ruseng@iesd.dk (Carsten Ruseng Jakobsen) (05/10/89)
Major change of TCP performance when sending packets with sizes close
to 4096 and 8192 bytes.
While evaluating the performance of SUN-RPC, I detected the TCP
protocol slowing dramatical down when sending packets with sizes close
to the magical 4096 and 8192 bytes. The tests I ran was a simple
RPC-program with a client calling it's server with an array of some
specified size. The server routine was a null routine, doing nothing.
When the array-size was between (4045 - 4952) and (8090 - 8960) the
time used for the communication was 5 - 10 times slower than expected.
It was only the TCP protocol that suddenly had this overhead, the UDP
protocol seemed unaffected, which makes me suspect the implementation
of TCP. I tried to figure out what happened, by monitoring the net
with "traffic". I observed that
1] there was NO collisions on the ethernet (10Mbit)
2] there were NO errors detected.
3] none of the cpu's were working very much
4] none of the machines were swapping
5] the number of context-switches weren't very high.
6] #packets/sec was 25% - 50% of what I saw when
sending eg 512 bytes.
From a profile of the program it can be seen that it is in the
function _write called from write_tcp that 78% of the time is used. In
the "normal" cases i expected 10% there.
The experiment was performed on 3 sun3/60 workstations running
SUNOS-4.0.1. They were totally isolated from anything else in the
world. They were not running yp, neither syslog or cron. Except from
the operating system, the only processes running was the server and
the client processes.
The tests I performed was to measure the time used for 100 RPC calls.
For each argument size the time for the 100 calls were measured 20
times, and an average calculated. The RPC-call was made using TCP and
UDP protocols. Further was it performed with server and client on the
same machine, and server and client on two different machines. The
client called the server with an argument of N bytes. The results I
got was:
-----------------------------------------------------------------------------
argument size| T C P | UDP
N | local remote | local remote
--------------------------------------------------------------------------
0 | 6,9 7,1 ||| 5,6 5,8
512 | 8,2 9,1 ||| 6,7 7,6
1024 | 10,6 11,6 ||| 7,9 9,1
2048 | 14,9 15,7 ||| 10,7 12,8
4000 | 23,6 24,2 ||| 15,6 20,0
* 4096 |174,5 199,8 ||| 16,1 20,3
8000 | 40,2 40,8 ||| 26,2 34,5
* 8192 |182,8 199,9 ||| 26,8 35,1
| |||
-------------|---------------------|||-------------------------------------
local = server-process and client-process reside on the same machine.
remote= server and client reside on 2 different nodes.
If you have any suggestions which explains these results, then please *please*
e-mail them to me.
--
------------------------------------------------------------------------------
| Carsten Ruseng Jakobsen,ruseng@iesd.dk(uucp), {...}!mcvax!dkuug!iesd!ruseng|
| Systems programmer |
| _ / |
| Department of Mathematics and Computer Science - / ( |
| Aalborg University (AUC) / | \ |
| DENMARK A U C |
------------------------------------------------------------------------------