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SubjectRe: zero-copy TCP
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>>>>> "Ingo" == Ingo Molnar <mingo@elte.hu> writes:

Ingo> On 2 Sep 2000, Jes Sorensen wrote:

>> You can't DMA directly from a file cache page unless you have a
>> network card that does scatter/gather DMA and surprise surprise,
>> 80-90% of the cards on the market don't support this. [...]

Ingo> exactly. The TUX patch solves this by copying 'multi-fragment
Ingo> skbs' into a temporary single-fragment skb, if the card doesnt
Ingo> support scatter-gather, 64-bit DMA. This way the copying is
Ingo> delayed as much as possible, to the point where we queue the
Ingo> packet to the network device.

Ok, I need to go look at your code. Sounds like you started doing a
lot of the stuff we discussed during OLS.

>> Besides that you need to do copy-on-write if you want to be able to
>> do zero copy on write() from user space [...]

Ingo> i agree that this is hard - i'm not sure wether we want to go
Ingo> the pain to enable anonymous-buffer write()s do zero-copy. My
Ingo> plan is to enable sendfile() first - it should cover all the
Ingo> important high-performance server cases. The point is that a
Ingo> write() is only used if some sort of dynamic data is generated
Ingo> on the fly. If data is generated once and sent once then there
Ingo> is no much win in zero-copy. If data is generated once and
Ingo> reused multiple times afterwards then it should rather be
Ingo> written into a temporary file and then it can be sent out via
Ingo> sendfile().

At CERN we had a bunch of applications where this would be a win, data
aquisition servers taking data in from on some custom hardware and
sending out data over the wire on another card. You never really want
to touch data in memory with the CPU but because of the lack of
write() zero copy you end up having to do so.

Jes
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