HP NonStop SSH Reference Manual Performance Considerations • 299
Performance Considerations
Introduction
As the saying goes, "there is no such thing as a free lunch": using SSH2 to encrypt traffic will consume some CPU cycles
on your NonStop host. The natural question "how much CPU resources does encryption consume" has no simple answer,
it will depend on many factors:
• In general:
o How many SSH connections are created—the initial setup of an SSH session involves a public-key
operation, which require some CPU intensive calculations.
o The key sizes used for the public/private key pairs both on the host and on the client—using a more secure
1024 bit key pair will cause more overhead for the initial setup than a 512 bit RSA key pair.
o The selected cipher for bulk encryption—for example, a cipher using 168 bit 3DES will consume more
CPU cycles than a 128 bit ARCFOUR based cipher suite.
• For SFTP traffic:
o The throughput of the transmitted data. How many files of which size are transmitted in which time?
o Type of data read (structured or non-structured files)
o The SFTP client used and the system it is run on.
o Speed of file listings depends on the way an SFTP client makes use of the file attributes received from the
SFTP server.
So there is no general answer to the question; the answer will depend on your individual system use.
However, measurements show that today's NonStop systems aren’t as bad in number crunching (and that’s what
encrypting and decrypting is basically about) as one would think.
The following sections will show the results of some selected measurements. The conclusions drawn from these can be
used to estimate what performance behavior you can expect on your system.
Note: All measurements referred to in this chapter have been performed on a 2 processor S7600. HP provides
performance metrics that allow you to extrapolate those results to other systems. These metrics can be provided upon
request.
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