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.Dd July 23, 2014
.Dt PCBGROUP 9
.Os
.Sh NAME
.Nm PCBGROUP
.Nd Distributed Protocol Control Block Groups
.Sh SYNOPSIS
.Cd "options PCBGROUP"
.Pp
.In sys/param.h
.In netinet/in.h
.In netinet/in_pcb.h
.Ft void
.Fo in_pcbgroup_init
.Fa "struct inpcbinfo *pcbinfo" "u_int hashfields" "int hash_nelements"
.Fc
.Ft void
.Fn in_pcbgroup_destroy "struct inpcbinfo *pcbinfo"
.Ft struct inpcbgroup *
.Fo in_pcbgroup_byhash
.Fa "struct inpcbinfo *pcbinfo" "u_int hashtype" "uint32_t hash"
.Fc
.Ft struct inpcbgroup *
.Fn in_pcbgroup_byinpcb "struct inpcb *inp"
.Ft void
.Fn in_pcbgroup_update "struct inpcb *inp"
.Ft void
.Fn in_pcbgroup_update_mbuf "struct inpcb *inp" "struct mbuf *m"
.Ft void
.Fn in_pcbgroup_remove "struct inpcb *inp"
.Ft int
.Fn in_pcbgroup_enabled "struct inpcbinfo *pcbinfo"
.In netinet6/in6_pcb.h
.Ft struct inpcbgroup *
.Fo in6_pcbgroup_byhash
.Fa "struct inpcbinfo *pcbinfo" "u_int hashtype" "uint32_t hash"
.Fc
.Sh DESCRIPTION
This implementation introduces notions of affinity
for connections and distribute work so as to reduce lock contention,
with hardware work distribution strategies
such as RSS.
In this construction, connection groups supplement, rather than replace,
existing reservation tables for protocol 4-tuples, offering CPU-affine
lookup tables with minimal cache line migration and lock contention
during steady state operation.
.Pp
Internet protocols like UDP and TCP register to use connection groups
by providing an ipi_hashfields value other than IPI_HASHFIELDS_NONE.
This indicates to the connection group code whether a 2-tuple or
4-tuple is used as an argument to hashes that assign a connection to
a particular group.
This must be aligned with any hardware-offloaded distribution model,
such as RSS or similar approaches taken in embedded network boards.
Wildcard sockets require special handling, as in Willmann 2006, and
are shared between connection groups while being protected by
group-local locks.
Connection establishment and teardown can be signficantly more
expensive than without connection groups, but that steady-state
processing can be significantly faster.
.Pp
Enabling PCBGROUP in the kernel only provides the infrastructure
required to create and manage multiple PCB groups.
An implementation needs to fill in a few functions to provide PCB
group hash information in order for PCBs to be placed in a PCB group.
.Ss Operation
By default, each PCB info block (struct pcbinfo) has a single hash for
all PCB entries for the given protocol with a single lock protecting it.
This can be a significant source of lock contention on SMP hardware.
When a PCBGROUP is created, an array of separate hash tables are
created, each with its own lock.
A separate table for wildcard PCBs is provided.
By default, a PCBGROUP table is created for each available CPU.
The PCBGROUP code attempts to calculate a hash value from the given
PCB or mbuf when looking up a PCBGROUP.
While processing a received frame,
.Fn in_pcbgroup_byhash
can be used in conjunction with either a hardware-provided hash
value
.Po
eg the
.Xr RSS 9
calculated hash value provided by some NICs
.Pc
or a software-provided hash value in order to choose a PCBGROUP
table to query.
A single table lock is held while performing a wildcard match.
However, all of the table locks are acquired before modifying the
wildcard table.
The PCBGROUP tables operate in conjunction with the normal single PCB list
in a PCB info block.
Thus, inserting and removing a PCB will still incur the same costs
as without PCBGROUP.
A protocol which uses PCBGROUP should fall back to the normal PCB list
lookup if a call to the PCBGROUP layer does not yield a lookup hit.
.Ss Usage
Initialize a PCBGROUP in a PCB info block
.Pq Vt "struct pcbinfo"
by calling
.Fn in_pcbgroup_init .
.Pp
Add a connection to a PCBGROUP with
.Fn in_pcbgroup_update .
Connections are removed by with
.Fn in_pcbgroup_remove .
These in turn will determine which PCBGROUP bucket the given PCB
is placed into and calculate the hash value appropriately.
.Pp
Wildcard PCBs are hashed differently and placed in a single wildcard
PCB list.
If
.Xr RSS 9
is enabled and in use, RSS-aware wildcard PCBs are placed in a single
PCBGROUP based on RSS information.
Protocols may look up the PCB entry in a PCBGROUP by using the lookup
functions
.Fn in_pcbgroup_byhash
and
.Fn in_pcbgroup_byinpcb .
.Sh IMPLEMENTATION NOTES
The PCB code in
.Pa sys/netinet
and
.Pa sys/netinet6
is aware of PCBGROUP and will call into the PCBGROUP code to do
PCBGROUP assignment and lookup, preferring a PCBGROUP lookup to the
default global PCB info table.
.Pp
An implementor wishing to experiment or modify the PCBGROUP assignment
should modify this set of functions:
.Bl -tag -width "12345678" -offset indent
.It Fn in_pcbgroup_getbucket No and Fn in6_pcbgroup_getbucket
Map a given 32 bit hash value to a PCBGROUP.
By default this is hash % number_of_pcbgroups.
However, this distribution may not align with NIC receive queues or
the
.Xr netisr 9
configuration.
.It Fn in_pcbgroup_byhash No and Fn in6_pcbgroup_byhash
Map a 32 bit hash value and a hash type identifier to a PCBGROUP.
By default, this simply returns NULL.
This function is used by the
.Xr mbuf 9
receive path in
.Pa sys/netinet/in_pcb.c
to map an mbuf to a PCBGROUP.
.It Fn in_pcbgroup_bytuple No and Fn in6_pcbgroup_bytuple
Map the source and destination address and port details to a PCBGROUP.
By default, this does a very simple XOR hash.
This function is used by both the PCB lookup code and as a fallback in
the
.Xr mbuf 9
receive path in
.Pa sys/netinet/in_pcb.c .
.El
.Sh SEE ALSO
.Xr mbuf 9 ,
.Xr netisr 9 ,
.Xr RSS 9
.Rs
.%A Paul Willmann
.%A Scott Rixner
.%A Alan L. Cox
.%T "An Evaluation of Network Stack Parallelization Strategies in Modern Operating Systems"
.%J "2006 USENIX Annual Technical Conference"
.%D 2006
.%U http://www.ece.rice.edu/~willmann/pubs/paranet_usenix.pdf
.Re
.Sh HISTORY
PCBGROUP first appeared in
.Fx 9.0 .
.Sh AUTHORS
.An -nosplit
The PCBGROUP implementation was written by
.An Robert N. M. Watson Aq Mt rwatson@FreeBSD.org
under contract to Juniper Networks, Inc.
.Pp
This manual page written by
.An Adrian Chadd Aq Mt adrian@FreeBSD.org .
.Sh NOTES
The
.Xr RSS 9
implementation currently uses
.Ic #ifdef
blocks to tie into PCBGROUP.
This is a sign that a more abstract programming API is needed.
.Pp
There is currently no support for re-balancing the PCBGROUP assignment,
nor is there any support for overriding which PCBGROUP a socket/PCB
should be in.
.Pp
No statistics are kept to indicate how often PCBGROUP lookups
succeed or fail.