Re: [PATCH v7 1/4] qrwlock: A queue read/write lock implementation

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

On 11/22/2013 02:14 PM, Linus Torvalds wrote:
> On Fri, Nov 22, 2013 at 11:04 AM, Waiman Long<Waiman.Long@hp.com>  wrote:
>> In term of single-thread performance (no contention), a 256K
>> lock/unlock loop was run on a 2.4GHz and 2.93Ghz Westmere x86-64
>> CPUs. The following table shows the average time (in ns) for a single
>> lock/unlock sequence (including the looping and timing overhead):
>>
>> Lock Type                   2.4GHz      2.93GHz
>> ---------                   ------      -------
>> Ticket spinlock              14.9        12.3
>> Read lock                    17.0        13.5
>> Write lock                   17.0        13.5
>> Queue read lock              16.0        13.4
>> Queue write lock              9.2         7.8
> Can you verify for me that you re-did those numbers? Because it used
> to be that the fair queue write lock was slower than the numbers you
> now quote..
>
> Was the cost of the fair queue write lock purely in the extra
> conditional testing for whether the lock was supposed to be fair or
> not, and now that you dropped that, it's fast? If so, then that's an
> extra argument for the old conditional fair/unfair being complete
> garbage.

Yes, the extra latency of the fair lock in earlier patch is due to the 
need to do a second cmpxchg(). That can be avoided by doing a read 
first, but that is not good for good cache.  So I optimized it for the 
default unfair lock. By supporting only one version, there is no need to 
do a second cmpxchg anymore.

> Alternatively, maybe you just took the old timings, and the above
> numbers are for the old unfair code, and *not* for the actual patch
> you sent out?
>
> So please double-check and verify.
>
>                Linus

I reran the timing test on the 2.93GHz processor. The timing is the 
practically the same. I reused the old one for the 2.4GHz processor.

Regards,
Longman