netdev - Re: [WIP][PATCHES] Network xmit batching

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Message-Id: <1181302267.4063.33.camel@localhost>
Date:	Fri, 08 Jun 2007 07:31:07 -0400
From:	jamal <hadi@...erus.ca>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	Gagan Arneja <gaagaan@...il.com>, netdev@...r.kernel.org,
	Rick Jones <rick.jones2@...com>,
	Sridhar Samudrala <sri@...ibm.com>,
	David Miller <davem@...emloft.net>,
	Robert Olsson <Robert.Olsson@...a.slu.se>
Subject: Re: [WIP][PATCHES] Network xmit batching

On Fri, 2007-08-06 at 12:38 +0400, Evgeniy Polyakov wrote:
> On Thu, Jun 07, 2007 at 06:23:16PM -0400, jamal (hadi@...erus.ca) wrote:

> > I believe both are called with no lock. The idea is to avoid the lock
> > entirely when unneeded. That code may end up finding that the packet
[..]
> +	netif_tx_lock_bh(odev);
> +	if (!netif_queue_stopped(odev)) {
> +
> +		idle_start = getCurUs();
> +		pkt_dev->tx_entered++;
> +		ret = odev->hard_batch_xmit(&odev->blist, odev);

[..]
> The same applies to *_gso case.
> 

You missed an important piece which is grabbing of
__LINK_STATE_QDISC_RUNNING


> Without lock that would be wrong - it accesses hardware.

We are achieving the goal of only a single CPU entering that path. Are
you saying that is not good enough?

> I only saw results Krishna posted, 

Ok, sorry - i thought you saw the git log or earlier results where
other things are captured.

> and i also do not know, what service demand is :)

>>From the explanation seems to be how much cpu was used while sending. Do
you have any suggestions for computing cpu use?
in pktgen i added code to count how many microsecs were used in
transmitting.

> Result looks good, but I still do not understand how it appeared, that
> is why I'm not that excited about idea - I just do not know it in
> details.

To add to KKs explanation on other email:
Essentially the value is in amortizing the cost of barriers and IO per
packet. For example the queue lock is held/released only once per X
packets. DMA kicking which includes both a PCI IO write and mbs is done
only once per X packets. There are still a lot of room for improvement
of such IO;

cheers,
jamal

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