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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> On Wed, 20 Oct 2004, Keir Fraser wrote:
> 
> > Yes, so the answer is that we 'flip' about as often as the current
> > code emulates (e.g., about 2.5 million flips/emulations to boot a Red
> > Hat system).
> >
> > The performance is very bad, but the flipping code is both simpler and
> > more robust than emulation so I will go with the new technique.
> 
> 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.

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. :-(

> > But I will still print a warning message from Linux to tell the user to
> > remove /lib/tls.
> 
> I've heard that this will actually break some things, like
> db4 locking and the RPM database consistency...

Strange: do the non /lib/tls libraries deal with thread-local state
in some incompatible way?

One fix is to distribute an alternative /lib/tls that is built with
'virtualisation-happy' GS accesses: e.g.,
   mov %%gs:0,%0
   mov <offset>(%0),%1
So we get the same externally-observable semantics (db4 and so on
shouldn't break) but this won't cause Xen to choke.

This may need to be one part of a general move to having two versions
of many executables. GCC now defaults to producing -ve accesses to
thread-local state --- if lots of apps start using the new 'thread'
keyword then this is going to cause problems for Xen unless versions
are built with the approriate GCC command-line switch to produce
virtualisation-happy code (no -ve accesses).

 -- Keir


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