netdev - RE: [PATCH net-next 2/2] net/mlx4: Revert "mlx4: set maximal number of default RSS queues"

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Date:	Wed, 15 Jan 2014 12:54:27 +0000
From:	Yuval Mintz <yuvalmin@...adcom.com>
To:	Ido Shamai <idos@....mellanox.co.il>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Or Gerlitz <or.gerlitz@...il.com>
CC:	Amir Vadai <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eugenia Emantayev <eugenia@...lanox.com>,
	Ido Shamay <idos@...lanox.com>
Subject: RE: [PATCH net-next 2/2] net/mlx4: Revert "mlx4: set maximal number
 of default RSS queues"

> > Anyway, I believe 8/16 are simply strict limitations without any true
> meaning;
> > To judge what's more important, default `slimness' or default performance
> > is beyond me.
> > Perhaps the numa approach will prove beneficial (and will make some
> sense).
> 
> After reviewing all that was said, I feel there is no need to enforce
> vendors with this strict limitation without any true meaning.
> 
> The reverted commit you applied forces the driver to use 8 rings at max
> at all time, without the possibility to change in flight using ethtool,
> as it's enforced on the PCI driver at module init (restarting the en
> driver with different of requested rings will not affect).
> So it's crucial for performance oriented applications using mlx4_en.

The rational is to prevent default misusage of resources, be them irq vectors
memories for rings.
Notice this is just the default - You can always re-request interrupts if the
user explicitly requests a large number of rings;
Although, if the driver is incapable of that (e.g., hw limitations), perhaps you
should allocate a larger number of irq vectors during init and use a limitation
on your default number of rings 
(that's assuming that irq vectors are really inexpensive on all machines).

> Going through all Ethernet vendors I don't see this limitation enforced,
> so this limitation has no true meaning (no fairness).
> I think this patch should go in as is.
> Ethernet vendors should use it this limitation when they desire.

Might be true, but two wrongs don't make a right.
I believe that either this limitation should be mandatory, or the functionality
Should Not be included in the kernel as communal code and each driver
should do as it pleases.

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