rtld optimizations

[David Naylor]

On Wednesday 26 January 2011 06:49:11 Alexander Kabaev wrote:
> On Tue, 25 Jan 2011 21:40:42 -0500
> 
> Mark Saad <nonesuch at longcount.org> wrote:
> > Hello Hackers
> > 
> > The NetBSD folks have a nice improvement with the rtld-elf subsystem,
> > known as "Negative Symbol Cache" .
> > 
> > http://blog.netbsd.org/tnf/entry/netbsd_runtime_linker_gains_negative
> > 
> >  Roy Marples roy@ has a simple write up of the change.
> > 
> > I took the basic idea from FreeBSD, but improved the performance
> > drastically. Basically, the huge win is by caching both breadth and
> > depth of the needed/weak symbol lookup.
> > Easiest to think of a,b,c,d as a matrix and FreeBSD just cache a row
> > where we cache both rows and columns.
> > 
> > Has anyone looked into porting the changes back to FreeBSD ?  The
> > improvement on load time for things like firefox, openoffice, and java
> > is huge on NetBSD. It looks like this change could improve load times
> > on FreeBSD in the same ways.
> 
> This is a second time someone posts this to public mailing list and
> curiously enough is a second time it suggested that someone else is to
> do the investigation. From the quick look, the commit in question is
> more or less a direct rip-off of Donelists we had for ages and as
> such is completely over-hyped. The only extra quirk that said commit
> does is an optimization of a dlsym() call, which is hardly ever in
> critical performance path. Said optimization is trivial and easy to
> try. Here you have it:
> http://people.freebsd.org/~kan/rtld-symlook-depth.diff
> 
> Since it only applies to dlsym, it only affects programs that are heavy
> plugin users, which I suppose is the category OpenOffice and firefox
> both fall into. Care to do some benchmarks with and without the
> patch and report the results? I frankly doubt that you'll see any
> noticeable difference compared to our stock rtld's performance.

I benchmarked the impact said patch has on the boot-time of my system.  I 
timed the boot-time to when KDE launches autostart programs and once all 
programs have loaded (I run a few extra programs, such as amarok).  The latter 
measure requires human action thus it has extra, human, variance in its 
measure.  

I tried an older version of rtld (about 2 months old), current version of rtld 
and the new (patched) rtld.  I ran each test three times.  There was little 
variance in the tests and I am confident that there is no difference between 
the different rtld versions and my boot-time.  

Here is a summary of my boot times (in seconds).  First measure is when KDE 
autostarts programs, the latter is when I determined when all programs had 
launched.  
rtld-old: 69 96
rtld:     69 94
rtld-new: 69 94

Please note that kernel boot time is approximately 10 seconds and kdm is 
delayed by about 10 seconds thus 20 seconds can be removed from above numbers 
to determine non-kernel boot wall-time.  

I would like to add that the blog entry claims a substantial improvement for 
some use cases.  Is it not worth to optimise these fringe cases as one mans 
fringe case is another mans normal case (or woman as one prefers)?  
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 196 bytes
Desc: This is a digitally signed message part.
Url : http://lists.freebsd.org/pipermail/freebsd-hackers/attachments/20110127/edd233fe/attachment.pgp