While investigating complains on bad HTTP server performance, I've discovered these lines in dmesg of my Xen XCP host that contains a guest OS with said server:

[11458852.811070] net_ratelimit: 321 callbacks suppressed
[11458852.811075] nf_conntrack: table full, dropping packet.
[11458852.819957] nf_conntrack: table full, dropping packet.
[11458852.821083] nf_conntrack: table full, dropping packet.
[11458852.822195] nf_conntrack: table full, dropping packet.
[11458852.824987] nf_conntrack: table full, dropping packet.
[11458852.825298] nf_conntrack: table full, dropping packet.
[11458852.825891] nf_conntrack: table full, dropping packet.
[11458852.826225] nf_conntrack: table full, dropping packet.
[11458852.826234] nf_conntrack: table full, dropping packet.
[11458852.826814] nf_conntrack: table full, dropping packet.

Complains are repeated every five seconds (number of suppressed callbacks is different each time).

What can these sympthoms mean? Is that bad? Any hints?

(Note that the question is more narrow than "how to solve specific case of bad HTTP server performance", so I do not give more details on that.)

Additional info:

$ uname -a
Linux MYHOST 3.2.0-24-generic #37-Ubuntu SMP Wed Apr 25 08:43:22 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 12.04 LTS
Release:    12.04
Codename:   precise

$ cat /proc/sys/net/netfilter/nf_conntrack_max 
1548576

The server is under about 10M hits / day load.