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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7029) Investigate alternative transport protocols for both client and inter-server communications
Date Tue, 21 Apr 2015 13:22:00 GMT


Benedict commented on CASSANDRA-7029:

QUIC is exactly the kind of thing I'm talking about, although it may be more complex than
we need (and we may not want to wait for a Java implementation to appear for general consumption).

bq. Do we even have an idea of how much overhead the transport is for us? If I were to guess
I'd say probably around 10% which is in "not worth optimizing yet" territory for me.

I've yet to obtain datapoints on a server, but when benchmarking CASSANDRA-4718 locally I
found networking calls to be a really significant portion of the CPU time (>30%) for the
small in-memory workloads we were testing. It is likely this would be lower on server-grade
hardware since some of the work would be offloaded to the NIC, but: we know we need many netty
threads to reach peak performance (most likely for interrupt queues, but it seems also because
the cost of the kernel calls to manage connection states overload a single thread) - if we
can drive the entire networking of the box from a single thread (which should be possible
over a UDP protocol) I am optimistic we could see really significant dividends.

The problem with that is that the highest yield improvements for benchmarking are likely to
come by replacing the client connections, since we generally test small clusters, and refactoring
that to support multiple protocols is trickier than for MessagingService. It is probably worth
doing, though, so that we can easily explore all of the ideas we have for efficient transport.

> Investigate alternative transport protocols for both client and inter-server communications
> -------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-7029
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>              Labels: performance
>             Fix For: 3.0
> There are a number of reasons to think we can do better than TCP for our communications:
> 1) We can actually tolerate sporadic small message losses, so guaranteed delivery isn't
essential (although for larger messages it probably is)
> 2) As shown in \[1\] and \[2\], Linux can behave quite suboptimally with regard to TCP
message delivery when the system is under load. Judging from the theoretical description,
this is likely to apply even when the system-load is not high, but the number of processes
to schedule is high. Cassandra generally has a lot of threads to schedule, so this is quite
pertinent for us. UDP performs substantially better here.
> 3) Even when the system is not under load, UDP has a lower CPU burden, and that burden
is constant regardless of the number of connections it processes. 
> 4) On a simple benchmark on my local PC, using non-blocking IO for UDP and busy spinning
on IO I can actually push 20-40% more throughput through loopback (where TCP should be optimal,
as no latency), even for very small messages. Since we can see networking taking multiple
CPUs' worth of time during a stress test, using a busy-spin for ~100micros after last message
receipt is almost certainly acceptable, especially as we can (ultimately) process inter-server
and client communications on the same thread/socket in this model.
> 5) We can optimise the threading model heavily: since we generally process very small
messages (200 bytes not at all implausible), the thread signalling costs on the processing
thread can actually dramatically impede throughput. In general it costs ~10micros to signal
(and passing the message to another thread for processing in the current model requires signalling).
For 200-byte messages this caps our throughput at 20MB/s.
> I propose to knock up a highly naive UDP-based connection protocol with super-trivial
congestion control over the course of a few days, with the only initial goal being maximum
possible performance (not fairness, reliability, or anything else), and trial it in Netty
(possibly making some changes to Netty to mitigate thread signalling costs). The reason for
knocking up our own here is to get a ceiling on what the absolute limit of potential for this
approach is. Assuming this pans out with performance gains in C* proper, we then look to contributing
to/forking the udt-java project and see how easy it is to bring performance in line with what
we can get with our naive approach (I don't suggest starting here, as the project is using
blocking old-IO, and modifying it with latency in mind may be challenging, and we won't know
for sure what the best case scenario is).
> \[1\]
> \[2\];filename=Performance%20Analysis%20of%20Linux%20Networking%20-%20Packet%20Receiving%20(Official).pdf;version=2
> Further related reading:

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