Linux networking and disk IO issues
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Quoted 7 lines · Mark Hayden
* The Linux networking stack requires all skbuff buffers to be contiguous. As far as I can tell, this makes it impossible to write high-bandwidth UDP applications on Linux. For instance, the kernel will drop a fragmented 8KB message if it cannot allocate 8KB of contiguous memory to reassemble it into. I have found that it is relatively easy to enter regimes where this can cause massive packet loss.
2.4.4+ supports fragmented packets and packet lists.
You're probably seeing the 8K allocation problem for incoming packets which need to be allocated by the driver on interrupt time with GFP_ATOMIC. GFP_ATOMIC memory is limited. The 2.4 VM unfortunately has no way to keep more GFP_ATOMIC free ATM and tune for heavy interrupt load (2.2 allowed this by increasing the freepages sysctl). Hopefully this VM bug will be fixed in the not too far future.
A workaround in the driver would be to use the 2.4.4 fragmented buffers (of course you'll still run into GFP_ATOMIC limits without manual tuning) or allocate RX memory from a thread with GFP_KERNEL.
-Andi