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what's a huge buffer miss?

Posted on 2012-04-04
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I'm looking in solarwinds at a switch and under buffer misses there are several categories. I'm having alot of misses under the Huge Buffer Misses. Can someone tell me what are Huge Buffer Misses? this is a cisco switch.
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Question by:knfitz
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Nayyar HH (CCIE RS) earned 2000 total points
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In summary - A huge buffer miss occurs when the Interface processor asks for a huge buffer in order to pass a "huge" packet to the RP (route processor or central processor) and there is no buffer space available - packet is likely to be dropped

Detailed explanation from Cisco site

Buffer Misses and Failures

The RP divides its processor memory into pools. Each pool contains a number of memory blocks of equal size. These memory blocks are called buffers.

Buffer Pools

There are six buffer pools:

Small—104 bytes buffers

Middle—600 byte buffers

Big—1524 byte buffers

VeryBig—4520 byte buffers

Large—5024 byte buffers

Huge—18024 byte buffers

For example, if an interface processor needs to pass a 20 byte packet to the RP, it “asks” for a Small buffer. If an interface processor needs to pass a 500 byte packet to the RP, it asks for a Middle buffer, and so forth.

Note: The interface processor must ask for a buffer of a certain size.

When the interface processor asks for a buffer, this occurs:

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 .

When IOS fails to get a Small buffer, it does not drop the packet. It increments the failed counter and falls through to the next level buffer, which is the Middle buffer and requests a buffer there. If it fails to get a Middle buffer, it requests the next level buffer, which is a Big buffer. This process continues until it hits the Huge buffer pool. If it fails to get a Huge buffer, then it drops the packet.

When you use the IBM feature set, a miss almost always generates a failure.

Although the IBM features may be process-switched, the code to get a buffer to pass a packet from an interface to the RP executes at interrupt level.

Buffers can not be created at interrupt level; consequently, a miss queues its request for more buffers to the RP.

Because an additional buffer can not 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, this occurs:

After a buffer failure, the RP has an outstanding request to create more buffers of the appropriate size for the particular pool.

While the RP is servicing 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 when the extra buffers are required.

Essentially, the create buffers operation could take several microseconds, in 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 on buffer creation than on packet processing.

This may cause the RP to begin to drop packets so quickly that performance degrades and sessions are lost.
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