arp -na performance w/ many permanent entries

[Nick Rogers]

On Mon, May 31, 2010 at 10:54 PM, Nick Rogers <ncrogers at gmail.com> wrote:

>
> [root@ ~]# time arp -na > /dev/null
>
> real 0m12.761s
> user 0m2.959s
> sys 0m9.753s
> [root@ ~]#
>
>
> Notice that "arp -na" takes about 13s to execute even though there is no
> other load. This can get a lot worse by a few orders of magnitude on a
> loaded machine in a production environment, and seems to scale up linearly
> when more aliases are added to the interface (permanent ARP entries
> created).
>
> Is this a reasonable problem that can be fixed/improved, or am I stuck with
> the slow arp -na output? Any help or comments is greatly appreciated.
>

I tried the same scenario on 8.1-BETA1 and it still takes a very long time
for arp(8) to complete.

I was able to isolate the performance bottleneck to a small piece of the
arp(8) code. It seems that looking up the interface for an ARP entry is a
very heavy operation when that entry corresponds to an alias assigned to the
interface. Permanent ARP entries that do not correspond with an interface
alias do not seem to cause arp(8) to puke on the interface lookup.

The following commands and code diff illustrates how arp(8) can be modified
to run a lot faster in this scenario, but obviously the associated interface
is no longer printed for each entry.

[root@ /usr/src/usr.sbin/arp]# uname -a
FreeBSD .localdomain 8.1-BETA1 FreeBSD 8.1-BETA1 #0: Thu May 27 15:03:30 UTC
2010     root at mason.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64
[root@ /usr/src/usr.sbin/arp]# time /usr/sbin/arp -na | wc -l
    4100

real 0m14.903s
user 0m3.133s
sys 0m11.519s
[root@ /usr/src/usr.sbin/arp]# pwd
/usr/src/usr.sbin/arp
[root@ /usr/src/usr.sbin/arp]# !diff
diff -ruN arp.c.orig arp.c
--- arp.c.orig 2010-06-05 18:25:24.000000000 +0000
+++ arp.c 2010-06-05 18:28:19.000000000 +0000
@@ -562,7 +562,7 @@
  const char *host;
  struct hostent *hp;
  struct iso88025_sockaddr_dl_data *trld;
- char ifname[IF_NAMESIZE];
+ //char ifname[IF_NAMESIZE];
  int seg;

  if (nflag == 0)
@@ -591,8 +591,8 @@
  }
  } else
  printf("(incomplete)");
- if (if_indextoname(sdl->sdl_index, ifname) != NULL)
- printf(" on %s", ifname);
+ //if (if_indextoname(sdl->sdl_index, ifname) != NULL)
+ //printf(" on %s", ifname);
  if (rtm->rtm_rmx.rmx_expire == 0)
  printf(" permanent");
  else {
[root@ /usr/src/usr.sbin/arp]# make clean && make
rm -f arp arp.o arp.4.gz arp.8.gz arp.4.cat.gz arp.8.cat.gz
Warning: Object directory not changed from original /usr/src/usr.sbin/arp
cc -O2 -pipe  -std=gnu99 -fstack-protector -Wsystem-headers -Werror -Wall
-Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototypes
-Wmissing-prototypes -Wpointer-arith -Wno-uninitialized -Wno-pointer-sign -c
arp.c
cc -O2 -pipe  -std=gnu99 -fstack-protector -Wsystem-headers -Werror -Wall
-Wno-format-y2k -W -Wno-unused-parameter -Wstrict-prototypes
-Wmissing-prototypes -Wpointer-arith -Wno-uninitialized -Wno-pointer-sign
 -o arp arp.o
gzip -cn arp.4 > arp.4.gz
gzip -cn arp.8 > arp.8.gz
[root@ /usr/src/usr.sbin/arp]# time ./arp -na | wc -l
    4099

real 0m0.036s
user 0m0.015s
sys 0m0.021s
[root@ /usr/src/usr.sbin/arp]#

Notice that 0.036s without the interface lookup is a heck of a lot faster
than 14.903s when doing the interface lookup.

Is there something that can be done to speedup the call to if_indextoname(),
or would it be worthwhile for me to submit a patch that adds the ability to
skip the interface lookup as an arp(8) option?