Why no zero-copy networking in linux kernel?
I'd like to know the reasons for why the linux kernel (or any other mainstream OS) does not have a feature for zero copy networking ? By zero copy I mean, that an packet/datastream does not get copied for passing to an application in userspace but e.g. uses a memory-pool type of allocator to share the memory between kernel and userspace. I've came up with 3 theory's on my own:
a) I guess there are security concerns. But is there really no way of making memory shared securily between userspace and kernel when they are just used as a buffer ?
b) I guess there are stability concerns. But can't we assume that whoever uses zero-copy networking and e.g. needs to instanciate and pass a memory-pool for the kernel call is aware of memory management? Aware enough to avoid leaks ?
c) It just haven't been done/needed so far. I can't really imagine that nobody requested this feature, as everybody who is using small packet sizes is typically bottlenecked by the "slow" TCP-stack implementation and there are 3rd party tools out there offered for 0-copy networking for usage with special network cards.
Feel free to post any guesses, but please mark whether you are assuming or have a deeper knowledge of the reasons to keep StackOverflow-quality :-)
Answer
There are a few options nowadays for zero-copy networking:
- PF_RING ZC (Zero Copy).
- Linux native zero-copy TCP.
- Intel Data Plane Development Kit.
- OpenOnload for proprietary hardware.
Note, that zero-copy with TCP may be inconvenient, because TCP segments carry headers and payload, but applications are only concerned with payload, so that payload but not headers must be copied into a contiguous buffer for your application.