ipsec null encryption slower than AES / was Re: 2.6 IPSec Throughput puzzle

[bert hubert <ahu@ds9a.nl>]

[added James Morris, resident crypto api guru, to the CC list] 

On Wed, Dec 29, 2004 at 03:50:34PM -0800, Shekhar Kshirsagar wrote:
> I played with oprofile for a while, and it seems that in case of null
> encryption, scatterwalk related code takes most of the cpu cycles.

Odd - the 'scatterlist' is what people were most proud of in the ipsec work
in 2.6. I recall that it was implemented as a natural way to represent the
encryption needs of ipsec. 

>From your numbers below it is clear all ipsec benchmarks have maxed out your
CPU, but aes/sha1 still has some hits in default_idle. Is this an SMP
system?

scatterwalk_done consists of crypto_kunmap, which in turn calls
kunmap_atomic (inline), which is defined as nothing sometimes and as a real
function otherwise which is not likely to be inlined, so should show up in
the profile if it were a large load.

The other part of scatterwalk_done is scatterwalk_page_done, which looks
like it could cause further (inlined) work.

But, in the end, I can't really help you further. All this scatterlist stuff
looks like something is really badly tuned for null-encryption and
well-tuned for encryption.

> Is there any place where I can find documentation about what exactly
> scatterwalk does?

http://www.certconf.org/presentations/2004/Tuesday/TS2.pdf - the concept is
called 'the scatterlist'.

http://lwn.net/Articles/14010/?format=printable is also nice.

Good luck!

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