linux-kernel - Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

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Message-ID: <20060918210321.GA4780@ms2.inr.ac.ru>
Date:	Tue, 19 Sep 2006 01:03:21 +0400
From:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
To:	Andi Kleen <ak@...e.de>
Cc:	"Vladimir B. Savkin" <master@...torb.msk.ru>,
	Jesper Dangaard Brouer <hawk@...u.dk>,
	Harry Edmon <harry@...os.washington.edu>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: Network performance degradation from 2.6.11.12 to 2.6.16.20

Hello!

> But that never happens right? 

Right.

Well, not right. It happens. Simply because you get packet
with newer timestamp after previous handler saw this packet
and did some actions. I just do not see any bad consequences.

> And do you have some other prefered way to solve this? Even if the timer
> was fast it would be still good to avoid it in the fast path when DHCPD
> is running.

No. The way, which you suggested, seems to be the best.

1. It even does not disable possibility to record timestamp inside
   driver, which Alan was afraid of. The sequence is:

	if (!skb->tstamp.off_sec)
                net_timestamp(skb);

2. Maybe, netif_rx() should continue to get timestamp in netif_rx().

3. NAPI already introduced almost the same inaccuracy. And it is really
   silly to waste time getting timestamp in netif_receive_skb() a few
   moments before the packet is delivered to a socket.

4. ...but clock source, which takes one of top lines in profiles
   must be repaired yet. :-)

Alexey
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