kswapd taking 100% CPU - only there's no swap?

[Don MacAskill <don smugmug com>]

We have an 8-way RHEL4 AS system (4 dual-core Opterons) with no swap 
(because swapping is still broken for DB loads.  It swaps the DB out in 
preference of disk cache).

However, 1 of the CPUs is 100% used by kswapd:


Mem:  32792132k total, 32751292k used,    40840k free,   124256k buffers
Swap:        0k total,        0k used,        0k free,  3439624k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND 

 5654 mysql     16   0 28.1g  27g 3784 S 31.6 88.1  13239:29 mysqld 

   95 root      17   0     0    0    0 R 12.1  0.0   7009:01 kswapd3 

   97 root      15   0     0    0    0 S  0.0  0.0  55:03.42 kswapd1


We're obviously not starving for RAM (3.4GB in disk cache), and we don't 
have any swap enabled, so what on earth is it doing and why is it 
pegging my CPU?

(It's always kswapd3, btw, not any of the others).

Does anyone have any ideas?

Thanks,

Don