AltiVec in the kernel

Linas Vepstas linas at
Thu Jul 20 04:57:59 EST 2006

On Wed, Jul 19, 2006 at 08:38:21PM +0200, Johannes Berg wrote:
> On Thu, 2006-07-20 at 04:19 +1000, Paul Mackerras wrote:
> > Linas Vepstas writes:
> > 
> > > But perhaps, in principle, couldn't one run four independent streams 
> > > in parallel?  Thus, for example, on an SSL-enabled web server, one 
> > > could service multiple encryption/decryption threads at once. 
> > 
> > Generally that would work.  If one had 4 separate streams to compute a
> > SHA1 of, one could do all 4 at once with altivec.  It would have to be
> > 4 separate streams though, not 4 parts of a single stream.
> I'd think it'd be pretty hard to get a real benefit from this because
> the data is going to come from 4 totally different places, hence you
> can't just load a single vector register and get data for all 4
> streams...

Dohh. Right. I actually thought that while writing the email, and then
it eveporated from my head before I hit the send button.  One would 
have to copy the incoming data into vectors, and the mem access latency
would probably overwhelm the performance gain (per "hot cache" as Paul


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