Re: [REVIEW][PATCH 11/11] ipc/sem: Fix semctl(..., GETPID, ...) between pid namespaces

[Davidlohr Bueso]

On Fri, 30 Mar 2018, Eric W. Biederman wrote:

> Davidlohr Bueso <dave@xxxxxxxxxxxx> writes:
>
> > I ran this on a 40-core (no ht) Westmere with two benchmarks. The first
> > is Manfred's sysvsem lockunlock[1] program which uses _processes_ to,
> > well, lock and unlock the semaphore. The options are a little
> > unconventional, to keep the "critical region small" and the lock+unlock
> > frequency high I added busy_in=busy_out=10. Similarly, to get the
> > worst case scenario and have everyone update the same semaphore, a single
> > one is used. Here are the results (pretty low stddev from run to run)
> > for doing 100,000 lock+unlock.
> >
> > - 1 proc:
> >   * vanilla
> > 	total execution time: 0.110638 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 0.120144 seconds for 100000 loops
> >
> > - 2 proc:
> >   * vanilla
> > 	total execution time: 0.379756 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 0.477778 seconds for 100000 loops
> >
> > - 4 proc:
> >   * vanilla
> > 	total execution time: 6.749710 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 4.651872 seconds for 100000 loops
> >
> > - 8 proc:
> >   * vanilla
> >        total execution time: 5.558404 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 7.143329 seconds for 100000 loops
> >
> > - 16 proc:
> >   * vanilla
> > 	total execution time: 9.016398 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 9.412055 seconds for 100000 loops
> >
> > - 32 proc:
> >   * vanilla
> > 	total execution time: 9.694451 seconds for 100000 loops
> >   * dirty
> > 	total execution time: 9.990451 seconds for 100000 loops
> >
> > - 64 proc:
> >   * vanilla
> > 	total execution time: 9.844984 seconds for 100032 loops
> >   * dirty
> > 	total execution time: 10.016464 seconds for 100032 loops
> >
> > Lower task counts show pretty massive performance hits of ~9%, ~25%
> > and ~30% for single, two and four/eight processes. As more are added
> > I guess the overhead tends to disappear as for one you have a lot
> > more locking contention going on.
>
> Can you check your notes on the 4 process case?  As I read the 4 process
> case above it is ~30% improvement.  Either that is a typo or there is the
> potential for quite a bit of noise in the test case.

Yeah, sorry that was a typo. Unlike the second benchmark I didn't have
this one automated but it's always the vanilla kernel that outperforms
the dirty.

Thanks,
Davidlohr