
+	/* not sure this is necessary any more, but its safe */
  	adapter->alloc_rx_buf(adapter);

  	return cleaned;
@@ -3209,12 +3212,15 @@ e1000_clean_rx_irq_ps(struct e1000_adapt
  next_desc:
  		rx_desc->wb.middle.status_error &= ~0xFF;
  		buffer_info->skb = NULL;
+		if(unlikely((i & ~(E1000_RX_BUFFER_WRITE - 1)) == i))
+			adapter->alloc_rx_buf(adapter);
  		if(unlikely(++i == rx_ring->count)) i = 0;

  		rx_desc = E1000_RX_DESC_PS(*rx_ring, i);
  		staterr = le32_to_cpu(rx_desc->wb.middle.status_error);
  	}
  	rx_ring->next_to_clean = i;
+	/* not sure this is necessary any more, but its safe */
  	adapter->alloc_rx_buf(adapter);

  	return cleaned;

PS e1000-6.0.60 is posted on sf.net/projects/e1000 now.