Re: Preemptable Ticket Spinlock

[Rik van Riel]

On 04/22/2013 05:56 PM, Andi Kleen wrote:
> Rik van Riel <riel@xxxxxxxxxx> writes:
> > If we always incremented the ticket number by 2 (instead of 1), then
> > we could use the lower bit of the ticket number as the spinlock.
>
> Spinning on a single bit is very inefficient, as you need to do
> try lock in a loop which is very unfriendly to the MESI state protocol.
> It's much better to have at least three states and allow
> spinning-while-reading-only.
>
> This is typically very visible on systems with >2S.

Absolutely, the spinning should be read-only, until the CPU
sees that the desired bit is clear.  MESI-friendly spinning
is essential.

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