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Re: RFC: NAPI packet weighting patch

To: "David S. Miller" <davem@xxxxxxxxxxxxx>
Subject: Re: RFC: NAPI packet weighting patch
From: jamal <hadi@xxxxxxxxxx>
Date: Thu, 23 Jun 2005 08:14:11 -0400
Cc: Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx>, davidm@xxxxxxxxxx, netdev <netdev@xxxxxxxxxxx>, dada1@xxxxxxxxxxxxx, ak@xxxxxxx, leonid.grossman@xxxxxxxxxxxx, becker@xxxxxxxxx, rick.jones2@xxxxxx, davem@xxxxxxxxxx
In-reply-to: <20050622.152325.15263910.davem@xxxxxxxxxxxxx>
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References: <20050622180654.GX14251@xxxxxxxxxxxxx> <20050622.132241.21929037.davem@xxxxxxxxxxxxx> <42B9DA4D.5090103@xxxxxxxxxxxxx> <20050622.152325.15263910.davem@xxxxxxxxxxxxx>
Reply-to: hadi@xxxxxxxxxx
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On Wed, 2005-22-06 at 15:23 -0700, David S. Miller wrote:
> From: Eric Dumazet <dada1@xxxxxxxxxxxxx>
> Date: Wed, 22 Jun 2005 23:38:21 +0200
> > Then maybe we could also play with prefetchw() in the case the
> > incoming frame is small enough to be copied to a new skb.
> That's a good idea too.  In fact, this would deal with platforms
> that use non-temporal stores in their memcpy() implementation.

For the fans of the e1000 (or even the tg3 deprived people), heres a 
patch which originated from David Mosberger that i played around (about
9 months back) - it will need some hand patching for the latest driver.
Similar approach: prefetch skb->data,twiddle twiddle not little star,
touch header.

I found the aggressive mode effective on a xeon but i belive David is
using this on x86_64. So Lennert, I lied to you saying it was never
effective on x86. You just have to do the right juju such as factoring
in the memory load-latency and how much cache you have on your specific
CCing davidm (in addition To: davem of course ;->) so he may provide
more insight on his tests.

Interesting of course is if you miss the "twiddle here" (as i saw in my
experiments) and do the obvious (such as defining AGGRESSIVE to 0), you
infact end up paying a penalty in performance.


===== drivers/net/e1000/e1000_main.c 1.134 vs edited =====
--- 1.134/drivers/net/e1000/e1000_main.c        2004-09-12 16:52:48
+++ edited/drivers/net/e1000/e1000_main.c       2004-09-30 06:05:11
@@ -2278,12 +2278,30 @@
        uint8_t last_byte;
        unsigned int i;
        boolean_t cleaned = FALSE;
+#define AGGRESSIVE 1
        i = rx_ring->next_to_clean;
+       prefetch(rx_ring->buffer_info[i].skb->data);
        rx_desc = E1000_RX_DESC(*rx_ring, i);
        while(rx_desc->status & E1000_RXD_STAT_DD) {
                buffer_info = &rx_ring->buffer_info[i];
+               {
+                       struct e1000_rx_desc *next_rx;
+                       unsigned int j = i + 1;
+                       if (j == rx_ring->count)
+                               j = 0;
+                       next_rx = E1000_RX_DESC(*rx_ring, j);
+                       if (next_rx->status & E1000_RXD_STAT_DD)
+                               prefetch(rx_ring->buffer_info[j].skb->data);
+               }
+# else
+               prefetch(buffer_info->skb->data);
+# endif
 #ifdef CONFIG_E1000_NAPI
                if(*work_done >= work_to_do)

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