Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

[Waiman Long <waiman.long@hp.com>]

On 07/22/2013 06:34 AM, Ingo Molnar wrote:
> * Waiman Long<waiman.long@hp.com>  wrote:
>
>> I had run some performance tests using the fserver and new_fserver
>> benchmarks (on ext4 filesystems) of the AIM7 test suite on a 80-core
>> DL980 with HT on. The following kernels were used:
>>
>> 1. Modified 3.10.1 kernel with mb_cache_spinlock in fs/mbcache.c
>>     replaced by a rwlock
>> 2. Modified 3.10.1 kernel + modified __read_lock_failed code as suggested
>>     by Ingo
>> 3. Modified 3.10.1 kernel + queue read/write lock
>> 4. Modified 3.10.1 kernel + queue read/write lock in classic read/write
>>     lock behavior
>>
>> The last one is with the read lock stealing flag set in the qrwlock
>> structure to give priority to readers and behave more like the classic
>> read/write lock with less fairness.
>>
>> The following table shows the averaged results in the 200-1000
>> user ranges:
>>
>> +-----------------+--------+--------+--------+--------+
>> |  Kernel         |    1   |    2   |    3   |   4    |
>> +-----------------+--------+--------+--------+--------+
>> | fserver JPM     | 245598 | 274457 | 403348 | 411941 |
>> | % change from 1 |   0%   | +11.8% | +64.2% | +67.7% |
>> +-----------------+--------+--------+--------+--------+
>> | new-fserver JPM | 231549 | 269807 | 399093 | 399418 |
>> | % change from 1 |   0%   | +16.5% | +72.4% | +72.5% |
>> +-----------------+--------+--------+--------+--------+
> So it's not just herding that is a problem.
>
> I'm wondering, how sensitive is this particular benchmark to fairness?
> I.e. do the 200-1000 simulated users each perform the same number of ops,
> so that any smearing of execution time via unfairness gets amplified?
>
> I.e. does steady-state throughput go up by 60%+ too with your changes?

For this particular benchmark, there are interplay of different locks 
that determine the overall performance of the system. Yes, I got steady 
state performance gain of 60%+ with the qrwlock change with the modified 
mbcache.c. Without the modified mbcache.c file, the performance gain 
drop to 20-30%. I am still trying to find out more about the performance 
variations in different situations.

Regards,
Longman