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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> >>However, I *do* agree with you that it's simpler to just squash this patch
> >>into 01/11.
> >Uh, did I say that? Oh I said why don't make it right the first time!
> >
> >I meant in terms of seperating the slowpath (aka the bytelock on the pending
> >bit) from the queue (MCS code). Or renaming the function to be called
> >'complex' instead of 'slowpath' as it is getting quite hairy.
> >
> >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

Aha!

> 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.

That should be mentioned in the commit description as the rationale for
the patch "qspinlock: Add pending bit" and also in the code.

Thank you!

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