Foundry NetIron M2404C and M2404F Metro Access Switches
Troubleshooting and Monitoring (Rev. 03)
Buffer Tuning
© 2008 Foundry Networks, Inc.
Page 49 of 55
Buffer Tuning
The OS provides a special buffer tuning feature that allows the user to modify the way the device
allocates buffers from its available memory, and helps prevent packet drops during a temporary
burst of traffic.
Table 23
lists the buffer tuning commands.
The number of buffers “
in free list
” is the number of available buffers. When a buffer request
comes in, a buffer from the “
in free list
” is allocated. If there are no buffers available there is a
buffer failure and the packet is dropped. When the buffer pool manager process detects a buffer
failure, it “creates” a new buffer to avoid future failures. The device does not create a new buffer if
the number of “
in free list
” buffers equals the maximum allowed buffers.
When the Routing Processor (RP) requests a buffer, the following takes place:
•
If a free buffer exists within the requested pool, the buffer is granted. Otherwise, the
request generates a “
miss
” and the buffer algorithm tries to “
create
” more buffers for that
pool.
•
If additional buffers cannot be created, the request generates a “
failure
” and the packet is
dropped.
•
Buffers cannot be created at interrupt level; consequently, a “
miss
” queues its request for
more buffers to the RP.
•
Because an additional buffer cannot be created on the spot, the buffer request fails, and the
packet is dropped.
Buffer failures are one of the most common reasons for packet drops. When packet drops occur
because of buffer failure, the following things happen:
•
After a buffer failure, the RP has an outstanding request to “
create
” more buffers of the
appropriate size for the particular pool.
•
During the time it takes for the RP to service the “
create buffers
” request, there may be
additional failures in the pool.
•
The RP may even fail to create more buffers because of memory constraints in the system
at the time the extra buffers are required.
•
Essentially, the “
create buffers
” operation could take several microseconds during which
packets are continually dropped because of the buffer shortage.
•
In addition, if buffers are used as quickly as they are created, the RP could be forced to
spend more time creating buffers than processing packets.
•
This may cause the RP to begin dropping packets so quickly that performance degrades
and sessions are lost.
Table 23: Buffer Tuning Commands
Command
Description
buffers
Changes the permanent buffer settings.
show buffers
Displays the current buffer tuning settings and usage.