TCP performance problem (hme, 100 Mbps Fast ethernet)

TCP performance problem (hme, 100 Mbps Fast ethernet)

Post by N-Duc » Thu, 05 Nov 1998 04:00:00



Hi All,
  I am just wondering if anyone has seen any kind of TCP
  throughput on Ultra 5 machine. The problem I have is
  the TCP throughput is very low (30 percent of the link
  throughput).

  The test environment:

   2 Ultra_5 machines
       256 MB RAM
       270 MHz CPU
       100 Mbps Ethernet card (hme card)

   These two machines are connected back-to-back and the ethernet
   cards are running in full duplex mode.

   TCP environment:
      Sender and Receiver:
         Buffer size:            32KB
         tcp_xmit_hiwat:         64KB
         tcp_recv_hiwat:         64KB
         tcp_cwnd_max:           64KB

   The test tool used:
      ttcp (from US army lab)

   The test case:
       Receiver side:
           ttcp -r -s -b 32KB

       Sender side:
           ttcp -t -l 100 -b 32KB receiver_IP < input

           packet size == 100 bytes (each write is 100 bytes)
           socket size == 32KB

   The resulted TCP throughput:
       20 Mbps

       This result is only 20% of the link throughput. This is very low.
       I am puzzled. 80% overhead for small packet is very high and
       rather unlikely.

 The same test done on the 10Mbps achieved a throughput of around 7Mbps
 which is 70% of the link throughput.

 Again, I am puzzled as to why the throughput is too low on the hme card.
 May be that the Ultra_5 is not powerful enough to drive the hme card. But
 I kind of doubt it. If anyone has any suggestion, comments, ideas or
 questions, please let me know. Please email me directly. Thanks for your
 help and time.

Regards
Le

 
 
 

TCP performance problem (hme, 100 Mbps Fast ethernet)

Post by Rick Jon » Sat, 07 Nov 1998 04:00:00


:            packet size == 100 bytes (each write is 100 bytes)

This means that you are running into the Nagle algorithm. The first
100byte send will go out, but the next sends will accumulate until
either the first 100 bytes are ACKed, or ~14-15 sends have "queued-up"
as it were.

Generally speaking, trying to get bulk data throughput with 100 bytes
sends is not a hot idea - it might be necessary in certain situations,
but it is still not very pleasant.

If you were running netperf TCP_STREAM (www.netperf.org) you could add
the -D option to your test and disable the Nagle algorithm. While
interesting in the abstract, having to disable nagle in a real
application often implies that the application itself is broken.

on the remote:

$ netserver -p 12865

before:

$ netperf -H <remote> -t TCP_STREAM -- -s 32K -S 32K -m 100

after

$ netperf -H <remote> -t TCP_STREAM -- -s 32K -S 32K -m 100 -D

:        This result is only 20% of the link throughput. This is very
:        low.  I am puzzled. 80% overhead for small packet is very
:        high and rather unlikely.

The TCP connection was possibly stuck waiting for delayed ACK timers
on the receiver.

:  The same test done on the 10Mbps achieved a throughput of around
:  7Mbps which is 70% of the link throughput.

The delayed ACK timers in most (all?) TCP stacks are fixed and do not
change with the link types. So, the impact of sitting waiting and
doing nothing on a 100 Mbit/s link would be much greater than a 10
Mbit/s link.

Of course, it would also be a good idea to check the CPU util on the
machine...

rick jones
--
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to email, or post, but please do not do both...
my email address is raj in the cup.hp.com domain...

 
 
 

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