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Date: Wed, 18 Mar 2015 14:04:52 +0300
From: Solar Designer <solar@...nwall.com>
To: john-dev@...ts.openwall.com
Subject: Re: [GSoC] building JtR for MIC

On Wed, Mar 18, 2015 at 01:38:45PM +0300, Solar Designer wrote:
> On Wed, Mar 18, 2015 at 05:27:45PM +0800, Lei Zhang wrote:
> > 2. There're two kind of numbers in the output, real and virtual. What's their difference?
> 
> This is in the FAQ:
> 
> Q: What are the "real" and "virtual" c/s rates as reported by "--test"
> (on Unix-like operating systems)?
> A: These correspond to real and virtual (processor) time, respectively.
> The two results would differ when the system is under other load, with
> the "virtual" c/s rate indicating roughly what you could expect to get
> from the same machine if it were not loaded.
> 
> ... but this FAQ entry is outdated and needs to be revised.  As written,
> it applies to single-threaded builds only (I wrote it before we
> introduced OpenMP support).  Clearly, the numbers also differ greatly in
> multi-threaded builds.  When running a multi-threaded build on an
> otherwise idle system, the "real" speed will be roughly equal to the
> "virtual" speed times the number of threads.

I've just revised this FAQ entry to be:

Q: What are the "real" and "virtual" c/s rates as reported by "--test"?
A: These correspond to real and virtual (processor) time, respectively.
When running single-threaded, the two results are normally almost the
same, but the "real" c/s rate becomes smaller when the system is under
other load, with the "virtual" c/s rate indicating roughly what you
could expect to get from the same system if it were not loaded.  When
running multi-threaded, the "real" c/s rate is normally much higher than
the "virtual" c/s rate, with the latter roughly indicating performance
of one thread on an otherwise idle system.

Alexander

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