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Re: [MirageOS-devel] Parallelizing writing to network devices

Thanks Thomas for the great tracing tool!

The following is a very simple unikernel with two interfaces, which redirects frames captured on the first interface to the second one:


The problem is that in a high packet rate (more than 80'000 pps), switch stops receiving. The goal is to spot the problem and enhance the throughput of Mirage netif.

Test environment consists of another vm running a traffic generator and sending frames of a specific pattern (UDP frames of size 100 bytes) over the bridge that connects to the first interface of the unikernel. Unikernel forwards frames by collecting a number of frames from input queue and running the same number of threads that write them to the output interface.

Two trace files are uploaded to the repo. The first file is the output of this configuration. This trace shows that each netif write locks until the thread that writes on the front-end connection to the ring is returned (function write_already_locked.)

For the second trace, the return of the thread is ignored (commenting out "lwt () = th in" in write_already_locked). This considerably increases switching speed, but after some running time, it looks that after garbage collection, similar problem happens.

Thomas and Anil, any idea from given traces, and how it is possible to make the traces more informative?


On 28/11/14 16:55, Thomas Leonard wrote:
On 28 November 2014 at 16:24, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
On 28 Nov 2014, at 16:03, Masoud Koleini <masoud.koleini@xxxxxxxxxxxxxxxx> 

Thanks Anil.

- graph the ring utilisation to see if it's always full (Thomas Leonard's 
profiling patches should help here)
Would you please point me out to the profiling patches?
See: http://roscidus.com/blog/blog/2014/10/27/visualising-an-asynchronous-monad/
The installation instructions here are for the previous version
(though they should still work). If you want to try the latest
version, the current Git mirage allows you to pass a ~tracing argument
to "register" in your config.ml, e.g.

let tracing = mprof_trace ~size:1000000 () in
register "myunikernel" ~tracing [
   main $ ...

This uses a newer version of the profiling API. You should generally
"opam pin" the #tracing2 branches rather than #tracing to use it.

Note also that it doesn't currently record ring utilisation, so you'll
still need to do some work to get that. You could use the
MProf.Counter interface, in which case the GUI will display it as a
graph over the trace.

- try to reduce the parallelisation to see if some condition there alleviates 
the issue to track it down.
Reducing the maximum number of threads running in parallel reduced CPU 
utilization, and vm was functioning for a much longer time, but the same 
problem occurred at the end.

It might be more useful looking at the code. Please have a look at the function 
"f_thread" in the file uploaded on the following repo:

That's a lot of code to try and distill down a test case.  Try to cut it down 
significantly by building a minimal Ethernet traffic generator that outputs 
frames with a predictable pattern in the frame, and a receiver that will check 
that the pattern is received as expected.

Then you can try out your parallel algorithm variations on the simple Ethernet 
sender/receiver and narrow down the problem without all the other concerns.

Once the bug is tracked down, we can add the sender/receiver into 
mirage-skeleton and use it as a test case to ensure that this functional never 
regresses in the future.  Line rate Ethernet transmission has worked in the 
past, but we never added a test case to ensure it stays working.

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