On Mon, Jul 01, 2013 at 02:49:34PM +0530, Raghavendra KT wrote:
> On Sun, Jun 23, 2013 at 11:23 PM, Raghavendra KT
> <raghavendra.kt.li...@gmail.com> wrote:
> >
> >
> > On Wed, Jun 12, 2013 at 9:10 PM, Paul E. McKenney
> > <paul...@linux.vnet.ibm.com> wrote:
> >>
> >> Breaking up locks is better than implementing high-contention locks, but
> >> if we must have high-contention locks, why not make them automatically
> >> switch between light-weight ticket locks at low contention and queued
> >> locks at high contention? After all, this would remove the need for
> >> the developer to predict which locks will be highly contended.
> >>
> >> This commit allows ticket locks to automatically switch between pure
> >> ticketlock and queued-lock operation as needed. If too many CPUs are
> >> spinning on a given ticket lock, a queue structure will be allocated
> >> and the lock will switch to queued-lock operation. When the lock becomes
> >> free, it will switch back into ticketlock operation. The low-order bit
> >> of the head counter is used to indicate that the lock is in queued mode,
> >> which forces an unconditional mismatch between the head and tail counters.
> >> This approach means that the common-case code path under conditions of
> >> low contention is very nearly that of a plain ticket lock.
> >>
> >> A fixed number of queueing structures is statically allocated in an
> >> array. The ticket-lock address is used to hash into an initial element,
> >> but if that element is already in use, it moves to the next element. If
> >> the entire array is already in use, continue to spin in ticket mode.
> >>
> >> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> >> [ paulmck: Eliminate duplicate code and update comments (Steven Rostedt).
> >> ]
> >> [ paulmck: Address Eric Dumazet review feedback. ]
> >> [ paulmck: Use Lai Jiangshan idea to eliminate smp_mb(). ]
> >> [ paulmck: Expand ->head_tkt from s32 to s64 (Waiman Long). ]
> >> [ paulmck: Move cpu_relax() to main spin loop (Steven Rostedt). ]
> >> [ paulmck: Reduce queue-switch contention (Waiman Long). ]
> >> [ paulmck: __TKT_SPIN_INC for __ticket_spin_trylock() (Steffen Persvold).
> >> ]
> >> [ paulmck: Type safety fixes (Steven Rostedt). ]
> >> [ paulmck: Pre-check cmpxchg() value (Waiman Long). ]
> >> [ paulmck: smp_mb() downgrade to smp_wmb() (Lai Jiangshan). ]
> >>
> > [...]
> >
> > I did test this on 32 core machine with 32 vcpu guests.
> >
> > This version gave me around 20% improvement fro sysbench and 36% improvement
> > for ebizzy, for 1x commit though other overcommited results showed
> > degradation. I have not tested Lai Jiangshan's patches on top of this yet.
> > Will report any findings.
>
> Sorry for late report.
Not a problem, thank you for running these numbers!
> With Lai's patch I see few percentage of improvement in ebizzy 1x and
> reduction in degradation in dbench 1x.
OK, good! But my guess is that even pushing the lock-acquisition
slowpath out of line, we still would not reach parity for the less-good
results. Still seems like I should add Lai Jiangshan's patches
and post them somewhere in case they are helpful in some other context.
Thanx, Paul
> But over-commit degradation seem to still persist. seeing this, I
> feel it is more of qmode overhead itself for large guests,
>
> +---+-----------+-----------+-----------+------------+-----------+
> ebizzy (rec/sec higher is better)
> +---+---+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improvement
> +---+-----------+-----------+-----------+------------+-----------+
> 1x 5574.9000 237.4997 7851.9000 148.6737 40.84378
> 2x 2741.5000 561.3090 1620.9000 410.8299 -40.87543
> 3x 2146.2500 216.7718 1751.8333 96.5023 -18.37702
> +---+-----------+-----------+-----------+------------+-----------+
> +---+-----------+-----------+-----------+------------+-----------+
> dbench (throughput higher is better)
> +---+-----------+-----------+-----------+------------+-----------+
> base stdev patched stdev %improvement
> +---+-----------+-----------+-----------+------------+-----------+
> 1x 14111.5600 754.4525 13826.5700 1458.0744 -2.01955
> 2x 2481.6270 71.2665 1549.3740 245.3777 -37.56620
> 3x 1510.2483 31.8634 1116.0158 26.4882 -26.10382
> +---+-----------+-----------+-----------+------------+-----------+
>
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