Messages in this thread | | | Date | Mon, 22 Apr 2013 19:13:48 -0400 | From | Rik van Riel <> | Subject | Re: Preemptable Ticket Spinlock |
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On 04/22/2013 05:56 PM, Andi Kleen wrote: > Rik van Riel <riel@redhat.com> 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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