I think the idea is not so much to prevent cache trashing, which is after all inevitable, but to make it happen less often. If kswapd pre-zeroes a big sack of pages, then that's more efficient than zeroing them one at a time. And of course if the hardware can be persuaded to zero chunks of RAM without touching the processor cache, then you've got a huge win.
In a couple of years it might even be possible to buy a PCI Express "/dev/null" card to accelerate your server.
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