Endace Measurement Systems Limited
http://www.endace.com
EDM01.05-04r1 DAG 3.6GE User Manual
Copyright, all rights reserved.
17
Revision 7. 22 September 2005.
4.2 High Load Performance
Description
As the DAG 3.6GE card captures packets from the network link, it writes
a record for each packet into a large buffer in the host PC’s main memory.
Avoiding
packet loss
In order to avoid packet loss, the user application reading the record, such
as
dagsnap
, must be able to read records out of the buffer faster than they
arrive, otherwise the buffer eventually fills, and packet records are lost.
For Linux and FreeBSD, when the PC buffer becomes full, the message:
kernel: dagN: pbm safety net reached
is displayed on the PC screen, and printed to
log /var/log/messages
.
The “Data capture” LED also goes out. This may be visibly indicated as
flashing or flickering.
Detecting
packet losses
Until some data is read out of the buffer to free some space, any arriving
packets subsequently are discarded by the DAG 3.6GE card.
Any loss can be detected in-band by observing the Loss Counter
lctr
field of the Extensible Record Format [ERF]. The Endace ERF is detailed
in Chapter 6 of this document.
Increasing
buffer size
The host PC buffer can be increased to deal with bursts of high traffic load
on the network link.
By default the dagmem driver reserves 32MB of memory per DAG card in
the system. Capture at OC-12/STM-4 (622Mbps) rates and above may
require a larger buffer.
128MB or more is suggested for Linux/FreeBSD.
For the DAG 3.6GE card Windows operating system the upper limit is
32MB.
In Debian Linux the amount of memory reserved is changed by editing the
file
/etc/modules
.
# For DAG 3.x, default 32MB/card
dagmem
#
# For DAG 4.x or 6.x, use more memory per card, E.G.
# dagmem dsize=128m
The option
dsize
sets the amount of memory used per DAG card in the
system.
The value of
dsize
multiplied by the number of DAG cards must be less
than the amount of physical memory installed, and less than 890MB.
