From: "David S. Miller" <davem@redhat.com>
To: ak@suse.de
Cc: jgarzik@pobox.com, netdev@oss.sgi.com
Subject: Re: reasons for dev_alloc_skb +16?
Date: Wed, 09 Jul 2003 17:20:12 -0700 (PDT) [thread overview]
Message-ID: <20030709.172012.41656849.davem@redhat.com> (raw)
In-Reply-To: <20030709175355.422545b5.ak@suse.de>
From: Andi Kleen <ak@suse.de>
Date: Wed, 9 Jul 2003 17:53:55 +0200
But it's not clear it is still a good idea because it leads to cache line
misalignment of the beginning of the packet, forcing the card to do a
costly Read-Modify-Write cycle.
Only "dumb cards" do that, smart ones rewind to the beginning of
the current cache line and ask for the whole thing instead of
pieces.
The +16 is actually needed to align the first hunk of the outgoing
packet so we can do a 16-byte aligned memcpy of the hard-header
cache as we build the packet.
Jeff, look at LL_RESERVED_SPACE() and the comment above it in
include/linux/netdevice.h
next prev parent reply other threads:[~2003-07-10 0:20 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-07-09 15:25 reasons for dev_alloc_skb +16? Jeff Garzik
2003-07-09 15:53 ` Andi Kleen
2003-07-09 16:06 ` Jeff Garzik
2003-07-09 16:13 ` Andi Kleen
2003-07-10 0:20 ` David S. Miller [this message]
-- strict thread matches above, loose matches on Subject: below --
2003-07-09 15:35 Hen, Shmulik
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