udp.7: Document /proc files udp_mem, udp_rmem_min, and udp_wmem_min

All of these are new in Linux 2.6.25

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/udp.7

@@ -5,7 +5,7 @@
 .\" of the modification is added to the header.
 .\" $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $
 .\"
-.TH UDP  7 2008-11-20 "Linux" "Linux Programmer's Manual"
+.TH UDP  7 2008-11-21 "Linux" "Linux Programmer's Manual"
 .SH NAME
 udp \- User Datagram Protocol for IPv4
 .SH SYNOPSIS
@@ -128,6 +128,45 @@ and can be received by
 with the
 .B MSG_ERRQUEUE
 flag set.
+.SS /proc interfaces
+System-wide UDP parameter settings can be accessed by files in the directory
+.IR /proc/sys/net/ipv4/ .
+.TP
+.IR udp_mem " (since Linux 2.6.25)"
+This is a vector of three integers governing the number
+of pages allowed for queueing by all UDP sockets.
+.RS
+.TP 10
+.I min
+Below this number of pages, UDP is not bothered about its
+memory appetite.
+When the amount of memory allocated by UDP exceeds
+this number, UDP starts to moderate memory usage.
+.TP
+.I pressure
+This value was introduced to follow the format of
+.IR tcp_mem 
+(see
+.BR tcp (7)).
+.TP
+.I max
+Number of pages allowed for queueing by all UDP sockets.
+.RE
+.IP
+Defaults values for these three items are
+calculated at boot time from the amount of available memory.
+.TP
+.IR udp_rmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
+Minimal size, in bites, of receive buffer used by UDP sockets in moderation.
+Each UDP socket is able to use the size for receiving data,
+even if total pages of UDP sockets exceed udp_mem pressure.
+.TP
+.IR udp_wmem_min " (integer; default value: PAGE_SIZE; since Linux 2.6.25)"
+Minimal size, in bytes, of send buffer used by UDP sockets in moderation.
+Each UDP socket is able to use the size for sending data,
+even if total pages of UDP sockets exceed
+.I udp_mem
+pressure.
 .SS "Socket Options"
 To set or get a UDP socket option, call
 .BR getsockopt (2)