[OpenAFS] Re: WAN speed

Simon Wilkinson simonxwilkinson@gmail.com
Thu, 22 Mar 2012 15:28:24 +0000

On 22 Mar 2012, at 15:23, Andrew Deason wrote:

> On Thu, 22 Mar 2012 15:20:20 +0000
> Simon Wilkinson <simonxwilkinson@gmail.com> wrote:
>> That limit is imposed because it is the point at which the current RX
>> implementation loses the queue efficiency/throughput tradeoff. You =
>> run with a larger window size, but it will actually make things go
>> slower.
>> It was naively increasing the maximum window size to 255 that caused
>> the huge performance problems in the 1.5.x series that Andrei
>> highlighted at the 2010 European AFS workshop.
> Yes, I know, that's what I'm talking about. Even if we didn't have =
> problem, a window size of 255 is still prohibitively small for many
> uses.

Yeah, that's a bigger problem. We can't extend the window beyond 255, =
because that's the maximum size of the RX ACK packet.

Large windows only help up to a point, though. We go back in to slow =
start with every new call, (this is similar to TCP, which re-enters slow =
start if RTT has passed since the last packet was sent). So, you will =
only use the full window for Store or Read Data if you are writing =
enough data in single call that you can grow the window quickly enough.

Ultimately, we either need rx/tcp, or a bigger revamp of rx/udp than =
anyone is currently considering.