Saturday, November 9, 2013

Journeys in Networks

After stress testing my pet project, JStringServer, I initially thought I'd made a big boo-boo as the performance was appalling. However, it turned out that my home router was not up to the job. So, on a friend's recommendation, I bought a TP-Link 5-Port Gigbait Desktop Switch. Performance was better but not that great. A quick Google showed I needed Cat 6 cables to make full use of it. D'oh.

OK, so after a few trips to my local electronics store, I set up a newish 1.8GHz i7 Mac Book Air (with a USB network adaptor) and an old 2009 Mac Book Pro trying to hammer my 16 core Linux desktop running my Java NIO server code.

The strategy JStringServer was using was one-thread-does-everything (code here). That is, it listens to the selector associated with the server socket, associates any clients who have connected with a second selector dedicated to clients, checks this second selector for any activity and services them. Although htop shows this thread to be very busy, the rest of the system was doing next to nothing.

The clients were averaging about 6 000 calls per second between them. Now, with a back-of-a-beer-mat calculation,  a payload of about 10 000 bytes (ignoring the 2 bytes return from the server) and 6 000 calls per second, this means the network was taking something like 480 gigabits/second (10 000 * 6 000 * 8 / 1 000 000). Not bad, but why not better?


Since JStringServer is currently using just TCP, it turns out that there is a lot of overhead on the network acknowledging the packets the client is sending the server.

If we run tcpdump and capture its output thus:

$ sudo tcpdump -nn host and -i p7p1 > tcpdump_jstringserver_2machines_normalUse.txt 

we see as many packets are going to the server ( as the other way:

$ grep -c " >" tcpdump_jstringserver_2machines_normalUse.txt 
$ grep -c ">" tcpdump_jstringserver_2machines_normalUse.txt 

So, the figure of 480 gigabits/second seems to be as good as we're going to get on this particular hardware using TCP (2 * 480 ~ 1 gigabit limit).

The return packets that carry the acknowledgement can also carry data [1]. There show up in tcpdump as [P.] where P stands for a PUSH of data and '.' represents an acknowledgement [2]. But since in this particular example, our server replies with very terse responses compared to very verbose requests, this doesn't save us much. A lot of packets are wasted just acknowledging:

$ grep -c -P " \>.* \[\.\], ack \\d\\d" tcpdump_jstringserver_2machines_normalUse.txt 

That's about 70% of all traffic from the server to the client.

Another problem with TCP is the handshake uses a lot of packets (as a percentage of the total package used in a connection).

For SYN:

$ grep -c " \[S\]" tcpdump_jstringserver_2machines_normalUse.txt 


$ grep -c " \[S\.\]" tcpdump_jstringserver_2machines_normalUse.txt 

and for ACK (handshake only):

$ grep -c -P "\[\.\], ack 1," tcpdump_jstringserver_2machines_normalUse.txt 

That totals 17% of the total traffic. In this particular example, this connection pooling would solve this. 

[1] Does tcp send a syn-ack on every packet or only on the first connection StackOverflow.
[2] tcpdump man pages

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