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SubjectRe: zero-copy TCP
Alan Cox wrote:
> > read/recv block while the NIC DMAs into user space main memory.
>
> Thats actually not always a win either. A DMA to user space flushes
> those pages out of cache which isnt so ideal if the CPU wants
> them. Some of the results are suprisingly counter-intuitive like this

Does it flush the CPU cache? I thought the CPU just snooped the bus and
updated its cache with new data.

> > The NIC memory is never accessed directly. It's a cache of skbuff data
> > payloads, and the only access is by DMA, in those places where the
> > kernel stack normally does copy_to_user/copy_from_user.
>
> Yep. Unfortunately to make it work you need to do a bit more than that - you
> have to pin pages in memory, flush caches on some cpus, lock those pages
> against being truncated by another process (on the other cpu) and you have
> to deal with queue reclamation when the nic gets short of buffers.

Ugh. User space DMA gets complicated quickly. The performance question
is, perhaps, can you do this without a TLB flush (but with locking the
struct page of course). Note that it doesn't matter if another thread,
and this includes truncate/write in another thread, clobbers the page
data. That's just the normal effect of two concurrent writers to the
same memory.

-- Jamie
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