[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Hashing function for PSAMP

Are router development folks happy with the computational requirement
for BOB (or CRC32) to be computed on every packet, if it is used as the
selection hash?

The short answer is NO ):

A lot depends on where you imagine this will be deployed.
On a pure s/w edge router there will be a measurable
headline performance hit with either of these, but perhaps
that does not matter in that environment. On a hardware
/microcoded core router, I would say that the chances
of getting either in the main path of existing hardware
are for all practical purposes zero.

If you were thinking that they would be run after
some primary sampler at a relatively low packet rate
in the export process, then there is less of an issue.

You canot use the existing CRC32 hardware to perform the
hash. So both BOB and CRC32 would need new hardware or
would need to be performed in software/microcode.

Software CRC32 implementations trade lookup table space
for compute cycles. Both of these resources are in
critically short supply on a high-end forwarder.
Even if you did find the resources (and on many
implementations they would simply not be available)
the result would be a crippling hit on headline
forwarding performance.

I have not done a detailed comparison of BOB and CRC32
execution speeds, but my first impression is that BOB
takes even more cycles to execute than CRC32 although
perhaps not if you take into account the cache stalls
in doing the table fetch.

- Stewart

to unsubscribe send a message to psamp-request@ops.ietf.org with
the word 'unsubscribe' in a single line as the message text body.
archive: <http://ops.ietf.org/lists/psamp/>