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Date: Fri, 04 Apr 2008 18:09:05 +0400
From: Pavel Emelyanov <xemul@...nvz.org>
To: Linux Netdev List <netdev@...r.kernel.org>
Subject: [RFC][PATCH 0/3] Make loopback device weight twice as little.
I noticed, that on a 32bit box with a non-debug config the
sizeof(struct net_device) is slightly more than 1024. This
means, that for example loopback device (with sizeof_priv == 0)
is allocated from the size-2048 cache and thus we have ~1000
wasted bytes.
I know, that this is not that much, all the more so on most of
the setups the lo device is single :) but with the net-namespaced
kernel each namespace has its own loopback device, so this problem
becomes more relevant.
I also guess, that the net_device struct allows many ways to get
shrunk, but I noticed, that is has ~20 pointers on functions for
different purposes, like init, hard_start_xmit, neigh_setup, etc.
The proposal is to _move_ all these callbacks on a new structure
called net_device_ops and put a pointer on it on the net_device.
I've checked how much will the net_device weigh after this and
it turned out, that this difference (80 bytes) is enough to get
the 1024-sized net_device and thus a halved loopback.
To make this change smooth, I propose the following plan (however,
three patches I sent will demonstrate the idea better).
* First - declare an empty nd_default_ops and make all new devices
point on this new ops temporarily.
* Then replace each call to dev->xxx() with the dev->nd_ops->xxx()
one step by step. The ns_ops->xxx() will be the nd_default_ops->xxx
then and they all will (temporarily) look like
static void nd_default_xxx(struct net_device *dev, ...)
{
if (dev->xxx)
dev->xxx(dev, ...)
}
* After this patch switch all the drivers (lots of work :( but it can
be done driver-by-driver, i.e. without HUGE patches) from on-device
ops to nd_ops pointer.
* Finally - remove all the ops from the net_device and the default
stubs described above.
Does this idea worth being developed further?
Thanks,
Pavel
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