user:sys time ratio

Robert Watson rwatson at
Sun Nov 30 18:06:00 PST 2003

On Sun, 30 Nov 2003, Jeff Roberson wrote:

> On Sun, 30 Nov 2003, Colin Percival wrote:
> >    Robert Watson suggested that I compare performance from UP and SMP kernels:
> >
> > # /usr/bin/time -hl sh -c 'make -s buildworld 2>&1' > /dev/null
> >                    Real          User        Sys
> >    UP kernel       38m33.29s     27m10.09s   10m59.15s
> >       (retest)     38m33.18s     27m04.40s   11m05.73s
> >    SMP w/o HTT     41m01.54s     27m10.27s   13m29.82s
> >       (retest)     39m47.50s     27m08.05s   12m12.20s
> >    SMP w/HTT       42m17.16s     28m12.82s   14m04.93s
> >       (retest)     44m09.61s     28m15.31s   15m44.86s
> >
> >    That enabling HTT degrades performance is not surprising, since I'm not
> > passing the -j option to make; but a 5% performance delta between UP and
> > SMP kernels is rather surprising (to me, at least), and the fact that the
> > system time varies so much on the SMP kernel also seems peculiar.
> So you have enabled SMP on a system with one physical core and two
> logical cores?  Looks like almost a 20% slowdown in system time with the
> SMP kernel.  It's too bad it's enabled by default now.  I suspect that
> some of this is due to using the lock prefix on P4 cores.  It makes the
> cost of a mutex over 300 cycles vs 50.  It might be interesting to do an
> experiment without HTT, but with SMP enabled and the lock prefix
> commented out. 

It would be interesting to compare the relative costs of:

(a) De-inlining of mutex calls so that you can "plug" mutex
    implementations at link-time, with SMP and non-SMP versions of mutex
    operations, perhaps indicating with a loader which approach to take. 

(b) Using inlined SMP mutexes for all kernels. 

Would also be very interesting to have mutex contention information, and
in particular, to know how much time was spent contending on Giant during
the build...

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at      Senior Research Scientist, McAfee Research

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