netdev - Re: Regarding bottlenecks for high speed packet generation

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Message-ID: <1351268139.2584.9.camel@bwh-desktop.uk.solarflarecom.com>
Date:	Fri, 26 Oct 2012 17:15:39 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Ajith Adapa <adapa.ajith@...il.com>
CC:	David Miller <davem@...emloft.net>, <danborkmann@...earbox.net>,
	<netdev@...r.kernel.org>
Subject: Re: Regarding bottlenecks for high speed packet generation

On Fri, 2012-10-26 at 11:42 +0530, Ajith Adapa wrote:
> > >
> > > But does NAPI comes into picture in case of transmission of packets ?
> >
> > Reclaim of TX descriptors occurs via NAPI.
> 
> You mean for every packet sent by the NIC card it drops a interrupt to
> reclaim the TX buffers used ?

Not necessarily; completions may be batched in some way.

> I am not able to understand why does NIC has to interrupt back instead
> of freeing the memory in background.
>
> Might be it does add some bottleneck in my case of transmitting traffic bursts.

The network core needs to account and limit how much memory is allocated
to socket buffers.  Completed TX buffers should therefore be freed
promptly, to avoid blocking the sending socket unnecessarily.  It
doesn't strictly matter whether this is triggered by an IRQ from the NIC
or by a timer, but it certainly must not be triggered by the next TX
initiation as this may result in a kind of deadlock.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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