Avi Kivity wrote:
Chris Wright wrote:
There's been a number of different discussions re: getting copyless
virtio
net (esp. for KVM). This is just a poke in that general direction to
stir the discussion. I'm interested to hear current thoughts
I believe that copyless networking is absolutely essential.
For transmit, copyless is needed to properly support sendfile() type
workloads - http/ftp/nfs serving. These are usually high-bandwidth,
cache-cold workloads where a copy is most expensive.
For receive, the guest will almost always do an additional copy, but
it will most likely do the copy from another cpu. Xen netchannel2
mitigates this somewhat by having the guest request the hypervisor to
perform the copy when the rx interrupt is processed, but this may
still be too early (the packet may be destined to a process that is on
another vcpu), and the extra hypercall is expensive.
In my opinion, it would be ideal to linux-aio enable taps and packet
sockets. io_submit() allows submitting multiple buffers in one
syscall and supports scatter/gather. io_getevents() supports
dequeuing multiple packet completions in one syscall.
splice() has some nice properties too. It disconnects the notion of
moving around packets from the actually copy them. It also fits well
into a more performant model of interguest IO. You can't publish
multiple buffers with splice but I don't think we can do that today
practically speaking because of mergable RX buffers. You would have to
extend the linux-aio interface to hand it a bunch of buffers and for it
to tell you where the packet boundaries were.
Regards,
Anthony Liguroi
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