Re: [Xen-devel] Re: NPTL/TLS "emulation" idea (fwd)

[Rik van Riel <riel@xxxxxxxxxx>]

On Wed, 20 Oct 2004, Keir Fraser wrote:

> > How bad is the performance?  A 10% performance penalty, 30% ?
> 
> My benchmark is 'time /bin/ls -R /usr/lib >/dev/null' with a warm
> buffer cache.
>
> With no /lib/tls this takes ~180ms. With emulation it takes
> ~300ms. With the new technique it's ~390ms -- so about a further 30%
> slowdown, or 115% slowdown overall.

Considering how system heavy this workload is, that's
probably not even that bad.

> The extra cost is due to the fact that we fault nearly twice as often
> because -ve and +ve accesses seem pretty neatly interleaved. So we
> fault on all GS accesses, rather than just the -ve ones. :-(

IIRC the glibc private data is accessed once per system
call, or possibly on both system call entrance and exit.

Less system heavy tasks probably do not have an overhead
as bad as ls -R.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan



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