What about other headers?

Posted Jul 24, 2006 15:55 UTC (Mon) by dlang (guest, #313)
In reply to: What about other headers? by christian.convey
Parent article: OLS: A proposal for a new networking API

not nessasarily, the kernel support scatter-gather internally which would let you identify the TCP header (in one area of memory) and the data (in another area or areas of memory) and tell the driver where they are, the driver will get all the pieces in one operation (this is even easier with an IOMMU, with that the kernel can program the address space so that the driver doesn't know that the pieces are seperated)

I don't know if this can be done with the userspace DMA work, but I'm not seeing a reason it couldnt'.

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What about other headers?

Posted Jul 26, 2006 18:33 UTC (Wed) by mikov (guest, #33179) [Link] (3 responses)

Do all (most) Ethernet adapters support scatter-gather DMA ?

What about other headers?

Posted Jul 26, 2006 19:11 UTC (Wed) by dlang (guest, #313) [Link] (2 responses)

the higher-end cards support scatter-gather, but with an Opteron and it's IOMMU this functionality is available without the card needing to support it (the IOMMU remaps the address space so that the card thinks it's doing a old-fashiond single-area DMA, when it's really pulling/pushing the info to multiple sections of ram)

David Lang

What about other headers?

Posted Jul 27, 2006 1:11 UTC (Thu) by mikov (guest, #33179) [Link] (1 responses)

Thanks for pointing this - I must be falling behind the times because I didn't know about Opteron's IOMMU. I found a good description here: http://www.amd.com/us-en/assets/content_type/white_papers...

Is there equivalent functionality in Intel's chipsets (since they can't put it in the CPU) ? If not, it doesn't seem prudent to rely on it for zero-copy.

What about other headers?

Posted Jul 27, 2006 1:51 UTC (Thu) by dlang (guest, #313) [Link]

My understanding is that the Intel chips do not have an IOMMU at this time.

however this just means that the kernel and drivers have to fall back to the non-zero-copy mechanism if the hardware doesn't support scatter-gather nativly

it's a matter of useing it if it's there, and doing the best you can if it's not (and remember that if you compile the kernel for Opteron-only as opposed to K8 architecture you get more performance becouse you get to leave out the tests for the IOMMU and the code to handle it, so it doesn't always cost you :-)