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Re: [Xen-devel] [PATCH 04/11] qspinlock: Extract out the exchange of tail code word

Il 18/06/2014 17:46, Waiman Long ha scritto:

The #1 patch is nice by itself - as it lays out the foundation of the
MCS-similar code - and if Ingo decides he does not want this pending
byte-lock bit business - it can be easily reverted or dropped.

The pending bit code is needed for performance parity with ticket
spinlock for light load. My own measurement indicates that the queuing
overhead will cause the queue spinlock to be slower than ticket spinlock
with 2-4 contending tasks. The pending bit solves the performance
problem with 2 contending tasks, leave only the 3-4 tasks cases being a
bit slower than the ticket spinlock which should be more than
compensated by its superior performance with heavy contention and
slightly better performance with no contention.

Note that this patch is not related to the pending bit, only to the trylock bit which is already in patch 1. It serializes two previously-parallel checks for transitions. This is why I thought it could already belong in patch 1.


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