[Ntop-misc] why is PF_RING limited to only allocating up to 4MB of ring slots

[Rob Willis]

(Hopefully, this doesnt turn into a double post since I sent an earlier email via my home email account by accident).

I've been playing with PF_RING (with positive results).  However it seems to be limited to only allocating enough slots up to 4MB of memory.  I'm on FedoraCore4, 2.6.12.

I took a look at the RF_RING code and it is calling __get_free_pages(priority,order) where order represents the number of kernel memory pages to return: 2^order.  So pages returned:

order=0:  1 page
order=1:  2 pages
order=2:  4 pages
...
And in my case, PF_RING decides on an order of 10:
order=10: 1024 pages

Since each page is 4kb, that means it returns 4 MB of memory for use as ring slots.  My ring slot size is set to 1518 (just big enough for an 802.1q packet).

Anyways, even when I request 8MB or 16 MB, this function always returns 4 MB (ie it always uses an order of 10).  Is there another option to get PF_RING to allocate more memory?  I've got 8 GB on this system and need to do some fairly indepth analysis of packets.  So would like to have a very large buffer allocated.

If there is some reason that __get_free_pages can only return 4 MB blocks, perhaps PF_RING could be reworked to manage multiple large blocks.  Then could call __get_free_pages multiple times and put all of the blocks in a array.  Would mean an extra level of indirection when referencing a ring slot, but that doesnt seem like that much more overhead.  

I might even be willing to dive in and come up with a patch to do just that if someone told me that was even possible.  And if they said there wasnt an easier way to get PF_RING to allocate more memory.

-Rob
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