Swapping and Kernel 2.6

Hi,

Environment :
 - Fedora Core 2 (kernel 2.6.10)
 - postgresql 7.4.7

I am running a huge query with several joins that needs around 900
Mbytes of Memory (System RAM is 512 Mbytes).

When the system starts to swap the postgres process, CPU consumed drops
to around 2% (instead of around 50% on another system with kernel 2.4).
The query was still working after more than 4 hours, spending the time
with 98% if IO wait.

So I tried to run the postmaster with the environment variable
LD_ASSUME_KERNEL=2.4.1. With this, the query last 7 minutes !!!

Why swapping is so bad with the new kernel ?

Thanks,

Guillaume