[Wylug-discuss] BBC speeds up TCP video streaming on Linux (paraphrase)

[Graham Whaley]

On 15 October 2015 at 17:18, Darren Menachem Drapkin
<darren.drapkin at ntlworld.com> wrote:
> Isn't this the same principal as Direct Memory Access that was used to speed
> up hard drive to ram transfer.

Not quite - DMA is done at the device/driver level to get the data
in/out from the 'last level buffer', so will be used in both the
through-kernel and bypass-kernel setups, but in the kernel-bypass
setup there is no buffer copying, so the buffer being DMAd into/out-of
is the same one that the userspace sees. Note, there are other ways to
achieve that (mmap'ing buffers to userspace for instance), but I
suspect for one reason or another they may not be suitable for the
below streaming solution - I skimmed some of the docs, but did not
study the technology in detail.

DMA is quite fascinating really - we can discuss scatter-gather,
IOMMUs and the recent(ish) addition of CMA (Contiguous Memory
Allocation) to the kernel - which is to help out where some devices
really really want or benefit from physically linear chunks of memory
to play with, which goes against the whole fundamental underpinnings
of a virtual memory system such as *nix.


>
>
> On 10/15/2015 01:45 PM, Graham Whaley wrote:
>>
>> RobS, is this you and yours?
>>
>>
>> http://www.theregister.co.uk/2015/10/12/linux_networking_api_showing_its_age/
>>
>> something we can discuss at the next meet maybe. I'd be interested if
>> it is just the buffer share/zero copy that gets the major speedup, or
>> the buffer batching, or the reduction in syscalls and context switches
>> - or a combination of all three.
>>
>> If I remember I'll also bring along some hacker boards for folks to
>> stare at - an early Minnow version (now generally surpassed), an ARM64
>> Dragon 410c I just won and a MIPS CI20 for comparison. All run Linux.
>>
>>   Graham
>>
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>
> --
> --
> Darren Menachem Drapkin
>
>
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