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SubjectRe: [PATCH v4 3/3] qspinlock, x86: Add x86 specific optimization for 2 contending tasks
On 02/21/2014 12:28 PM, Peter Zijlstra wrote:
> On Mon, Feb 17, 2014 at 03:41:24PM -0500, Waiman Long wrote:
>> + struct {
>> + u8 lock; /* Lock bit */
>> + u8 wait; /* Waiting bit */
>> + u16 qcode; /* Queue code */
>> + };
> 16 bit code would result in 14 bits for the CPU number, that's only 16k,
> I think SGI actually had a machine with that many CPUs in.
>

If NR_CPUS >= 16k, the 2-task optimized code path will be disabled.
However, with that many CPUs, it is high spinlock contention behavior
that is more important than a bit better performance at low contention
level.

-Longman


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