netdev - Re: Socket buffer sizes with autotuning

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Date:	Fri, 25 Apr 2008 00:42:03 -0700
From:	"Tom Herbert" <therbert@...gle.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	andi@...stfloor.org, johnwheffner@...il.com, rick.jones2@...com,
	netdev@...r.kernel.org
Subject: Re: Socket buffer sizes with autotuning

On Thu, Apr 24, 2008 at 11:36 PM, David Miller <davem@...emloft.net> wrote:
> From: "Tom Herbert" <therbert@...gle.com>
> Date: Thu, 24 Apr 2008 22:34:50 -0700
>
>
> > Queuing in the host as opposed to in the TX NIC might be beneficial to make
> > work conserving queuing disciplines more effective, since once a packet is
> > queued in the NIC the host can no longer perform any more scheduling on
> > it.    In fact, we have been working on a mechanism to dynamically limit
> > queuing in the NIC (by number of bytes), so that more packets will be queued
> > in host.  This seems to give better results with some of the qdiscs (I can
> > post a patch if there's interest in this).
>
> This work is interesting, but doesn't it make more sense to limit by
> number of packets instead of bytes?
>

If NIC is interrupt driven, enough data must be queued to span
consecutive interrupts.  So for instance if a 10G NIC is generating an
interrupt every 100us, about 125,000 bytes needs to queued at each
interrupt to prevent starvation-- this doesn't translate to a fixed
number of packets.  Limiting by packets seems somewhat ad hoc; if the
limit is to small and the link will be starved, too big and
over-queuing results.

> No intermediate node that I know of drops bytes, they drop packets
> instead :-)
>
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