Date: Thu, 21 Nov 2002 11:17:09 -0800 From: Luigi Rizzo <rizzo@icir.org> To: current@freebsd.org Subject: mbuf header bloat ? Message-ID: <20021121111709.A23435@xorpc.icir.org>
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[Bcc to -net because it is relevant there. This email has been
triggered by a private discussion i was having with other committers
(who will easily recognise themselves :) which suggested the
possibility of adding more fields to mbuf headers]
Just recently came up to my attention that we have the following
code in <sys/_label.h>
#define MAC_MAX_POLICIES 4
struct label {
int l_flags;
union {
void *l_ptr;
long l_long;
} l_perpolicy[MAC_MAX_POLICIES];
};
(what are l_perpolicy[], ints ? Could this be written a bit better ?)
and then in <sys/mbuf.h>
struct pkthdr {
struct ifnet *rcvif; /* rcv interface */
int len; /* total packet length */
/* variables for ip and tcp reassembly */
void *header; /* pointer to packet header */
/* variables for hardware checksum */
int csum_flags; /* flags regarding checksum */
int csum_data; /* data field used by csum routines */
SLIST_HEAD(packet_tags, m_tag) tags; /* list of packet tags */
struct label label; /* MAC label of data in packet */
};
The label is 5 ints, the pkthdr a total of 11 ints (and m_hdr takes
another 6, for a total of 136 bytes of header info on 64-bit architectures).
Of the pkthdr, only 3 fields (rcvif, len, tags) are of really general
use, the rest being used only in certain cases and for very specific
purposes (e.g. reassembly of fragments, or hw capabilities, or MAC).
Now that Sam has done the excellent work of integrating packet tags
to carry annotations around, i really believe that we should try
to move out of the pkthdr all non-general fields, and move them to
m_tags so we only pay the cost when needed and not in all cases.
Also this pays a lot in terms of ABI compatibility and extensibility.
I understand that for 5.0 it is a bit late to act, but i do hope
that we can reconsider this issue for 5.1 and pull out of the pkthdr
at least the MAC label, and possibly also the csum_* fields, much
in the same way it has been done for VLAN labels.
cheers
luigi
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