mutex: Queue mutex spinners with MCS lock to reduce cacheline contention [Waiman Long]

DateMon, 15 Apr 2013 10:37:58 -0400

The current mutex spinning code (with MUTEX_SPIN_ON_OWNER option turned
on) allow multiple tasks to spin on a single mutex concurrently. A
potential problem with the current approach is that when the mutex
becomes available, all the spinning tasks will try to acquire the
mutex more or less simultaneously. As a result, there will be a lot of
cacheline bouncing especially on systems with a large number of CPUs.

This patch tries to reduce this kind of contention by putting the
mutex spinners into a queue so that only the first one in the queue
will try to acquire the mutex. This will reduce contention and allow
all the tasks to move forward faster.

The queuing of mutex spinners is done using an MCS lock based
implementation which will further reduce contention on the mutex
cacheline than a similar ticket spinlock based implementation. This
patch will add a new field into the mutex data structure for holding
the MCS lock. This expands the mutex size by 8 bytes for 64-bit system
and 4 bytes for 32-bit system. This overhead will be avoid if the
MUTEX_SPIN_ON_OWNER option is turned off.

The following table shows the jobs per minute (JPM) scalability data
on an 8-node 80-core Westmere box with a 3.7.10 kernel. The numactl
command is used to restrict the running of the fserver workloads to
1/2/4/8 nodes with hyperthreading off.

+-----------------+-----------+-----------+-------------+----------+
|  Configuration  | Mean JPM  | Mean JPM  |  Mean JPM   | % Change |
|ge 1100 - 2000   |
+-----------------+------------------------------------------------+
| 8 nodes, HT off |  227972   |  227237   |   305043    |  +34.2%  |
| 4 nodes, HT off |  393503   |  381558   |   394650    |   +3.4%  |
| 2 nodes, HT off |  334957   |  325240   |   338853    |   +4.2%  |
| 1 node , HT off |  198141   |  197972   |   198075    |   +0.1%  |
+-----------------+------------------------------------------------+
|ge 200 - 1000   |
+-----------------+------------------------------------------------+
| 8 nodes, HT off |  282325   |  312870   |   332185    |   +6.2%  |
| 4 nodes, HT off |  390698   |  378279   |   393419    |   +4.0%  |
| 2 nodes, HT off |  336986   |  326543   |   340260    |   +4.2%  |
| 1 node , HT off |  197588   |  197622   |   197582    |    0.0%  |
+-----------------+-----------+-----------+-------------+----------+
At low user range 10-100, the JPM differences were within +/-1%. So
they are not that interesting.

The fserver workload uses mutex spinning extensively. With just
the mutex change in the first patch, there is no noticeable change
in performance.  Rather, there is a slight drop in performance. This
mutex spinning patch more than recovers the lost performance and show
a significant increase of +30% at high user load with the full 8 nodes.
Similar improvements were also seen in a 3.8 kernel.

The table below shows the %time spent by different kernel functions
as reported by perf when running the fserver workload at 1500 users
with all 8 nodes.

+-----------------------+-----------+---------+-------------+
|        Function       |  % time   | % time  |   % time    |
|                       | w/o patch | patch 1 | patches 1&2 |
+-----------------------+-----------+---------+-------------+
| __read_lock_failed    |  34.96%   | 34.91%  |   29.14%    |
| __write_lock_failed   |  10.14%   | 10.68%  |    7.51%    |
| mutex_spin_on_owner   |   3.62%   |  3.42%  |    2.33%    |
| mspin_lock            |    N/A    |   N/A   |    9.90%    |
| __mutex_lock_slowpath |   1.46%   |  0.81%  |    0.14%    |
| _raw_spin_lock        |   2.25%   |  2.50%  |    1.10%    |
+-----------------------+-----------+---------+-------------+
The fserver workload for an 8-node system is dominated by the
contention in the read/write lock. Mutex contention also plays a
role. With the first patch only, mutex contention is down (as shown by
the __mutex_lock_slowpath figure) which help a little bit. We saw only
a few percents improvement with that.

By applying patch 2 as well, the single mutex_spin_on_owner figure is
now split out into an additional mspin_lock figure. The time increases
from 3.42% to 11.23%. It shows a great reduction in contention among
the spinners leading to a 30% improvement. The time ratio 9.9/2.33=4.3
indicates that there are on average 4+ spinners waiting in the spin_lock
loop for each spinner in the mutex_spin_on_owner loop. Contention in
other locking functions also go down by quite a lot.

The table below shows the performance change of both patches 1 & 2 over
patch 1 alone in other AIM7 workloads (at 8 nodes, hyperthreading off).

+--------------+---------------+----------------+-----------------+
|   Workload   | mean % change | mean % change  | mean % change   |
|              | 10-100 users  | 200-1000 users | 1100-2000 users |
+--------------+---------------+----------------+-----------------+
| alltests     |      0.0%     |     -0.8%      |     +0.6%       |
| five_sec     |     -0.3%     |     +0.8%      |     +0.8%       |
| high_systime |     +0.4%     |     +2.4%      |     +2.1%       |
| new_fserver  |     +0.1%     |    +14.1%      |    +34.2%       |
| shared       |     -0.5%     |     -0.3%      |     -0.4%       |
| short        |     -1.7%     |     -9.8%      |     -8.3%       |
+--------------+---------------+----------------+-----------------+
The short workload is the only one that shows a decline in performance
probably due to the spinner locking and queuing overhead.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>